What is Ecology?
Ecology is the scientific study of interactions of organisms with one another and with the physical and chemical environment. Although it includes the study of environmental problems such as pollution, the science of ecology mainly involves research on the natural world from many viewpoints, using many techniques. Modern ecology relies heavily on experiments, both in laboratory and in field settings. These techniques have proved useful in testing ecological theories, and in arriving at practical decisions concerning the management of natural resources.
An understanding of ecology is essential for the survival of the human species. Our populations are increasing rapidly, all around the world, and we are in grave danger of outstripping the earth’s ability to supply the resources that we need for our long-term survival. Furthermore, social, economic and political factors often influence the short-term distribution of resources needed by a specific human population. An understanding of ecological principles can help us understand the global and regional consequences of competition among humans for the scarce natural resources that support us.
Ecology is a science that contributes considerably to our understanding of evolution, including our own evolution as a species. All evolutionary change takes place in response to ecological interactions that operate on the population, community, ecosystem, biome and biosphere levels. Studies conducted within the scientific discipline of ecology may therefore focus on one or more different levels: on populations of a single species, on an interacting community involving populations of many species, on the movement of matter and energy through a community within and ecosystem, on large scale processes within a biome, or on global patterns within the biosphere.
In the standard 8 syllabus, we consider the basic principles that allow us to understand the structure and function of each level of organisation in nature, examining the levels from the biosphere down to the population. The central theme in our studies will be to develop an appreciation that an understanding of the structure and function of the various levels and the relationships between them is vitally important for the well-being of humanity and of life in general.
Wednesday, November 21, 2007
Flatworms
The flatworm’s cephalized soft body is ribbon-shaped, flattened dorso-ventrally (from top to bottom), and bilaterally symmetric. Flatworms are the simplest triploblastic animals with organs. This means their organ systems form out of three germ layers: an outer ectoderm and an inner endoderm with a mesoderm between them. Turbellarians generally have a ciliated epidermis, while cestodes and trematodes covered with a cuticle. There is also no true body cavity (coelom) except the gut; hence, flatworms are classified as acoelomates. The interior of the acoelomate body is filled with somewhat loosely spaced mesodermal tissue called parenchyma tissue.
Flatworms exhibit an undulating form of locomotion.
Depending on species and age, individuals can range in size from almost microscopic to over 20 m long. The longest ever recorded flatworm was a tapeworm over 90 ft (27 m) long.[3]
[edit] Circulation and nervous system
There is no true circulatory or respiratory system, but like all other animals, flatworms do take in oxygen. Extracellular body fluids (interstitial fluids) percolate between cells to help distribute nutrients, gases, and waste products.
Flatworms respire at their integument; gasses diffuse directly across their moist outer surface. This type of system is called integumentary exchange.
However, flatworms do have a bilateral nervous system; they are the simplest animals to have one. Two cordlike nerves branch repeatedly in an array resembling a ladder. The head end of some species even has a collection of ganglia acting as a rudimentary brain to integrate signals from sensory organs such as eyespots.
[edit] Feeding
Usually the digestive tract has one opening, so the animal can't feed, digest, and eliminate undigested particles of food simultaneously, as most animals with tubular guts are able to do. This blind-ended gastrovascular cavity functions similarly to that of the Cnidaria. However, in a few particularly long flatworms or those with highly branched guts, there may be one or more anuses. A small group where the gut is absent or non-permanent, called acoel flatworms, appear to be unrelated to the other Platyhelminthes (see below).
Despite the simplicity of the digestive chamber, they are significantly more complex than cnidarians in that they possess numerous organs, and are therefore said to show an organ level of organization. Mesoderm allows for the development of these organs, and true muscle. Major sense organs are concentrated in the front end of the animals for species who possess these organs.
Muscular contraction in the gut causes a strong sucking force which allows flatworms to ingest food.
[edit] Reproduction
Flatworms reproduce sexually, thus Flatworm reproduction is hermaphroditic, meaning each individual produces eggs and sperm. When two flatworms mate, they exchange sperm so both become fertilized. Some flatworms, such as Pseudobiceros hancockanus engage in penis fencing, in which two individuals fight,trying to pierce the skin of the other with their penises; the first to succeed inseminates the other, which must then carry and nourish the eggs.[4] Flatworms usually do not fertilize their own eggs.
Turbellarians classified as planarians (usually freshwater, non-parasitic) can also reproduce asexually by transverse fission. The body constricts at the midsection, and the posterior end grips a substrate. After a few hours of tugging, the body rips apart at the constriction. Each half grows replacements of the missing pieces to form two whole flatworms. This also means that if one of these planarian flatworms is cut in half, each half will regenerate, forming two separate, fully-functioning flatworms.
[edit] Classes
"Platodes" from Ernst Haeckel's Kunstformen der Natur, 1909
Flatworms were formerly considered to be basal among the protostomes. Molecular evidence suggests that this is only true of the orders Acoela and Nemertodermatida, which are thus given their own phylum Acoelomorpha. These findings, however, are still not accepted by all biologists. The systematic position of Catenulida seems uncertain, although Donoghue and Cracraft would place it as a sister group to all other non-Acoelomorpha flatworms.[5] Xenoturbella was at first believed to be a flatworm as well, but it is now obvious that it belongs in its own phyla. The remaining and true flatworms form a monophyletic group that developed from more complex ancestors, and grouped with several other phyla as the Platyzoa. The traditional classifications of flatworms is primarily based on differing degrees of parasitism and divided into three monophyletic classes:
Trematoda - flukes, probably paraphyletic to Cestoda.
Cestoda - tapeworms
Monogenea - ectoparasitic flukes with simpler life cycles than Trematode flukes. They live an exclusively parasitic existence.
The remaining flatworms are grouped together for convenience as the class Turbellaria, now comprising the following orders:
Catenulida
Macrostomida
Lecithoepitheliata
Rhabdocoela
Prolecithophora
Proseriata
Tricladida
Polycladida - Marine flatworms
Most of these groups include free-living forms. The flukes and tapeworms, though, are parasitic, and a few cause massive damage to humans and other animals.
[edit] Biochemical memory experiments
In 1955, Thompson and James V. McConnell conditioned planarian flatworms by pairing a bright light with an electric shock. After repeating this several times they took away the electric shock, and only exposed them to the bright light. The flatworms would react to the bright light as if they had been shocked. Thompson and McConnell found that if they cut the worm in two, and allowed both worms to regenerate each half would develop the light-shock reaction. In 1962, McConnell repeated the experiment, but instead of cutting the trained flatworms in two he ground them into small pieces and fed them to other flatworms. Incredibly these flatworms learned to associate the bright light with a shock much faster than flatworms who had not been fed trained worms.
This experiment intended to show that memory could perhaps be transferred chemically. The experiment was repeated with mice, fish, and rats, but it always failed to produce the same results, even when it was performed by other scientists who also used planaria. The perceived explanation was that rather than memory being transferred to the other animals, it was the hormones in the ingested ground animals that changed its behaviour.[6] McConnell believed that this was evidence of a chemical basis for memory, which he identified as memory RNA. McConnell's results are now attributed to observer bias.[7] No double-blind experiment has
The flatworm’s cephalized soft body is ribbon-shaped, flattened dorso-ventrally (from top to bottom), and bilaterally symmetric. Flatworms are the simplest triploblastic animals with organs. This means their organ systems form out of three germ layers: an outer ectoderm and an inner endoderm with a mesoderm between them. Turbellarians generally have a ciliated epidermis, while cestodes and trematodes covered with a cuticle. There is also no true body cavity (coelom) except the gut; hence, flatworms are classified as acoelomates. The interior of the acoelomate body is filled with somewhat loosely spaced mesodermal tissue called parenchyma tissue.
Flatworms exhibit an undulating form of locomotion.
Depending on species and age, individuals can range in size from almost microscopic to over 20 m long. The longest ever recorded flatworm was a tapeworm over 90 ft (27 m) long.[3]
[edit] Circulation and nervous system
There is no true circulatory or respiratory system, but like all other animals, flatworms do take in oxygen. Extracellular body fluids (interstitial fluids) percolate between cells to help distribute nutrients, gases, and waste products.
Flatworms respire at their integument; gasses diffuse directly across their moist outer surface. This type of system is called integumentary exchange.
However, flatworms do have a bilateral nervous system; they are the simplest animals to have one. Two cordlike nerves branch repeatedly in an array resembling a ladder. The head end of some species even has a collection of ganglia acting as a rudimentary brain to integrate signals from sensory organs such as eyespots.
[edit] Feeding
Usually the digestive tract has one opening, so the animal can't feed, digest, and eliminate undigested particles of food simultaneously, as most animals with tubular guts are able to do. This blind-ended gastrovascular cavity functions similarly to that of the Cnidaria. However, in a few particularly long flatworms or those with highly branched guts, there may be one or more anuses. A small group where the gut is absent or non-permanent, called acoel flatworms, appear to be unrelated to the other Platyhelminthes (see below).
Despite the simplicity of the digestive chamber, they are significantly more complex than cnidarians in that they possess numerous organs, and are therefore said to show an organ level of organization. Mesoderm allows for the development of these organs, and true muscle. Major sense organs are concentrated in the front end of the animals for species who possess these organs.
Muscular contraction in the gut causes a strong sucking force which allows flatworms to ingest food.
[edit] Reproduction
Flatworms reproduce sexually, thus Flatworm reproduction is hermaphroditic, meaning each individual produces eggs and sperm. When two flatworms mate, they exchange sperm so both become fertilized. Some flatworms, such as Pseudobiceros hancockanus engage in penis fencing, in which two individuals fight,trying to pierce the skin of the other with their penises; the first to succeed inseminates the other, which must then carry and nourish the eggs.[4] Flatworms usually do not fertilize their own eggs.
Turbellarians classified as planarians (usually freshwater, non-parasitic) can also reproduce asexually by transverse fission. The body constricts at the midsection, and the posterior end grips a substrate. After a few hours of tugging, the body rips apart at the constriction. Each half grows replacements of the missing pieces to form two whole flatworms. This also means that if one of these planarian flatworms is cut in half, each half will regenerate, forming two separate, fully-functioning flatworms.
[edit] Classes
"Platodes" from Ernst Haeckel's Kunstformen der Natur, 1909
Flatworms were formerly considered to be basal among the protostomes. Molecular evidence suggests that this is only true of the orders Acoela and Nemertodermatida, which are thus given their own phylum Acoelomorpha. These findings, however, are still not accepted by all biologists. The systematic position of Catenulida seems uncertain, although Donoghue and Cracraft would place it as a sister group to all other non-Acoelomorpha flatworms.[5] Xenoturbella was at first believed to be a flatworm as well, but it is now obvious that it belongs in its own phyla. The remaining and true flatworms form a monophyletic group that developed from more complex ancestors, and grouped with several other phyla as the Platyzoa. The traditional classifications of flatworms is primarily based on differing degrees of parasitism and divided into three monophyletic classes:
Trematoda - flukes, probably paraphyletic to Cestoda.
Cestoda - tapeworms
Monogenea - ectoparasitic flukes with simpler life cycles than Trematode flukes. They live an exclusively parasitic existence.
The remaining flatworms are grouped together for convenience as the class Turbellaria, now comprising the following orders:
Catenulida
Macrostomida
Lecithoepitheliata
Rhabdocoela
Prolecithophora
Proseriata
Tricladida
Polycladida - Marine flatworms
Most of these groups include free-living forms. The flukes and tapeworms, though, are parasitic, and a few cause massive damage to humans and other animals.
[edit] Biochemical memory experiments
In 1955, Thompson and James V. McConnell conditioned planarian flatworms by pairing a bright light with an electric shock. After repeating this several times they took away the electric shock, and only exposed them to the bright light. The flatworms would react to the bright light as if they had been shocked. Thompson and McConnell found that if they cut the worm in two, and allowed both worms to regenerate each half would develop the light-shock reaction. In 1962, McConnell repeated the experiment, but instead of cutting the trained flatworms in two he ground them into small pieces and fed them to other flatworms. Incredibly these flatworms learned to associate the bright light with a shock much faster than flatworms who had not been fed trained worms.
This experiment intended to show that memory could perhaps be transferred chemically. The experiment was repeated with mice, fish, and rats, but it always failed to produce the same results, even when it was performed by other scientists who also used planaria. The perceived explanation was that rather than memory being transferred to the other animals, it was the hormones in the ingested ground animals that changed its behaviour.[6] McConnell believed that this was evidence of a chemical basis for memory, which he identified as memory RNA. McConnell's results are now attributed to observer bias.[7] No double-blind experiment has
Arthropods
Blue crab (Callinectes sapidus), a crustacean
The success of arthropods is related to their hard exoskeleton, segmentation, and jointed appendages. The appendages are used for feeding, sensory reception, defense, and locomotion. The muscle system is more or less assisted by hydraulics originated from the blood pressure created by the heart.[2] The hydraulic system in spiders is especially well developed.
Harpaphe haydeniana, a myriapod
Citrus root weevil, an insect
Aquatic arthropods use gills to exchange gases. These gills have an extensive surface area in contact with the surrounding water. Terrestrial arthropods have internal surfaces that are specialised for gas exchange. Insects and most other terrestrial species have tracheal systems: air sacs leading into the body from pores called spiracles in the epidermis cuticle. Others use book lungs, or gills modified for breathing air as seen in species like the coconut crab. Some areas of the legs of soldier crabs are covered with an oxygen absorbing membrane. The gill chambers in terrestrial crabs sometimes have two different structures: one that is gilled and used for breathing underwater, and another specially adapted to take up oxygen from the air (a pseudolung). Arthropods also have a complete digestive system with both a mouth and anus.
Arthropods have an open circulatory system. Haemolymph containing haemocyanin, a copper-based oxygen-carrying protein (the copper makes the blood blue, unlike humans that use hemoglobin which uses iron that makes it red), is propelled by a series of hearts into the body cavity where it comes in direct contact with the tissues. Arthropods are protostomes. There is a coelom, but it is reduced to a tiny cavity around the reproductive and excretory organs, and the dominant body cavity is a haemocoel, filled with haemolymph which bathes the organs directly. The arthropod body is divided into a series of distinct segments, plus a pre-segmental acron which usually supports compound and simple eyes and a post-segmental telson. These are grouped into distinct, specialised body regions called tagmata. Each segment, at least primitively, supports a pair of appendages.
The cuticle in arthropods forms a rigid exoskeleton, composed mainly of chitin, which is periodically shed as the animal grows. They contain an inner zone (procuticle) which is made of protein and chitin and is responsible for the strength of the exoskeleton. The outer zone (epicuticle) lies on the surface of the procuticle. It is nonchitinous and is a complex of proteins and lipids. It provides the moisture proofing and protection to the procuticle. The exoskeleton takes the form of plates called sclerites on the segments, plus rings on the appendages that divide them into segments separated by joints. This is in fact what gives arthropods their name — jointed feet — and separates them from their relatives, the Onychophora and Tardigrada, also called Lobopoda (and which is sometimes included in a group called Panarthropoda that also includes arthropods). The exoskeletons of arthropods strengthen them against attack by predators and are impermeable to water. In order to grow, an arthropod must shed its old exoskeleton and secrete a new one. This process, ecdysis, is expensive in terms of energy, and during the moulting period, an arthropod is vulnerable.
[edit] Classification of arthropods
Arthropoda
Paradoxopoda
Myriapoda
Chelicerata
Pancrustacea
Cirripedia
Remipedia
Collembola
Branchiopoda
Cephalocarida
Malacostraca
Insecta
Phylogenetic relationships of the major extant arthropod groups, derived from mitochondrial DNA sequences.[3] Taxa in pink are parts of the subphylum Crustacea.
Arthropods are typically classified into five subphyla, of which one is extinct:[4]
Trilobites are a group of formerly numerous marine animals that died in the mass extinction at the end of the Permian-Triassic extinction event.
Chelicerates include spiders, mites, scorpions and related organisms. They are characterised by the presence of chelicerae.
Myriapods comprise millipedes and centipedes and their relatives and have many body segments, each bearing one or two pairs of legs. They are sometimes grouped with the hexapods.
Hexapods comprise insects and three small orders of insect-like animals with six thoracic legs. They are sometimes grouped with the myriapods, in a group called Uniramia, though genetic evidence tends to support a closer relationship between hexapods and crustaceans.
Crustaceans are primarily marine (a notable exception being woodlice) and are characterised by having biramous appendages. They include lobsters, crabs, barnacles, and many others.
Aside from these major groups, there are also a number of fossil forms - mostly from the lower Cambrian - including anomalocarids, euthycarcinoids [5] and Arthrogyrinus which are difficult to place, either from lack of obvious affinity to any of the main groups or from clear affinity to several of them.
The phylogeny of the arthropods has been an area of considerable interest and dispute. The validity of many of the arthropod groups suggested by earlier authors is being questioned by recent studies; these include Mandibulata, Uniramia and Atelocerata. The most recent studies tend to suggest a paraphyletic Crustacea with different hexapod groups nested within it.[3][6] The remaining clade of Myriapoda and Chelicerata is referred to as Paradoxopoda or Myriochelata.
Since the International Code of Zoological Nomenclature recognises no priority above the rank of family, many of the higher groups can be referred to by a variety of different names.[7]
[edit] Evolution
A velvet worm
Sipuncula
Articulata
Mollusca
Euarticulata
Annelida
Panarthropoda
Onychophora
Tardigrada
Arthropoda
A phylogeny of the arthropods after Nielsen.[8]
Arthropods are today almost universally considered to be monophyletic, i.e. they only arose once, a view supported by both morphological and molecular studies. Such a view contradicts the widespread view in the 1970s that the arthropods had evolved on several occasions from soft-bodied, annelid-like ancestors.
The closest relatives of the arthropods are usually considered to be the Tardigrada and Onychophora, together forming the monophyletic group Panarthropoda (the crustaceans, myriapods, chelicerates and insects are often referred to as "Euarthropoda" to distinguish them from their soft-bodied relatives). Comparison between these groups suggests that the euarthropods evolved from a soft-bodied ancestor not too dissimilar to the living onychophorans, a view that has found some support from the fossil record.
Traditionally the Annelida have been considered the closest relatives of these three phyla, on account of their common segmentation. Molecular data however, are strongly against this grouping (known as the Articulata), suggesting instead that the panarthropods belong in a clade including both the arthropods and various pseudocoelomates such as roundworms and priapulids that share with them growth by moulting, or ecdysis, from which its name, the Ecdysozoa. is derived. If this new grouping is correct, then segmentation of arthropods and annelids has either evolved through convergence, or has been inherited from a very deep ancestor, and has been subsequently lost in several other lineages, such as the non-arthropod members of the Ecdysozoa
Blue crab (Callinectes sapidus), a crustacean
The success of arthropods is related to their hard exoskeleton, segmentation, and jointed appendages. The appendages are used for feeding, sensory reception, defense, and locomotion. The muscle system is more or less assisted by hydraulics originated from the blood pressure created by the heart.[2] The hydraulic system in spiders is especially well developed.
Harpaphe haydeniana, a myriapod
Citrus root weevil, an insect
Aquatic arthropods use gills to exchange gases. These gills have an extensive surface area in contact with the surrounding water. Terrestrial arthropods have internal surfaces that are specialised for gas exchange. Insects and most other terrestrial species have tracheal systems: air sacs leading into the body from pores called spiracles in the epidermis cuticle. Others use book lungs, or gills modified for breathing air as seen in species like the coconut crab. Some areas of the legs of soldier crabs are covered with an oxygen absorbing membrane. The gill chambers in terrestrial crabs sometimes have two different structures: one that is gilled and used for breathing underwater, and another specially adapted to take up oxygen from the air (a pseudolung). Arthropods also have a complete digestive system with both a mouth and anus.
Arthropods have an open circulatory system. Haemolymph containing haemocyanin, a copper-based oxygen-carrying protein (the copper makes the blood blue, unlike humans that use hemoglobin which uses iron that makes it red), is propelled by a series of hearts into the body cavity where it comes in direct contact with the tissues. Arthropods are protostomes. There is a coelom, but it is reduced to a tiny cavity around the reproductive and excretory organs, and the dominant body cavity is a haemocoel, filled with haemolymph which bathes the organs directly. The arthropod body is divided into a series of distinct segments, plus a pre-segmental acron which usually supports compound and simple eyes and a post-segmental telson. These are grouped into distinct, specialised body regions called tagmata. Each segment, at least primitively, supports a pair of appendages.
The cuticle in arthropods forms a rigid exoskeleton, composed mainly of chitin, which is periodically shed as the animal grows. They contain an inner zone (procuticle) which is made of protein and chitin and is responsible for the strength of the exoskeleton. The outer zone (epicuticle) lies on the surface of the procuticle. It is nonchitinous and is a complex of proteins and lipids. It provides the moisture proofing and protection to the procuticle. The exoskeleton takes the form of plates called sclerites on the segments, plus rings on the appendages that divide them into segments separated by joints. This is in fact what gives arthropods their name — jointed feet — and separates them from their relatives, the Onychophora and Tardigrada, also called Lobopoda (and which is sometimes included in a group called Panarthropoda that also includes arthropods). The exoskeletons of arthropods strengthen them against attack by predators and are impermeable to water. In order to grow, an arthropod must shed its old exoskeleton and secrete a new one. This process, ecdysis, is expensive in terms of energy, and during the moulting period, an arthropod is vulnerable.
[edit] Classification of arthropods
Arthropoda
Paradoxopoda
Myriapoda
Chelicerata
Pancrustacea
Cirripedia
Remipedia
Collembola
Branchiopoda
Cephalocarida
Malacostraca
Insecta
Phylogenetic relationships of the major extant arthropod groups, derived from mitochondrial DNA sequences.[3] Taxa in pink are parts of the subphylum Crustacea.
Arthropods are typically classified into five subphyla, of which one is extinct:[4]
Trilobites are a group of formerly numerous marine animals that died in the mass extinction at the end of the Permian-Triassic extinction event.
Chelicerates include spiders, mites, scorpions and related organisms. They are characterised by the presence of chelicerae.
Myriapods comprise millipedes and centipedes and their relatives and have many body segments, each bearing one or two pairs of legs. They are sometimes grouped with the hexapods.
Hexapods comprise insects and three small orders of insect-like animals with six thoracic legs. They are sometimes grouped with the myriapods, in a group called Uniramia, though genetic evidence tends to support a closer relationship between hexapods and crustaceans.
Crustaceans are primarily marine (a notable exception being woodlice) and are characterised by having biramous appendages. They include lobsters, crabs, barnacles, and many others.
Aside from these major groups, there are also a number of fossil forms - mostly from the lower Cambrian - including anomalocarids, euthycarcinoids [5] and Arthrogyrinus which are difficult to place, either from lack of obvious affinity to any of the main groups or from clear affinity to several of them.
The phylogeny of the arthropods has been an area of considerable interest and dispute. The validity of many of the arthropod groups suggested by earlier authors is being questioned by recent studies; these include Mandibulata, Uniramia and Atelocerata. The most recent studies tend to suggest a paraphyletic Crustacea with different hexapod groups nested within it.[3][6] The remaining clade of Myriapoda and Chelicerata is referred to as Paradoxopoda or Myriochelata.
Since the International Code of Zoological Nomenclature recognises no priority above the rank of family, many of the higher groups can be referred to by a variety of different names.[7]
[edit] Evolution
A velvet worm
Sipuncula
Articulata
Mollusca
Euarticulata
Annelida
Panarthropoda
Onychophora
Tardigrada
Arthropoda
A phylogeny of the arthropods after Nielsen.[8]
Arthropods are today almost universally considered to be monophyletic, i.e. they only arose once, a view supported by both morphological and molecular studies. Such a view contradicts the widespread view in the 1970s that the arthropods had evolved on several occasions from soft-bodied, annelid-like ancestors.
The closest relatives of the arthropods are usually considered to be the Tardigrada and Onychophora, together forming the monophyletic group Panarthropoda (the crustaceans, myriapods, chelicerates and insects are often referred to as "Euarthropoda" to distinguish them from their soft-bodied relatives). Comparison between these groups suggests that the euarthropods evolved from a soft-bodied ancestor not too dissimilar to the living onychophorans, a view that has found some support from the fossil record.
Traditionally the Annelida have been considered the closest relatives of these three phyla, on account of their common segmentation. Molecular data however, are strongly against this grouping (known as the Articulata), suggesting instead that the panarthropods belong in a clade including both the arthropods and various pseudocoelomates such as roundworms and priapulids that share with them growth by moulting, or ecdysis, from which its name, the Ecdysozoa. is derived. If this new grouping is correct, then segmentation of arthropods and annelids has either evolved through convergence, or has been inherited from a very deep ancestor, and has been subsequently lost in several other lineages, such as the non-arthropod members of the Ecdysozoa
sexual and asxual reproduction.
For other uses, see Reproduction (disambiguation).
Reproduction is the biological process by which new individual organisms are produced. Reproduction is a fundamental feature of all known life; each individual organism exists as the result of reproduction. The known methods of reproduction are broadly grouped into two main types: sexual and asexual.
In asexual reproduction, an individual can reproduce without involvement with another individual of that species. The division of a bacterial cell into two daughter cells is an example of asexual reproduction. Asexual reproduction is not, however, limited to single-celled organisms. Most plants have the ability to reproduce asexually.
Sexual reproduction requires the involvement of two individuals, typically one of each sex. Normal human reproduction is a common example of sexual reproduction
Asexual reproduction
Main article: Asexual reproduction
Asexual reproduction is the biological process by which an organism creates a genetically-similar or identical copy of itself without a contribution of genetic material from another individual. Bacteria divide asexually via binary fission; viruses take control of host cells to produce more viruses; Hydras (invertebrates of the order Hydroidea) and yeasts are able to reproduce by budding. These organisms do not have different sexes, and they are capable of "splitting" themselves into two or more individuals. Some 'asexual' species, like hydra and jellyfish, may also reproduce sexually. For instance, most plants are capable of vegetative reproduction—reproduction without seeds or spores—but can also reproduce sexually. Likewise, bacteria may exchange genetic information by conjugation. Other ways of asexual reproduction include parthogenesis, fragmentation and spore formation that involves only mitosis. Parthenogenesis (from the Greek παρθένος parthenos, "virgin", + γένεσις genesis, "creation") is the growth and development of embryo or seed without fertilization by a male. Parthenogenesis occurs naturally in some species, including lower plants, invertebrates (e.g. water fleas, aphids, some bees and parasitic wasps), and vertebrates (e.g. some reptiles,[1] fish, and, very rarely, birds[2] and sharks[3]). It is sometimes also used to describe reproduction modes in hermaphroditic species which can self-fertilize.
Sexual reproduction
Main article: Sexual reproduction
Hoverflies mating in midair flight
Sexual reproduction is a biological process by which organisms create descendants that have a combination of genetic material contributed from two (usually) different members of the species. Each of two parent organisms contributes half of the offspring's genetic makeup by creating haploid gametes. Most organisms form two different types of gametes. In these anisogamous species, the two sexes are referred to as male (producing sperm or microspores) and female (producing ova or megaspores). In isogamous species the gametes are similar or identical in form, but may have separable properties and then may be given other different names. For example, in the green alga, Chlamydomonas reinhardtii, there are so-called "plus" and "minus" gametes. A few types of organisms, such as ciliates, have more than two kinds of gametes.
Most animals (including humans) and plants reproduce sexually. Sexually reproducing organisms have two sets of genes for every trait (called alleles). Offspring inherit one allele for each trait from each parent, thereby ensuring that offspring have a combination of the parents' genes. Having two copies of every gene, only one of which is expressed, allows deleterious alleles to be masked, an advantage believed to have led to the evolutionary development of diploidy (Otto and Goldstein).
Allogamy
Main article: Allogamy
Allogamy is a term used in the field of biological reproduction describing the fertilization of an ovum from one individual with the spermatozoa of another.
Autogamy
Self-fertilization (also known as autogamy) occurs in hermaphroditic organisms where the two gametes fused in fertilization come from the same individual. They are bound and all the cells merge to form one new gamete.
Mitosis and Meiosis
Mitosis and meiosis are an integral part of cell division. Mitosis occurs in somatic cells, while meiosis occurs in gametes.
Mitosis The resultant number of cells in mitosis is twice the number of original cells. The number of chromosomes in the daughter cells is the same as that of the parent cell.Meiosis The resultant number of cells is four times the number of original cells. This results in cells with half the number of chromosomes present in the parent cell. A diploid cell duplicates itself, then undergoes two divisions (tetroid to diploid to haploid), in the process forming four haploid cells. This process occurs in two phases, meiosis I and meiosis II.
Reproductive strategies
There is a wide range of reproductive strategies employed by different species. Some animals, such as the human and Northern Gannet, do not reach sexual maturity for many years after birth and even then produce few offspring. Others reproduce quickly; but, under normal circumstances, most offspring do not survive to adulthood. For example, a rabbit (mature after 8 months) can produce 10–30 offspring per year, and a fruit fly (mature after 10–14 days) can produce up to 900 offspring per year. These two main strategies are known as K-selection (few offspring) and r-selection (many offspring). Which strategy is favoured by evolution depends on a variety of circumstances. Animals with few offspring can devote more resources to the nurturing and protection of each individual offspring, thus reducing the need for many offspring. On the other hand, animals with many offspring may devote fewer resources to each individual offspring; for these types of animals it is common for many offspring to die soon after birth, but enough individuals typically survive to maintain the population.
Other types of reproductive strategies
Polycyclic animals reproduce intermittently throughout their lives.
Semelparous organisms reproduce only once in their lifetime, such as annual plants. Often, they die shortly after reproduction. This is a characteristic of r-strategists.
Iteroparous organisms produce offspring in successive (e.g. annual or seasonal) cycles, such as perennial plants. Iteroparous animals survive over multiple seasons (or periodic condition changes). This is a characteristic of K-strategists.
Asexual vs. sexual reproduction
Organisms that reproduce through asexual reproduction tend to grow in number exponentially. However, because they rely on mutation for variations in their DNA, all members of the species have similar vulnerabilities. Organisms that reproduce sexually yield a smaller number of offspring, but the large amount of variation in their genes makes them less susceptible to disease.
Many organisms can reproduce sexually as well as asexually. Aphids, slime molds, sea anemones, some species of starfish (by fragmentation), and many plants are examples. When environmental factors are favorable, asexual reproduction is employed to exploit suitable conditions for survival such as an abundant food supply, adequate shelter, favorable climate, disease, optimum pH or a proper mix of other lifestyle requirements. Populations of these organisms increase exponentially via asexual reproductive strategies to take full advantage of the rich supply resources.
When food sources have been depleted, the climate becomes hostile, or individual survival is jeopardized by some other adverse change in living conditions, these organisms switch to sexual forms of reproduction. Sexual reproduction ensures a mixing of the gene pool of the species. The variations found in offspring of sexual reproduction allow some individuals to be better suited for survival and provide a mechanism for selective adaptation to occur. In addition, sexual reproduction usually results in the formation of a life stage that is able to endure the conditions that threaten the offspring of an asexual parent. Thus, seeds, spores, eggs, pupae, cysts or other "over-wintering" stages of sexual reproduction ensure the survival during unfavorable times and the organism can "wait out" adverse situations until a swing back to suitability occurs.
Life without reproduction
The existence of life without reproduction is the subject of some speculation. The biological study of how the origin of life led from non-reproducing elements to reproducing organisms is called abiogenesis. Whether or not there were several independent abiogenetic events, biologists believe that the last universal ancestor to all present life on earth lived about 3.5 billion years ago.
Today, some scientists have speculated about the possibility of creating life non-reproductively in the laboratory. Several scientists have succeeded in producing simple viruses from entirely non-living materials[4]. The virus is often regarded as not alive. Being nothing more than a bit of RNA or DNA in a protein capsule, they have no metabolism and can only replicate with the assistance of a hijacked cell's metabolic machinery.
The production of a truly living organism (e.g. a simple bacterium) with no ancestors would be a much more complex task, but may well be possible according to current biological knowledge.
For other uses, see Reproduction (disambiguation).
Reproduction is the biological process by which new individual organisms are produced. Reproduction is a fundamental feature of all known life; each individual organism exists as the result of reproduction. The known methods of reproduction are broadly grouped into two main types: sexual and asexual.
In asexual reproduction, an individual can reproduce without involvement with another individual of that species. The division of a bacterial cell into two daughter cells is an example of asexual reproduction. Asexual reproduction is not, however, limited to single-celled organisms. Most plants have the ability to reproduce asexually.
Sexual reproduction requires the involvement of two individuals, typically one of each sex. Normal human reproduction is a common example of sexual reproduction
Asexual reproduction
Main article: Asexual reproduction
Asexual reproduction is the biological process by which an organism creates a genetically-similar or identical copy of itself without a contribution of genetic material from another individual. Bacteria divide asexually via binary fission; viruses take control of host cells to produce more viruses; Hydras (invertebrates of the order Hydroidea) and yeasts are able to reproduce by budding. These organisms do not have different sexes, and they are capable of "splitting" themselves into two or more individuals. Some 'asexual' species, like hydra and jellyfish, may also reproduce sexually. For instance, most plants are capable of vegetative reproduction—reproduction without seeds or spores—but can also reproduce sexually. Likewise, bacteria may exchange genetic information by conjugation. Other ways of asexual reproduction include parthogenesis, fragmentation and spore formation that involves only mitosis. Parthenogenesis (from the Greek παρθένος parthenos, "virgin", + γένεσις genesis, "creation") is the growth and development of embryo or seed without fertilization by a male. Parthenogenesis occurs naturally in some species, including lower plants, invertebrates (e.g. water fleas, aphids, some bees and parasitic wasps), and vertebrates (e.g. some reptiles,[1] fish, and, very rarely, birds[2] and sharks[3]). It is sometimes also used to describe reproduction modes in hermaphroditic species which can self-fertilize.
Sexual reproduction
Main article: Sexual reproduction
Hoverflies mating in midair flight
Sexual reproduction is a biological process by which organisms create descendants that have a combination of genetic material contributed from two (usually) different members of the species. Each of two parent organisms contributes half of the offspring's genetic makeup by creating haploid gametes. Most organisms form two different types of gametes. In these anisogamous species, the two sexes are referred to as male (producing sperm or microspores) and female (producing ova or megaspores). In isogamous species the gametes are similar or identical in form, but may have separable properties and then may be given other different names. For example, in the green alga, Chlamydomonas reinhardtii, there are so-called "plus" and "minus" gametes. A few types of organisms, such as ciliates, have more than two kinds of gametes.
Most animals (including humans) and plants reproduce sexually. Sexually reproducing organisms have two sets of genes for every trait (called alleles). Offspring inherit one allele for each trait from each parent, thereby ensuring that offspring have a combination of the parents' genes. Having two copies of every gene, only one of which is expressed, allows deleterious alleles to be masked, an advantage believed to have led to the evolutionary development of diploidy (Otto and Goldstein).
Allogamy
Main article: Allogamy
Allogamy is a term used in the field of biological reproduction describing the fertilization of an ovum from one individual with the spermatozoa of another.
Autogamy
Self-fertilization (also known as autogamy) occurs in hermaphroditic organisms where the two gametes fused in fertilization come from the same individual. They are bound and all the cells merge to form one new gamete.
Mitosis and Meiosis
Mitosis and meiosis are an integral part of cell division. Mitosis occurs in somatic cells, while meiosis occurs in gametes.
Mitosis The resultant number of cells in mitosis is twice the number of original cells. The number of chromosomes in the daughter cells is the same as that of the parent cell.Meiosis The resultant number of cells is four times the number of original cells. This results in cells with half the number of chromosomes present in the parent cell. A diploid cell duplicates itself, then undergoes two divisions (tetroid to diploid to haploid), in the process forming four haploid cells. This process occurs in two phases, meiosis I and meiosis II.
Reproductive strategies
There is a wide range of reproductive strategies employed by different species. Some animals, such as the human and Northern Gannet, do not reach sexual maturity for many years after birth and even then produce few offspring. Others reproduce quickly; but, under normal circumstances, most offspring do not survive to adulthood. For example, a rabbit (mature after 8 months) can produce 10–30 offspring per year, and a fruit fly (mature after 10–14 days) can produce up to 900 offspring per year. These two main strategies are known as K-selection (few offspring) and r-selection (many offspring). Which strategy is favoured by evolution depends on a variety of circumstances. Animals with few offspring can devote more resources to the nurturing and protection of each individual offspring, thus reducing the need for many offspring. On the other hand, animals with many offspring may devote fewer resources to each individual offspring; for these types of animals it is common for many offspring to die soon after birth, but enough individuals typically survive to maintain the population.
Other types of reproductive strategies
Polycyclic animals reproduce intermittently throughout their lives.
Semelparous organisms reproduce only once in their lifetime, such as annual plants. Often, they die shortly after reproduction. This is a characteristic of r-strategists.
Iteroparous organisms produce offspring in successive (e.g. annual or seasonal) cycles, such as perennial plants. Iteroparous animals survive over multiple seasons (or periodic condition changes). This is a characteristic of K-strategists.
Asexual vs. sexual reproduction
Organisms that reproduce through asexual reproduction tend to grow in number exponentially. However, because they rely on mutation for variations in their DNA, all members of the species have similar vulnerabilities. Organisms that reproduce sexually yield a smaller number of offspring, but the large amount of variation in their genes makes them less susceptible to disease.
Many organisms can reproduce sexually as well as asexually. Aphids, slime molds, sea anemones, some species of starfish (by fragmentation), and many plants are examples. When environmental factors are favorable, asexual reproduction is employed to exploit suitable conditions for survival such as an abundant food supply, adequate shelter, favorable climate, disease, optimum pH or a proper mix of other lifestyle requirements. Populations of these organisms increase exponentially via asexual reproductive strategies to take full advantage of the rich supply resources.
When food sources have been depleted, the climate becomes hostile, or individual survival is jeopardized by some other adverse change in living conditions, these organisms switch to sexual forms of reproduction. Sexual reproduction ensures a mixing of the gene pool of the species. The variations found in offspring of sexual reproduction allow some individuals to be better suited for survival and provide a mechanism for selective adaptation to occur. In addition, sexual reproduction usually results in the formation of a life stage that is able to endure the conditions that threaten the offspring of an asexual parent. Thus, seeds, spores, eggs, pupae, cysts or other "over-wintering" stages of sexual reproduction ensure the survival during unfavorable times and the organism can "wait out" adverse situations until a swing back to suitability occurs.
Life without reproduction
The existence of life without reproduction is the subject of some speculation. The biological study of how the origin of life led from non-reproducing elements to reproducing organisms is called abiogenesis. Whether or not there were several independent abiogenetic events, biologists believe that the last universal ancestor to all present life on earth lived about 3.5 billion years ago.
Today, some scientists have speculated about the possibility of creating life non-reproductively in the laboratory. Several scientists have succeeded in producing simple viruses from entirely non-living materials[4]. The virus is often regarded as not alive. Being nothing more than a bit of RNA or DNA in a protein capsule, they have no metabolism and can only replicate with the assistance of a hijacked cell's metabolic machinery.
The production of a truly living organism (e.g. a simple bacterium) with no ancestors would be a much more complex task, but may well be possible according to current biological knowledge.
predators
Cats Are Predators in a Lithe, Supple Package
Like most predators, cats have keen senses. Our lovingly spoiled and mostly domesticated former hunters get food served to them these days, often in fancy bowls. But that doesn't mean they've lost the senses their wild kin rely on to survive.
People and cats live in completely different worlds when it comes to our respective sensory perceptions. When you imagine things from your pet's point of view, you'll be able to better understand what makes your cat tick.
Consider the feline sense of smell, which is many times more powerful than a human's puny abilities in this area. Once you know that your cat is so much more sensitive than you are capable of being, you shouldn't be surprised that the litter box you think is "tolerable" may be offensive to your cat. Same goes for those perfumed litters: We may love them, but they can be strong to the point of overpowering to our keen-nosed felines.
Of course, the litter box is a relatively modern convenience, and the cat's sense of smell is good for much more than deciding when it's not clean enough. Smell also plays a role in the establishment of territory. Cats like things in their home range to smell like them, be they people or furniture, and make them familiar by rubbing or scratching. The sense of smell is also important to free-roaming cats when it comes to finding prey, and in the determination of whether "found" food is safe enough to eat.
Dogs are scavengers who eat just about anything; cats are true predators. For them, fresh food, please, is their decided preference. Ever wonder why your cat turns up his nose at canned food that's been out a while? Simple: It doesn't smell right.
The sense of smell and taste are very closely connected in cats in part because the animals have a special anatomical feature called the vomeronasal organ, which allows them to process scent almost by tasting it. The organ is at the front of the roof of the mouth, and you can tell when your cat is using it: They open their mouths a crack and seem to be panting slightly. The facial expression that accompanies this behavior is so distinctive that it even has a name: the Flehmen response.
You can use your cat's well-developed sense of smell to your advantage if you're trying to entice a sick or just plain finicky cat to eat. If you warm your cat's wet food to just above room temperature before serving (about 85 degrees, or what we humans would call "lukewarm"), you'll make the odor more enticing, and so increase the appeal of the meal.
No matter what you do, though, you're not likely to get most cats interested in anything sweet. It's not for any lack in their sensory ability. Experts believe cats can identify foods that are bitter, salty, sweet or sour, although their appreciation of any of those qualities differs greatly from our own.
Not surprising, really, when you consider that were your cat to choose a gourmet meal, chances are he'd opt for a freshly killed mouse, hold the seasonings. Or maybe, knowing the skillful predator that lives in even the most pampered of pets, your cat would choose his meal live and easy to catch.
Dead or alive, a mouse is a meal preference no human would share. But then, our tongues also aren't adapted to clean meat off the bones of prey the way a cat's sandpaper-textured licker is. It all comes together so beautifully in the lithe, supple body of the perfect small predator.
When you think about how different cats and people are, it makes you wonder how we get along as well as we do. If nothing else, it should give you a sense of wonder at the superhuman senses of our most popular pet.
PETS ON THE WEB
I wish every chronic pet illness had a Web site as helpful and supportive as the one dedicated to feline diabetes (http://www.felinediabetes.com). Administered by a cat-loving physician, the site offers everything an owner needs (but maybe didn't think to ask the veterinarian) about caring for a pet with this disease. What is it like to live with such an animal? You'll find that here, along with tips on using syringes and monitoring your pet's sugar levels. A worthwhile read, without a doubt -- this is a site that's built with science but runs on love.
THE SCOOP
Winterizing your car or truck? Now may be the time to do it, but make sure when you're taking care of your vehicle that you're also watching out for your pet. The worry? Coolant made from ethylene glycol, a sweet-tasting liquid that can be lethal to your pet in dosages as small as a teaspoon, or less.
Safer alternatives exist to ethylene glycol, such as coolant made from propylene glycol. No matter what you use, though, be sure to clean up any spills promptly and thoroughly, and keep any stored product in leak-proof containers in a closed cupboard. If your pet laps even the smallest amount of coolant, see your veterinarian immediately. Your pet's only shot at survival is prompt treatment.
QUESTIONS FROM THE PACK
Q: Our puppy, a Lab mix from the shelter, is almost 4 months old now. She has a lot of energy, and sometimes we think we made a mistake in adopting her. But we know that puppies are a lot of work, and we're going to hang in there.
The biggest problem we have is that we can't get her to quit biting when she plays. It's really serious, to the extent that our 7-year-old son is afraid to play with the puppy now. Since we got the puppy for our son, this obviously is a bad situation. Can you help? -- N.N., via e-mail
A: Consider how human babies explore the world around them: They touch things, they grab things, and they taste what they grab. Puppies are much the same way, but since nature didn't equip them with fingers, they do their exploring with their mouths.
If you watch a litter of puppies play with each other, you might be surprised at how rough they can be. They nip -- hard. They grab hold of each other by the ears with needle-sharp teeth and pull. As puppies grow older, they learn from their littermates and their mother how to restrain those playful bites, which is one reason why it's so important to leave a puppy with his canine family until he's at least 7 weeks of age.
Some puppies don't get this critical early education, and some others are just slow learners. Others still are from breeds that are known to be "mouthier" than others -- retrievers are the classic example.
You can teach your puppy to keep his teeth to himself by attacking the problem from a couple of different directions. The first would be to redirect the behavior, giving your puppy a yummy toy and praising her for chewing on something that's not a family member.
Even as you're teaching the puppy what's OK to mouth, teach her how to leave family members unchewed by making the nipping unrewarding. Every time the puppy nips, cry "ouch" in a loud voice and immediately stop the play session. Turn away and ignore the puppy completely for a few minutes. Teach your son to do the same thing.
The message to get across: Play stops when she nips. If you're persistent and consistent, your puppy will get start getting the message soon and will learn to inhibit her bites.
If the behavior doesn't show any sign of ending, or if the biting seems more aggressive than playful, don't delay in asking your veterinarian for a referral to a behaviorist or trainer.
Q: Would you put in a word for greyhounds? There's a real need for homes for these dogs after their racing careers are over. Greyhounds are sweet, gentle and affectionate. They may be retired athletes, but they are committed couch potatoes. About three months ago, I adopted a sweet, loving 2-year-old greyhound. I am amazed at how mellow she is. -- B.W., via e-mail
A: I don't mind at all making the case for greyhounds. They're generally clean, quiet and easygoing, and they seem to be aware of how lucky they are to be in a loving home.
In adopting one, you need to work with a reputable rescue organization that'll match the dog with your household. One of the biggest problems: Some greyhounds -- but not all -- don't mix safely with cats.
A good place to start researching is the Greyhound Project Web site (http://www.adopt-a-greyhound.org). You'll find lots of information pro and con, as well as links to regional rescue groups.
Date Published: 11/15/2001
Cats Are Predators in a Lithe, Supple Package
Like most predators, cats have keen senses. Our lovingly spoiled and mostly domesticated former hunters get food served to them these days, often in fancy bowls. But that doesn't mean they've lost the senses their wild kin rely on to survive.
People and cats live in completely different worlds when it comes to our respective sensory perceptions. When you imagine things from your pet's point of view, you'll be able to better understand what makes your cat tick.
Consider the feline sense of smell, which is many times more powerful than a human's puny abilities in this area. Once you know that your cat is so much more sensitive than you are capable of being, you shouldn't be surprised that the litter box you think is "tolerable" may be offensive to your cat. Same goes for those perfumed litters: We may love them, but they can be strong to the point of overpowering to our keen-nosed felines.
Of course, the litter box is a relatively modern convenience, and the cat's sense of smell is good for much more than deciding when it's not clean enough. Smell also plays a role in the establishment of territory. Cats like things in their home range to smell like them, be they people or furniture, and make them familiar by rubbing or scratching. The sense of smell is also important to free-roaming cats when it comes to finding prey, and in the determination of whether "found" food is safe enough to eat.
Dogs are scavengers who eat just about anything; cats are true predators. For them, fresh food, please, is their decided preference. Ever wonder why your cat turns up his nose at canned food that's been out a while? Simple: It doesn't smell right.
The sense of smell and taste are very closely connected in cats in part because the animals have a special anatomical feature called the vomeronasal organ, which allows them to process scent almost by tasting it. The organ is at the front of the roof of the mouth, and you can tell when your cat is using it: They open their mouths a crack and seem to be panting slightly. The facial expression that accompanies this behavior is so distinctive that it even has a name: the Flehmen response.
You can use your cat's well-developed sense of smell to your advantage if you're trying to entice a sick or just plain finicky cat to eat. If you warm your cat's wet food to just above room temperature before serving (about 85 degrees, or what we humans would call "lukewarm"), you'll make the odor more enticing, and so increase the appeal of the meal.
No matter what you do, though, you're not likely to get most cats interested in anything sweet. It's not for any lack in their sensory ability. Experts believe cats can identify foods that are bitter, salty, sweet or sour, although their appreciation of any of those qualities differs greatly from our own.
Not surprising, really, when you consider that were your cat to choose a gourmet meal, chances are he'd opt for a freshly killed mouse, hold the seasonings. Or maybe, knowing the skillful predator that lives in even the most pampered of pets, your cat would choose his meal live and easy to catch.
Dead or alive, a mouse is a meal preference no human would share. But then, our tongues also aren't adapted to clean meat off the bones of prey the way a cat's sandpaper-textured licker is. It all comes together so beautifully in the lithe, supple body of the perfect small predator.
When you think about how different cats and people are, it makes you wonder how we get along as well as we do. If nothing else, it should give you a sense of wonder at the superhuman senses of our most popular pet.
PETS ON THE WEB
I wish every chronic pet illness had a Web site as helpful and supportive as the one dedicated to feline diabetes (http://www.felinediabetes.com). Administered by a cat-loving physician, the site offers everything an owner needs (but maybe didn't think to ask the veterinarian) about caring for a pet with this disease. What is it like to live with such an animal? You'll find that here, along with tips on using syringes and monitoring your pet's sugar levels. A worthwhile read, without a doubt -- this is a site that's built with science but runs on love.
THE SCOOP
Winterizing your car or truck? Now may be the time to do it, but make sure when you're taking care of your vehicle that you're also watching out for your pet. The worry? Coolant made from ethylene glycol, a sweet-tasting liquid that can be lethal to your pet in dosages as small as a teaspoon, or less.
Safer alternatives exist to ethylene glycol, such as coolant made from propylene glycol. No matter what you use, though, be sure to clean up any spills promptly and thoroughly, and keep any stored product in leak-proof containers in a closed cupboard. If your pet laps even the smallest amount of coolant, see your veterinarian immediately. Your pet's only shot at survival is prompt treatment.
QUESTIONS FROM THE PACK
Q: Our puppy, a Lab mix from the shelter, is almost 4 months old now. She has a lot of energy, and sometimes we think we made a mistake in adopting her. But we know that puppies are a lot of work, and we're going to hang in there.
The biggest problem we have is that we can't get her to quit biting when she plays. It's really serious, to the extent that our 7-year-old son is afraid to play with the puppy now. Since we got the puppy for our son, this obviously is a bad situation. Can you help? -- N.N., via e-mail
A: Consider how human babies explore the world around them: They touch things, they grab things, and they taste what they grab. Puppies are much the same way, but since nature didn't equip them with fingers, they do their exploring with their mouths.
If you watch a litter of puppies play with each other, you might be surprised at how rough they can be. They nip -- hard. They grab hold of each other by the ears with needle-sharp teeth and pull. As puppies grow older, they learn from their littermates and their mother how to restrain those playful bites, which is one reason why it's so important to leave a puppy with his canine family until he's at least 7 weeks of age.
Some puppies don't get this critical early education, and some others are just slow learners. Others still are from breeds that are known to be "mouthier" than others -- retrievers are the classic example.
You can teach your puppy to keep his teeth to himself by attacking the problem from a couple of different directions. The first would be to redirect the behavior, giving your puppy a yummy toy and praising her for chewing on something that's not a family member.
Even as you're teaching the puppy what's OK to mouth, teach her how to leave family members unchewed by making the nipping unrewarding. Every time the puppy nips, cry "ouch" in a loud voice and immediately stop the play session. Turn away and ignore the puppy completely for a few minutes. Teach your son to do the same thing.
The message to get across: Play stops when she nips. If you're persistent and consistent, your puppy will get start getting the message soon and will learn to inhibit her bites.
If the behavior doesn't show any sign of ending, or if the biting seems more aggressive than playful, don't delay in asking your veterinarian for a referral to a behaviorist or trainer.
Q: Would you put in a word for greyhounds? There's a real need for homes for these dogs after their racing careers are over. Greyhounds are sweet, gentle and affectionate. They may be retired athletes, but they are committed couch potatoes. About three months ago, I adopted a sweet, loving 2-year-old greyhound. I am amazed at how mellow she is. -- B.W., via e-mail
A: I don't mind at all making the case for greyhounds. They're generally clean, quiet and easygoing, and they seem to be aware of how lucky they are to be in a loving home.
In adopting one, you need to work with a reputable rescue organization that'll match the dog with your household. One of the biggest problems: Some greyhounds -- but not all -- don't mix safely with cats.
A good place to start researching is the Greyhound Project Web site (http://www.adopt-a-greyhound.org). You'll find lots of information pro and con, as well as links to regional rescue groups.
Date Published: 11/15/2001
What is population?
Population is about people, and the dwellings, locations and environments that people live in. Population can be defined in many ways, for example by age, ethnicity, type of housing, birthplace or location.
Population size
Population size can refer to the total number of people living within a defined area, or it can refer to a group of people from a defined area who have similar characteristics (eg children aged 0 to 4 years, people of Asian ethnicities, people who live in two-bedroom houses, people who live in Mangere or in the Southland region).
Population structure
The structure of a population describes the relative numbers of people with similar characteristics within a population, for example, age groups, sex, ethnicity. The structure of a population shows how the subgroups within it affect its composition and characteristics. For example, it shows the percentages making up the different age groups of the population.
Population structure changes over time as people age, but also because of births, deaths and migration. Changes to social, environmental and economic conditions can also influence population structure. For example, changes in migration reflect a number of these factors and have different effects on different age groups.
The interrelationship between population, society, economics and the environment defines a population’s future size and make-up.
Population distribution
Distribution of a population within a defined area can be an important factor to consider in planning and analysis work. For example, clusters of families in only some suburbs of a city could influence planning for future school placement.
Measuring population
Population can be measured in three ways: actual counts of people, estimates of changes in actual counts due to population growth from births, deaths and migration, and projections of changes to future numbers of inhabitants.
Defining population for analysis
Defining the population relevant to a particular situation is a fundamental first step in any policy development or planning exercise.
The next step is to ensure that the sources used to derive the required measures are as compatible with each other as possible.
For example, birth rates may be based on the actual number of births relative to the estimated number of women in child-bearing ages at the time. For this purpose it would be appropriate to use population estimates for the denominator used to derive birth rates, rather than other sources.
Population is about people, and the dwellings, locations and environments that people live in. Population can be defined in many ways, for example by age, ethnicity, type of housing, birthplace or location.
Population size
Population size can refer to the total number of people living within a defined area, or it can refer to a group of people from a defined area who have similar characteristics (eg children aged 0 to 4 years, people of Asian ethnicities, people who live in two-bedroom houses, people who live in Mangere or in the Southland region).
Population structure
The structure of a population describes the relative numbers of people with similar characteristics within a population, for example, age groups, sex, ethnicity. The structure of a population shows how the subgroups within it affect its composition and characteristics. For example, it shows the percentages making up the different age groups of the population.
Population structure changes over time as people age, but also because of births, deaths and migration. Changes to social, environmental and economic conditions can also influence population structure. For example, changes in migration reflect a number of these factors and have different effects on different age groups.
The interrelationship between population, society, economics and the environment defines a population’s future size and make-up.
Population distribution
Distribution of a population within a defined area can be an important factor to consider in planning and analysis work. For example, clusters of families in only some suburbs of a city could influence planning for future school placement.
Measuring population
Population can be measured in three ways: actual counts of people, estimates of changes in actual counts due to population growth from births, deaths and migration, and projections of changes to future numbers of inhabitants.
Defining population for analysis
Defining the population relevant to a particular situation is a fundamental first step in any policy development or planning exercise.
The next step is to ensure that the sources used to derive the required measures are as compatible with each other as possible.
For example, birth rates may be based on the actual number of births relative to the estimated number of women in child-bearing ages at the time. For this purpose it would be appropriate to use population estimates for the denominator used to derive birth rates, rather than other sources.
what is an ecosystem?
Within all species, individuals interact with each other - feeding together, mating together, and living together. Some species have a pecking order as well, and each individual has a role to play within it.
However, it is not only individuals within a species that interact. Different species of animals interact with each other all the time. For instance, animals eat other animals through their interactions in a food web. But plants are included in this web as well as they, too, are eaten by animals.
What would happen if the weather were really cold all the time? Well, not all species of animals, plants and bacteria would be able to survive. What differences are there between species who live in the Rocky Mountains and those who inhabit the Sahara desert? Landscape also determines where plants and animals might live. But what, exactly, is an ecosystem? An ecosystem is a geographical area of a variable size where plants, animals, the landscape and the climate all interact together.
Heritage Trail:About 1 in 10 of Alberta's towns, cities, rivers and lakes are named after animals. Not all of Alberta's animal-influenced place names are after local animals.
Animal Place Names 1: Everything from Jackfish to Beaver
Animal Place Names Part 2: Deer, Elk and Moose
Animal Place Names Part 3: Exotic species like Llama and Cattalo
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The whole earth's surface can be described by a series of interconnected ecosystems. All living beings form and are part of ecosystems. They are diverse and always changing. Within an ecosystem, all aspects of the environment (both living things and their non-living settings) interact and affect one another. Every species affects the lives of those around them.
A small ecosystem in the boreal forest might look something like this: in the summertime, trees in forests (that produce oxygen used by living things through photosynthesis) lower the temperature in the forest for communities in the hot summer months. In turn, some members of the communities will probably feed upon the tree to gain nourishment, thus affecting or stunting the tree's growth.
Different areas in the world house different ecosystems. For example, you won't find an elephant or a tropical rainforest in Alberta! The different world ecological units are called biomes and they each have different flora, fauna, landscapes and weather patterns. An ecosystem is not the same thing as a biome. A biome is a large unit that is home to many different ecosystems. Within Alberta, there are six different biomes that each have their own specific flora and fauna distribution. These regions are: Grassland, Parkland, Boreal Forest, Foothill, Rocky Mountain and the Canadian Shield, all indicated on the map of Alberta's Regions
Within all species, individuals interact with each other - feeding together, mating together, and living together. Some species have a pecking order as well, and each individual has a role to play within it.
However, it is not only individuals within a species that interact. Different species of animals interact with each other all the time. For instance, animals eat other animals through their interactions in a food web. But plants are included in this web as well as they, too, are eaten by animals.
What would happen if the weather were really cold all the time? Well, not all species of animals, plants and bacteria would be able to survive. What differences are there between species who live in the Rocky Mountains and those who inhabit the Sahara desert? Landscape also determines where plants and animals might live. But what, exactly, is an ecosystem? An ecosystem is a geographical area of a variable size where plants, animals, the landscape and the climate all interact together.
Heritage Trail:About 1 in 10 of Alberta's towns, cities, rivers and lakes are named after animals. Not all of Alberta's animal-influenced place names are after local animals.
Animal Place Names 1: Everything from Jackfish to Beaver
Animal Place Names Part 2: Deer, Elk and Moose
Animal Place Names Part 3: Exotic species like Llama and Cattalo
The free downloadable RealPlayer plugin from RealNetworks is required to listen to the audio.
The whole earth's surface can be described by a series of interconnected ecosystems. All living beings form and are part of ecosystems. They are diverse and always changing. Within an ecosystem, all aspects of the environment (both living things and their non-living settings) interact and affect one another. Every species affects the lives of those around them.
A small ecosystem in the boreal forest might look something like this: in the summertime, trees in forests (that produce oxygen used by living things through photosynthesis) lower the temperature in the forest for communities in the hot summer months. In turn, some members of the communities will probably feed upon the tree to gain nourishment, thus affecting or stunting the tree's growth.
Different areas in the world house different ecosystems. For example, you won't find an elephant or a tropical rainforest in Alberta! The different world ecological units are called biomes and they each have different flora, fauna, landscapes and weather patterns. An ecosystem is not the same thing as a biome. A biome is a large unit that is home to many different ecosystems. Within Alberta, there are six different biomes that each have their own specific flora and fauna distribution. These regions are: Grassland, Parkland, Boreal Forest, Foothill, Rocky Mountain and the Canadian Shield, all indicated on the map of Alberta's Regions
stars
prostor formation
Star formation
The formation of a star begins with a gravitational instability inside a molecular cloud, often triggered by shockwaves from supernovae (massive stellar explosions) or the collision of two galaxies (as in a starburst galaxy). Once a region reaches a sufficient density of matter to satisfy the criteria for Jeans Instability it begins to collapse under its own gravitational force.
Artist's conception of the birth of a star within a dense molecular cloud. NASA image
As the cloud collapses, individual conglomerations of dense dust and gas form what are known as Bok globules. These can contain up to 50 solar masses of material. As a globule collapses and the density increases, the gravitational energy is converted into heat and the temperature rises. When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium, a protostar forms at the core.[30] These pre-main sequence stars are often surrounded by a protoplanetary disk. The period of gravitational contraction lasts for about 10–15 million years.
Early stars of less than 2 solar masses are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. These newly-born stars emit jets of gas along their axis of rotation, producing small patches of nebulosity known as Herbig-Haro objects.[31]
[edit] Main sequence
Main article: Main sequence
Stars spend about 90% of their lifetime fusing hydrogen to produce helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence and are called dwarf stars. Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase. As a consequence, in order to maintain the required rate of nuclear fusion at the core, the star will slowly increase in temperature and luminosity.[32] The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion years ago.[33]
Every star generates a stellar wind of particles that causes a continual outflow of gas into space. For most stars, the amount of mass lost is negligible. The Sun loses 10−14 solar masses every year,[34] or about 0.01% of its total mass over its entire lifespan. However very massive stars can lose 10−7 to 10−5 solar masses each year, significantly affecting their evolution.[35] Stars that begin with more than 50 solar masses can lose over half their total mass while they remain on the main sequence.[36]
An example of a Hertzsprung-Russell diagram for a set of stars that includes the Sun (center). (See "Classification" below.)
The duration that a star spends on the main sequence depends primarily on the amount of fuel it has to burn and the rate at which it burns that fuel. In other words, its initial mass and its luminosity. For the Sun, this is estimated to be about 1010 years. Large stars burn their fuel very rapidly and are short-lived. Small stars (called red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years. At the end of their lives, they simply become dimmer and dimmer, fading into black dwarfs.[2] However, since the lifespan of such stars is greater than the current age of the universe (13.7 billion years), no black dwarfs are expected to exist yet.
Besides mass, the portion of elements heavier than helium can play a significant role in the evolution of stars. In astronomy all elements heavier than helium are considered a "metal", and the chemical concentration of these elements is called the metallicity. The metallicity can influence the duration that a star will burn its fuel, control the formation of magnetic fields[37] and modify the strength of the stellar wind.[38] Older, population II stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. (Over time these clouds become increasingly enriched in heavier elements as older stars die and shed portions of their atmospheres.)
[edit] Post-main sequence
As stars of at least 0.4 solar masses[2] exhaust their supply of hydrogen at their core, their outer layers expand and cool to form a red giant. In about 5 billion years, when the Sun is a red giant, it will be so large that it will consume Mercury and possibly Venus. Models predict that the Sun will expand out to about 99% of the distance to the Earth's present orbit (1 astronomical unit, or AU). By that time, however, the orbit of the Earth will expand to about 1.7 AUs due to mass loss by the Sun and thus the Earth will escape envelopment.[39] However, the Earth will be stripped of its oceans and atmosphere as the Sun's luminosity increases several thousandfold.
In a red giant of up to 2.25 solar masses, hydrogen fusion proceeds in a shell-layer surrounding the core.[40] Eventually the core is compressed enough to start helium fusion, and the star now gradually shrinks in radius and increases its surface temperature. For larger stars, the core region transitions directly from fusing hydrogen to fusing helium.[41]
After the star has consumed the helium at the core, fusion continues in a shell around a hot core of carbon and oxygen. The star then follows an evolutionary path that parallels the original red giant phase, but at a higher surface temperature.
[edit] Massive stars
Betelgeuse is a red supergiant star approaching the end of its life cycle
During their helium-burning phase, very high mass stars with more than nine solar masses expand to form red supergiants. Once this fuel is exhausted at the core, they can continue to fuse elements heavier than helium. The core contracts until the temperature and pressure are sufficient to fuse carbon. This process continues, with the successive stages being fueled by oxygen, neon, silicon, and sulfur. Near the end of the star's life, fusion can occur along a series of onion-layer shells within the star. Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth.[42]
The final stage is reached when the star begins producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, if they are fused they do not release energy — the process would, on the contrary, consume energy. Likewise, since they are more tightly bound than all lighter nuclei, energy cannot be released by fission.[40] In relatively old, very massive stars, a large core of inert iron will accumulate in the center of the star. The heavier elements in these stars can work their way up to the surface, forming evolved objects known as Wolf-Rayet stars that have a dense stellar wind which sheds the outer atmosphere.
[edit] Collapse
An evolved, average-size star will now shed its outer layers as a planetary nebula. If what remains after the outer atmosphere has been shed is less than 1.4 solar masses, it shrinks to a relatively tiny object (about the size of Earth) that is not massive enough for further compression to take place, known as a white dwarf.[43] The electron-degenerate matter inside a white dwarf is no longer a plasma, even though stars are generally referred to as being spheres of plasma. White dwarfs will eventually fade into black dwarfs over a very long stretch of time.
The Crab Nebula, remnants of a supernova that was first observed around 1050 AD
In larger stars, fusion continues until the iron core has grown so large (more than 1.4 solar masses) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons and neutrinos in a burst of inverse beta decay, or electron capture. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae are so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none existed before.[44]
Most of the matter in the star is blown away by the supernovae explosion (forming nebulae such as the Crab Nebula[44]) and what remains will be a neutron star (which sometimes manifests itself as a pulsar or X-ray burster) or, in the case of the largest stars (large enough to leave a stellar remnant greater than roughly 4 solar masses), a black hole.[45] In a neutron star the matter is in a state known as neutron-degenerate matter, with a more exotic form of degenerate matter, QCD matter, possibly present in the core. Within a black hole the matter is in a state that is not currently understood.
The blown-off outer layers of dying stars include heavy elements which may be recycled during new star formation. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.[44]
[edit] Distribution
A white dwarf star in orbit around Sirius (artist's impression). NASA image
It has been a long-held assumption that the majority of stars occur in gravitationally-bound, multiple-star systems, forming binary stars. This is particularly true for very massive O and B class stars, where 80% of the systems are believed to be multiple. However the portion of single star systems increases for smaller stars, so that only 25% of red dwarfs are known to have stellar companions. As 85% of all stars are red dwarfs, most stars in the Milky Way are likely single from birth.[46]
Larger groups called star clusters also exist. These range from loose stellar associations with only a few stars, up to enormous globular clusters with hundreds of thousands of stars.
Stars are not spread uniformly across the universe, but are normally grouped into galaxies along with interstellar gas and dust. A typical galaxy contains hundreds of billions of stars, and there are more than 100 billion (1011) galaxies in the observable universe.[47] While it is often believed that stars only exist within galaxies, intergalactic stars have been discovered.[48]
Astronomers estimate that there are at least 70 sextillion (7×1022) stars in the observable universe.[49] That is 230 billion times as many as the 300 billion in the Milky Way.
The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion (1012) kilometres, or 4.2 light-years away. Light from Proxima Centauri takes 4.2 years to reach Earth. Travelling at the orbital speed of the Space Shuttle (5 miles per second — almost 30,000 kilometres per hour), it would take about 150,000 years to get there.[50] Distances like this are typical inside galactic discs, including in the vicinity of the solar system.[51] Stars can be much closer to each other in the centres of galaxies and in globular clusters, or much farther apart in galactic halos.
Due to the relatively vast distances between stars outside the galactic nucleus, collisions between stars are thought to be rare. In denser regions such as the core of globular clusters or the galactic center, collisions can be more common.[52] Such collisions can produce what are known as blue stragglers. These abnormal stars have a higher surface temperature than the other main sequence stars with the same luminosity in the cluster .[53]
[edit] Characteristics
The Sun is the nearest star to Earth
Almost everything about a star is determined by its initial mass, including essential characteristics such as luminosity and size, as well as the star's evolution, lifespan, and eventual fate.
[edit] Age
Many stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.7 billion years old — the observed age of the universe. The oldest star yet discovered, HE 1523-0901, is an estimated 13.2 billion years old.[54]
The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of about one million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years.[55][56]
[edit] Chemical composition
See also: Metallicity
When stars form they are composed of about 70% hydrogen and 28% helium, as measured by mass, with a small fraction of heavier elements. Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere, as iron is a common element and its absorption lines are relatively easy to measure. Because the molecular clouds where stars form are steadily enriched by heavier elements from supernovae explosions, a measurement of the chemical composition of a star can be used to infer its age.[57] The portion of heavier elements may also be an indicator of the likelihood that the star has a planetary system.[58]
The star with the lowest iron content ever measured is the dwarf HE1327-2326, with only 1/200,000th the iron content of the Sun.[59]
[edit] Diameter
Due to their great distance from the Earth, all stars except the Sun appear to the human eye as shining points in the night sky that twinkle because of the effect of the Earth's atmosphere. The Sun is also a star, but it is close enough to the Earth to appear as a disk instead, and to provide daylight. Other than the Sun, the star with the largest apparent size is R Doradus, with an angular diameter of only 0.057 arcseconds.[60]
The disks of most stars are much too small in angular size to be observed with current ground-based optical telescopes, and so interferometer telescopes are required in order to produce images of these objects. Another technique for measuring the angular size of stars is through occultation. By precisely measuring the drop in brightness of a star as it is occulted by the Moon (or the rise in brightness when it reappears), the star's angular diameter can be computed.[61]
Stars range in size from neutron stars, which vary anywhere from 20 to 40 km in diameter, to supergiants like Betelgeuse in the Orion constellation, which has a diameter approximately 650 times larger than the Sun — about 0.9 billion kilometres. However, Betelgeuse has a much lower density than the Sun.[62]
[edit] Kinematics
The motion of a star relative to the Sun can provide useful information about the origin and age of a star, as well as the structure and evolution of the surrounding galaxy.
The proper motion of a star is the traverse velocity across the sky. This is determined by precise astrometric measurements in units of milli-arc seconds (mas) per year. By determining the parallax of a star, the proper motion can then be converted into units of velocity. Stars with high rates of proper motion are likely to be relatively close to the Sun, making them good candidates for parallax measurements.[63]
The radial velocity is the movement of the star toward or away from the Sun. This is determined by measurements in the doppler shift of spectral lines, and is given in units of km/s.
Once both rates of movement are known, the space velocity of the star relative to the Sun or the galaxy can be computed. Among nearby stars, it has been found that population I stars have generally lower velocities than older, population II stars. The latter have elliptical orbits that are inclined to the plane of the galaxy.[64] Comparison of the kinematics of nearby stars has also led to the identification of stellar associations. These are most likely groups of stars that share a common point of origin in giant molecular clouds. [65]
[edit] Magnetic field
Main article: Stellar magnetic field
The magnetic field of a star is generated within regions of the interior where convective circulation occurs. This movement of conductive plasma functions like a dynamo, generating magnetic fields that extend throughout the star. The strength of the magnetic field varies with the mass and composition of the star, and the amount of magnetic surface activity depends upon the star's rate of rotation. This surface activity produces starspots, which are regions of strong magnetic fields and lower than normal surface temperatures. Coronal loops are arching magnetic fields that reach out into the corona from active regions. Stellar flares are bursts of high-energy particles that are emitted due to the same magnetic activity.[66]
Young, rapidly rotating stars tend to have high levels of surface activity because of their magnetic field. The magnetic field can act upon a star's stellar wind, however, functioning as a brake to gladually slow the rate of rotation as the star grows older. Thus, older stars such as the Sun have a much slower rate of rotation and a lower level of surface activity. The activity levels of slowly-rotating stars tend to vary in a cyclical manner and can shut down altogether for periods.[67] During the Maunder minimum, for example, the Sun underwent a 70-year period with almost no sunspot activity.
[edit] Mass
One of the most massive stars known is Eta Carinae,[68] with 100–150 times as much mass as the Sun; its lifespan is very short — only several million years at most. A recent study of the Arches cluster suggests that 150 solar masses is the upper limit for stars in the current era of the universe.[69] The reason for this limit is not precisely known, but it is partially due to the Eddington luminosity which defines the maximum amount of luminosity that can pass through the atmosphere of a star without ejecting the gases into space.
The reflection nebula NGC 1999 is brilliantly illuminated by V380 Orionis (center), a variable star with about 3.5 times the mass of the Sun. NASA image
The first stars to form after the Big Bang may have been larger, up to 300 solar masses or more,[70] due to the complete absence of elements heavier than lithium in their composition. This generation of supermassive, population III stars is long extinct, however, and currently only theoretical.
With a mass only 93 times that of Jupiter, AB Doradus C, a companion to AB Doradus A, is the smallest known star undergoing nuclear fusion in its core.[71] For stars with similar metallicity to the Sun, the theoretical minimum mass the star can have, and still undergo fusion at the core, is estimated to be about 75 times the mass of Jupiter.[72][73] When the metallicity is very low, however, a recent study of the faintest stars found that the minimum star size seems to be about 8.3% of the solar mass, or about 87 times the mass of Jupiter.[74][73] Smaller bodies are called brown dwarfs, which occupy a poorly-defined grey area between stars and gas giants.
The combination of the radius and the mass of a star determines the surface gravity. Giant stars have a much lower surface gravity than main sequence stars, while the opposite is the case for degenerate, compact stars such as white dwarfs. The surface gravity can influence the appearance of a star's spectrum, with higher gravity causing a broadening of the absorption lines.[18]
[edit] Rotation
Main article: Stellar rotation
The rotation rate of stars can be approximated through spectroscopic measurement, or more exactly determined by tracking the rotation rate of starspots. Young stars can have a rapid rate of rotation greater than 100 km/s at the equator. The B-class star Achernar, for example, has an equatorial rotation velocity of about 225 km/s or greater, giving it an equatorial diameter that is more than 50% larger than the distance between the poles. This rate of rotation is just below the critical velocity of 300 km/s where the star would break apart.[75] By contrast, the Sun only rotates once every 25 – 35 days, with an equatorial velocity of 1.994 km/s. The star's magnetic field and the stellar wind serve to slow down a main sequence star's rate of rotation by a significant amount as it evolves on the main sequence.[76]
Degenerate stars have contracted into a compact mass, resulting in a rapid rate of rotation. However they have relatively low rates of rotation compared to what would be expected by conservation of angular momentum—the tendency of a rotating body to compensate for a contraction in size by increasing its rate of spin. A large portion of the star's angular momentum is dissipated as a result of mass loss through the stellar wind.[77] In spite of this, the rate of rotation for a pulsar can be very rapid. The pulsar at the heart of the Crab nebula, for example, rotates 30 times per second.[78] The rotation rate of the pulsar will gradually slow due to the emission of radiation.
[edit] Temperature
The surface temperature of a main sequence star is determined by the rate of energy production at the core and the radius of the star and is often estimated from the star's color index.[79] It is normally given as the effective temperature, which is the temperature of an idealized black body that radiates its energy at the same luminosity per surface area as the star. Note that the effective temperature is only a representative value, however, as stars actually have a temperature gradient that decreases with increasing distance from the core.[80] The temperature in the core region of a star is several million Kelvins.[81]
The stellar temperature will determine the rate of energization or ionization of different elements, resulting in characteristic absorption lines in the spectrum. The surface temperature of a star, along with its visual absolute magnitude and absorption features, is used to classify a star (see classification below).[18]
Massive main sequence stars can have surface temperatures of 50,000 K. Smaller stars such as the Sun have surface temperatures of a few thousand degrees. Red giants have relatively low surface temperatures of about 3,600 K, but they also have a high luminosity due to their large exterior surface area.
[edit] Radiation
The energy produced by stars, as a by-product of nuclear fusion, radiates into space as both electromagnetic radiation and particle radiation. The particle radiation emitted by a star is manifested as the stellar wind[82] (which exists as a steady stream of electrically charged particles, such as free protons, alpha particles, and beta particles, emanating from the star’s outer layers) and as a steady stream of neutrinos emanating from the star’s core.
The production of energy at the core is the reason why stars shine so brightly: every time two or more atomic nuclei of one element fuse together to form an atomic nucleus of a new heavier element, gamma ray photons are released from the nuclear fusion reaction. This energy is converted to other forms of electromagnetic energy, including visible light, by the time it reaches the star’s outer layers.
The color of a star, as determined by the peak frequency of the visible light, depends on the temperature of the star’s outer layers, including its photosphere.[83] Besides visible light, stars also emit forms of electromagnetic radiation that are invisible to the human eye. In fact, stellar electromagnetic radiation spans the entire electromagnetic spectrum, from the longest wavelengths of radio waves and infrared to the shortest wavelengths of ultraviolet, X-rays, and gamma rays. All components of stellar electromagnetic radiation, both visible and invisible, are typically significant.
Using the stellar spectrum, astronomers can also determine the surface temperature, surface gravity, metallicity and rotational velocity of a star. If the distance of the star is known, such as by measuring the parallax, then the luminosity of the star can be derived. The mass, radius, surface gravity, and rotation period can then be estimated based on stellar models. (Mass can be measured directly for stars in binary systems. The technique of gravitational microlensing will also yield the mass of a star.[84]) With these parameters, astronomers can also estimate the age of the star.[85]
[edit] Luminosity
In astronomy, luminosity is the amount of light, and other forms of radiant energy, a star radiates per unit of time. The luminosity of a star is determined by the radius and the surface temperature.
Surface patches with a lower temperature and luminosity than average are known as starspots. Small, dwarf stars such as the Sun generally have essentially featureless disks with only small starspots. Larger, giant stars have much bigger, much more obvious starspots,[86] and they also exhibit strong stellar limb darkening. That is, the brightness decreases towards the edge of the stellar disk.[87] Red dwarf flare stars such as UV Ceti may also possess prominent starspot features.[88]
[edit] Magnitude
Main articles: Apparent magnitude and Absolute magnitude
The apparent brightness of a star is measured by its apparent magnitude, which is the brightness of a star with respect to the star’s luminosity, distance from Earth, and the altering of the star’s light as it passes through Earth’s atmosphere.
Number of stars brighter than magnitude
Apparentmagnitude
Number of Stars[89]
0
4
1
15
2
48
3
171
4
513
5
1,602
6
4,800
7
14,000
Intrinsic or absolute magnitude is what the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs (32.6 light-years), and it is directly related to a star’s luminosity.
Both the apparent and absolute magnitude scales are logarithmic units: one whole number difference in magnitude is equal to a brightness variation of about 2.5 times[90] (the 5th root of 100 or approximately 2.512). This means that a first magnitude (+1.00) star is about 2.5 times brighter than a second magnitude (+2.00) star, and approximately 100 times brighter than a sixth magnitude (+6.00) star. The faintest stars visible to the naked eye under good seeing conditions are about magnitude +6.
On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; the larger the magnitude number, the fainter. The brightest stars, on either scale, have negative magnitude numbers. The variation in brightness between two stars is calculated by subtracting the magnitude number of the brighter star (mb) from the magnitude number of the fainter star (mf), then using the difference as an exponent for the base number 2.512; that is to say:
Δm = mf − mb
2.512Δm = variation in brightness
Relative to both luminosity and distance from Earth, absolute magnitude (M) and apparent magnitude (m) are not equivalent for an individual star;[90] for example, the bright star Sirius has an apparent magnitude of −1.44, but it has an absolute magnitude of +1.41.
The Sun has an apparent magnitude of −26.7, but its absolute magnitude is only +4.83. Sirius, the brightest star in the night sky as seen from Earth, is approximately 23 times more luminous than the Sun, while Canopus, the second brightest star in the night sky with an absolute magnitude of −5.53, is approximately 14,000 times more luminous than the Sun. Despite Canopus being vastly more luminous than Sirius, however, Sirius appears brighter than Canopus. This is because Sirius is merely 8.6 light-years from the Earth, while Canopus is much farther away at a distance of 310 light-years.
As of 2006, the star with the highest known absolute magnitude is LBV 1806-20, with a magnitude of −14.2. This star is at least 5,000,000 times more luminous than the Sun.[91] The least luminous stars that are currently known are located in the NGC 6397 cluster. The faintest red dwarfs in the cluster were magnitude 26, while a 28th magnitude white dwarf was also discovered. These faint stars are so dim that their light is as bright as a birthday candle on the Moon when viewed from the Earth.[92]
[edit] Classification
Surface Temperature Ranges forDifferent Stellar Classes[93]
Class
Temperature
Sample star
O
33,000 K or more
Zeta Ophiuchi
B
10,500–30,000 K
Rigel
A
7,500–10,000 K
Altair
F
6,000–7,200 K
Procyon A
G
5,500–6,000 K
Sun
K
4,000–5,250 K
Epsilon Indi
M
2,600–3,850 K
Proxima Centauri
Main article: Stellar classification
There are different classifications of stars according to their spectra ranging from type O, which are very hot, to M, which are so cool that molecules may form in their atmospheres. The main classifications in order of decreasing surface temperature are O, B, A, F, G, K, and M. A variety of rare spectral types have special classifications. The most common of these are types L and T, which classify the coldest low-mass stars and brown dwarfs.
Each letter has 10 sub-classifications numbered (hottest to coldest) from 0 to 9. This system matches closely with temperature, but breaks down at the extreme hottest end; class O0 and O1 stars may not exist.[94]
In addition, stars may be classified by the luminosity effects found in their spectral lines, which correspond to their spatial size and is determined by the surface gravity. These range from 0 (hypergiants) through III (giants) to V (main sequence dwarfs) and VII (white dwarfs). Most stars belong to the main sequence, which consists of ordinary hydrogen-burning stars. These fall along a narrow band when graphed according to their absolute magnitude and spectral type.[94] Our Sun is a main sequence G2V (yellow dwarf), being of intermediate temperature and ordinary size.
Additional nomenclature, in the form of lower-case letters, can follow the spectral type to indicate peculiar features of the spectrum. For example, an "e" can indicate the presence of emission lines; "m" represents unusually strong levels of metals, and "var" can mean variations in the spectral type.[94]
White dwarf stars have their own class that begins with the letter D. This is further sub-divided into the classes DA, DB, DC, DO, DZ, and DQ, depending on the types of prominent lines found in the spectrum. This is followed by a numerical value that indicates the temperature index.[95]
[edit] Variable stars
Main article: Variable star
The asymmetrical appearance of Mira, an oscillating variable star. NASA HST image
Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three principal groups.
During their stellar evolution, some stars pass through phases where they can become pulsating variables. Pulsating variable stars vary in radius and luminosity over time, expanding and contracting with periods ranging from minutes to years, depending on the size of the star. This category includes Cepheid and cepheid-like stars, and long-period variables such as Mira.[96]
Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.[96] This group includes protostars, Wolf-Rayet stars, and Flare stars, as well as giant and supergiant stars.
Cataclysmic or explosive variables undergo a dramatic change in their properties. This group includes novae and supernovae. A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions, including the nova and a Type 1a supernova.[4] The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion.[97] Some novae are also recurrent, having periodic outbursts of moderate amplitude.[96]
Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.[96] A notable example of an eclipsing binary is Algol, which regularly varies in magnitude from 2.3 to 3.5 over a period of 2.87 days.
[edit] Structure
Main article: Stellar structure
The interior of a stable, main sequence star is in a state of equilibrium in which the forces in any small volume almost exactly counterbalance each other. The balancing forces consist of inward directed gravitational force and the opposing pressure from the thermal energy of the plasma gas. For these forces to balance out, the temperature at the core of a typical star has to be on the order of 107 K or higher. The resulting temperature and pressure at the hydrogen-burning core of a main sequence star are sufficient for nuclear fusion to occur, and for sufficient energy to be produced to prevent further collapse of the star.[98]
As atomic nuclei are fused in the core, they emit energy in the form of gamma rays. These photons interact with the surrounding plasma, adding to the thermal energy at the core. Stars on the main sequence convert hydrogen into helium, creating a slowly but steadily increasing proportion of helium in the core. Eventually the helium content becomes predominant and energy production ceases at the core. Instead, for stars of more than 0.4 solar masses, fusion occurs in a slowly expanding shell around the degenerate helium core.[99]
In addition to hydrostatic equilibrium, the interior of a stable star will also maintain an energy balance of thermal equilibrium. There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior. The outgoing flux of energy leaving any layer within the star will exactly match the incoming flux from below.
This diagram shows a cross-section of a solar-type star. NASA image
The radiation zone is the region within the stellar interior where radiative transfer is sufficiently efficient to maintain the flux of energy. In this region the plasma will not be perturbed and any mass motions will die out. If this is not the case, however, then the plasma becomes unstable and convection will occur, forming a convection zone. This can occur, for example, in regions where very high energy fluxes occur, such as near the core or in areas with high opacity as in the outer envelope.[98]
The occurrence of convection in the outer envelope of a main sequence star depends on the spectral type. Stars with several times the mass of the Sun have a convection zone deep within the interior and a radiative zone in the outer layers. Smaller stars such as the Sun are just the opposite, with the convective zone located in the outer layers.[100] Red dwarf stars with less than 0.4 solar masses are convective throughout, which prevents the accumulation of a helium core.[2] For most stars the convective zones will also vary over time as the star ages and the constitution of the interior is modified.[98]
The portion of a main sequence star that is visible to an observer is called the photosphere. This is the layer at which the plasma gas of the star becomes transparent to photons of light. From here, the energy generated at the core becomes free to propagate out into space. It is within the photosphere that sun spots, or regions of lower than average temperature, appear.
Above the level of the photosphere is the stellar atmosphere. In a main sequence star such as the Sun, the lowest level of the atmosphere is the thin chromosphere region, where spicules appear and stellar flares begin. This is surrounded by a transition region, where the temperature rapidly increases within a distance of only 100 km. Beyond this is the corona, a volume of super-heated plasma that can extend outward to several million kilometres.[101] The existence of a corona appears to be dependent on a convective zone in the outer layers of the star.[100] Despite its high temperature, the corona emits very little light. The corona region of the Sun is normally only visible during a solar eclipse.
From the corona, a stellar wind of plasma particles expands outward from the star, propagating until it interacts with the interstellar medium. For the Sun, the influence of its solar wind extends throughout the bubble-shaped region of the heliosphere.[102]
prostor formation
Star formation
The formation of a star begins with a gravitational instability inside a molecular cloud, often triggered by shockwaves from supernovae (massive stellar explosions) or the collision of two galaxies (as in a starburst galaxy). Once a region reaches a sufficient density of matter to satisfy the criteria for Jeans Instability it begins to collapse under its own gravitational force.
Artist's conception of the birth of a star within a dense molecular cloud. NASA image
As the cloud collapses, individual conglomerations of dense dust and gas form what are known as Bok globules. These can contain up to 50 solar masses of material. As a globule collapses and the density increases, the gravitational energy is converted into heat and the temperature rises. When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium, a protostar forms at the core.[30] These pre-main sequence stars are often surrounded by a protoplanetary disk. The period of gravitational contraction lasts for about 10–15 million years.
Early stars of less than 2 solar masses are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. These newly-born stars emit jets of gas along their axis of rotation, producing small patches of nebulosity known as Herbig-Haro objects.[31]
[edit] Main sequence
Main article: Main sequence
Stars spend about 90% of their lifetime fusing hydrogen to produce helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence and are called dwarf stars. Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase. As a consequence, in order to maintain the required rate of nuclear fusion at the core, the star will slowly increase in temperature and luminosity.[32] The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion years ago.[33]
Every star generates a stellar wind of particles that causes a continual outflow of gas into space. For most stars, the amount of mass lost is negligible. The Sun loses 10−14 solar masses every year,[34] or about 0.01% of its total mass over its entire lifespan. However very massive stars can lose 10−7 to 10−5 solar masses each year, significantly affecting their evolution.[35] Stars that begin with more than 50 solar masses can lose over half their total mass while they remain on the main sequence.[36]
An example of a Hertzsprung-Russell diagram for a set of stars that includes the Sun (center). (See "Classification" below.)
The duration that a star spends on the main sequence depends primarily on the amount of fuel it has to burn and the rate at which it burns that fuel. In other words, its initial mass and its luminosity. For the Sun, this is estimated to be about 1010 years. Large stars burn their fuel very rapidly and are short-lived. Small stars (called red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years. At the end of their lives, they simply become dimmer and dimmer, fading into black dwarfs.[2] However, since the lifespan of such stars is greater than the current age of the universe (13.7 billion years), no black dwarfs are expected to exist yet.
Besides mass, the portion of elements heavier than helium can play a significant role in the evolution of stars. In astronomy all elements heavier than helium are considered a "metal", and the chemical concentration of these elements is called the metallicity. The metallicity can influence the duration that a star will burn its fuel, control the formation of magnetic fields[37] and modify the strength of the stellar wind.[38] Older, population II stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. (Over time these clouds become increasingly enriched in heavier elements as older stars die and shed portions of their atmospheres.)
[edit] Post-main sequence
As stars of at least 0.4 solar masses[2] exhaust their supply of hydrogen at their core, their outer layers expand and cool to form a red giant. In about 5 billion years, when the Sun is a red giant, it will be so large that it will consume Mercury and possibly Venus. Models predict that the Sun will expand out to about 99% of the distance to the Earth's present orbit (1 astronomical unit, or AU). By that time, however, the orbit of the Earth will expand to about 1.7 AUs due to mass loss by the Sun and thus the Earth will escape envelopment.[39] However, the Earth will be stripped of its oceans and atmosphere as the Sun's luminosity increases several thousandfold.
In a red giant of up to 2.25 solar masses, hydrogen fusion proceeds in a shell-layer surrounding the core.[40] Eventually the core is compressed enough to start helium fusion, and the star now gradually shrinks in radius and increases its surface temperature. For larger stars, the core region transitions directly from fusing hydrogen to fusing helium.[41]
After the star has consumed the helium at the core, fusion continues in a shell around a hot core of carbon and oxygen. The star then follows an evolutionary path that parallels the original red giant phase, but at a higher surface temperature.
[edit] Massive stars
Betelgeuse is a red supergiant star approaching the end of its life cycle
During their helium-burning phase, very high mass stars with more than nine solar masses expand to form red supergiants. Once this fuel is exhausted at the core, they can continue to fuse elements heavier than helium. The core contracts until the temperature and pressure are sufficient to fuse carbon. This process continues, with the successive stages being fueled by oxygen, neon, silicon, and sulfur. Near the end of the star's life, fusion can occur along a series of onion-layer shells within the star. Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth.[42]
The final stage is reached when the star begins producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, if they are fused they do not release energy — the process would, on the contrary, consume energy. Likewise, since they are more tightly bound than all lighter nuclei, energy cannot be released by fission.[40] In relatively old, very massive stars, a large core of inert iron will accumulate in the center of the star. The heavier elements in these stars can work their way up to the surface, forming evolved objects known as Wolf-Rayet stars that have a dense stellar wind which sheds the outer atmosphere.
[edit] Collapse
An evolved, average-size star will now shed its outer layers as a planetary nebula. If what remains after the outer atmosphere has been shed is less than 1.4 solar masses, it shrinks to a relatively tiny object (about the size of Earth) that is not massive enough for further compression to take place, known as a white dwarf.[43] The electron-degenerate matter inside a white dwarf is no longer a plasma, even though stars are generally referred to as being spheres of plasma. White dwarfs will eventually fade into black dwarfs over a very long stretch of time.
The Crab Nebula, remnants of a supernova that was first observed around 1050 AD
In larger stars, fusion continues until the iron core has grown so large (more than 1.4 solar masses) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons and neutrinos in a burst of inverse beta decay, or electron capture. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae are so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none existed before.[44]
Most of the matter in the star is blown away by the supernovae explosion (forming nebulae such as the Crab Nebula[44]) and what remains will be a neutron star (which sometimes manifests itself as a pulsar or X-ray burster) or, in the case of the largest stars (large enough to leave a stellar remnant greater than roughly 4 solar masses), a black hole.[45] In a neutron star the matter is in a state known as neutron-degenerate matter, with a more exotic form of degenerate matter, QCD matter, possibly present in the core. Within a black hole the matter is in a state that is not currently understood.
The blown-off outer layers of dying stars include heavy elements which may be recycled during new star formation. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.[44]
[edit] Distribution
A white dwarf star in orbit around Sirius (artist's impression). NASA image
It has been a long-held assumption that the majority of stars occur in gravitationally-bound, multiple-star systems, forming binary stars. This is particularly true for very massive O and B class stars, where 80% of the systems are believed to be multiple. However the portion of single star systems increases for smaller stars, so that only 25% of red dwarfs are known to have stellar companions. As 85% of all stars are red dwarfs, most stars in the Milky Way are likely single from birth.[46]
Larger groups called star clusters also exist. These range from loose stellar associations with only a few stars, up to enormous globular clusters with hundreds of thousands of stars.
Stars are not spread uniformly across the universe, but are normally grouped into galaxies along with interstellar gas and dust. A typical galaxy contains hundreds of billions of stars, and there are more than 100 billion (1011) galaxies in the observable universe.[47] While it is often believed that stars only exist within galaxies, intergalactic stars have been discovered.[48]
Astronomers estimate that there are at least 70 sextillion (7×1022) stars in the observable universe.[49] That is 230 billion times as many as the 300 billion in the Milky Way.
The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion (1012) kilometres, or 4.2 light-years away. Light from Proxima Centauri takes 4.2 years to reach Earth. Travelling at the orbital speed of the Space Shuttle (5 miles per second — almost 30,000 kilometres per hour), it would take about 150,000 years to get there.[50] Distances like this are typical inside galactic discs, including in the vicinity of the solar system.[51] Stars can be much closer to each other in the centres of galaxies and in globular clusters, or much farther apart in galactic halos.
Due to the relatively vast distances between stars outside the galactic nucleus, collisions between stars are thought to be rare. In denser regions such as the core of globular clusters or the galactic center, collisions can be more common.[52] Such collisions can produce what are known as blue stragglers. These abnormal stars have a higher surface temperature than the other main sequence stars with the same luminosity in the cluster .[53]
[edit] Characteristics
The Sun is the nearest star to Earth
Almost everything about a star is determined by its initial mass, including essential characteristics such as luminosity and size, as well as the star's evolution, lifespan, and eventual fate.
[edit] Age
Many stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.7 billion years old — the observed age of the universe. The oldest star yet discovered, HE 1523-0901, is an estimated 13.2 billion years old.[54]
The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of about one million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years.[55][56]
[edit] Chemical composition
See also: Metallicity
When stars form they are composed of about 70% hydrogen and 28% helium, as measured by mass, with a small fraction of heavier elements. Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere, as iron is a common element and its absorption lines are relatively easy to measure. Because the molecular clouds where stars form are steadily enriched by heavier elements from supernovae explosions, a measurement of the chemical composition of a star can be used to infer its age.[57] The portion of heavier elements may also be an indicator of the likelihood that the star has a planetary system.[58]
The star with the lowest iron content ever measured is the dwarf HE1327-2326, with only 1/200,000th the iron content of the Sun.[59]
[edit] Diameter
Due to their great distance from the Earth, all stars except the Sun appear to the human eye as shining points in the night sky that twinkle because of the effect of the Earth's atmosphere. The Sun is also a star, but it is close enough to the Earth to appear as a disk instead, and to provide daylight. Other than the Sun, the star with the largest apparent size is R Doradus, with an angular diameter of only 0.057 arcseconds.[60]
The disks of most stars are much too small in angular size to be observed with current ground-based optical telescopes, and so interferometer telescopes are required in order to produce images of these objects. Another technique for measuring the angular size of stars is through occultation. By precisely measuring the drop in brightness of a star as it is occulted by the Moon (or the rise in brightness when it reappears), the star's angular diameter can be computed.[61]
Stars range in size from neutron stars, which vary anywhere from 20 to 40 km in diameter, to supergiants like Betelgeuse in the Orion constellation, which has a diameter approximately 650 times larger than the Sun — about 0.9 billion kilometres. However, Betelgeuse has a much lower density than the Sun.[62]
[edit] Kinematics
The motion of a star relative to the Sun can provide useful information about the origin and age of a star, as well as the structure and evolution of the surrounding galaxy.
The proper motion of a star is the traverse velocity across the sky. This is determined by precise astrometric measurements in units of milli-arc seconds (mas) per year. By determining the parallax of a star, the proper motion can then be converted into units of velocity. Stars with high rates of proper motion are likely to be relatively close to the Sun, making them good candidates for parallax measurements.[63]
The radial velocity is the movement of the star toward or away from the Sun. This is determined by measurements in the doppler shift of spectral lines, and is given in units of km/s.
Once both rates of movement are known, the space velocity of the star relative to the Sun or the galaxy can be computed. Among nearby stars, it has been found that population I stars have generally lower velocities than older, population II stars. The latter have elliptical orbits that are inclined to the plane of the galaxy.[64] Comparison of the kinematics of nearby stars has also led to the identification of stellar associations. These are most likely groups of stars that share a common point of origin in giant molecular clouds. [65]
[edit] Magnetic field
Main article: Stellar magnetic field
The magnetic field of a star is generated within regions of the interior where convective circulation occurs. This movement of conductive plasma functions like a dynamo, generating magnetic fields that extend throughout the star. The strength of the magnetic field varies with the mass and composition of the star, and the amount of magnetic surface activity depends upon the star's rate of rotation. This surface activity produces starspots, which are regions of strong magnetic fields and lower than normal surface temperatures. Coronal loops are arching magnetic fields that reach out into the corona from active regions. Stellar flares are bursts of high-energy particles that are emitted due to the same magnetic activity.[66]
Young, rapidly rotating stars tend to have high levels of surface activity because of their magnetic field. The magnetic field can act upon a star's stellar wind, however, functioning as a brake to gladually slow the rate of rotation as the star grows older. Thus, older stars such as the Sun have a much slower rate of rotation and a lower level of surface activity. The activity levels of slowly-rotating stars tend to vary in a cyclical manner and can shut down altogether for periods.[67] During the Maunder minimum, for example, the Sun underwent a 70-year period with almost no sunspot activity.
[edit] Mass
One of the most massive stars known is Eta Carinae,[68] with 100–150 times as much mass as the Sun; its lifespan is very short — only several million years at most. A recent study of the Arches cluster suggests that 150 solar masses is the upper limit for stars in the current era of the universe.[69] The reason for this limit is not precisely known, but it is partially due to the Eddington luminosity which defines the maximum amount of luminosity that can pass through the atmosphere of a star without ejecting the gases into space.
The reflection nebula NGC 1999 is brilliantly illuminated by V380 Orionis (center), a variable star with about 3.5 times the mass of the Sun. NASA image
The first stars to form after the Big Bang may have been larger, up to 300 solar masses or more,[70] due to the complete absence of elements heavier than lithium in their composition. This generation of supermassive, population III stars is long extinct, however, and currently only theoretical.
With a mass only 93 times that of Jupiter, AB Doradus C, a companion to AB Doradus A, is the smallest known star undergoing nuclear fusion in its core.[71] For stars with similar metallicity to the Sun, the theoretical minimum mass the star can have, and still undergo fusion at the core, is estimated to be about 75 times the mass of Jupiter.[72][73] When the metallicity is very low, however, a recent study of the faintest stars found that the minimum star size seems to be about 8.3% of the solar mass, or about 87 times the mass of Jupiter.[74][73] Smaller bodies are called brown dwarfs, which occupy a poorly-defined grey area between stars and gas giants.
The combination of the radius and the mass of a star determines the surface gravity. Giant stars have a much lower surface gravity than main sequence stars, while the opposite is the case for degenerate, compact stars such as white dwarfs. The surface gravity can influence the appearance of a star's spectrum, with higher gravity causing a broadening of the absorption lines.[18]
[edit] Rotation
Main article: Stellar rotation
The rotation rate of stars can be approximated through spectroscopic measurement, or more exactly determined by tracking the rotation rate of starspots. Young stars can have a rapid rate of rotation greater than 100 km/s at the equator. The B-class star Achernar, for example, has an equatorial rotation velocity of about 225 km/s or greater, giving it an equatorial diameter that is more than 50% larger than the distance between the poles. This rate of rotation is just below the critical velocity of 300 km/s where the star would break apart.[75] By contrast, the Sun only rotates once every 25 – 35 days, with an equatorial velocity of 1.994 km/s. The star's magnetic field and the stellar wind serve to slow down a main sequence star's rate of rotation by a significant amount as it evolves on the main sequence.[76]
Degenerate stars have contracted into a compact mass, resulting in a rapid rate of rotation. However they have relatively low rates of rotation compared to what would be expected by conservation of angular momentum—the tendency of a rotating body to compensate for a contraction in size by increasing its rate of spin. A large portion of the star's angular momentum is dissipated as a result of mass loss through the stellar wind.[77] In spite of this, the rate of rotation for a pulsar can be very rapid. The pulsar at the heart of the Crab nebula, for example, rotates 30 times per second.[78] The rotation rate of the pulsar will gradually slow due to the emission of radiation.
[edit] Temperature
The surface temperature of a main sequence star is determined by the rate of energy production at the core and the radius of the star and is often estimated from the star's color index.[79] It is normally given as the effective temperature, which is the temperature of an idealized black body that radiates its energy at the same luminosity per surface area as the star. Note that the effective temperature is only a representative value, however, as stars actually have a temperature gradient that decreases with increasing distance from the core.[80] The temperature in the core region of a star is several million Kelvins.[81]
The stellar temperature will determine the rate of energization or ionization of different elements, resulting in characteristic absorption lines in the spectrum. The surface temperature of a star, along with its visual absolute magnitude and absorption features, is used to classify a star (see classification below).[18]
Massive main sequence stars can have surface temperatures of 50,000 K. Smaller stars such as the Sun have surface temperatures of a few thousand degrees. Red giants have relatively low surface temperatures of about 3,600 K, but they also have a high luminosity due to their large exterior surface area.
[edit] Radiation
The energy produced by stars, as a by-product of nuclear fusion, radiates into space as both electromagnetic radiation and particle radiation. The particle radiation emitted by a star is manifested as the stellar wind[82] (which exists as a steady stream of electrically charged particles, such as free protons, alpha particles, and beta particles, emanating from the star’s outer layers) and as a steady stream of neutrinos emanating from the star’s core.
The production of energy at the core is the reason why stars shine so brightly: every time two or more atomic nuclei of one element fuse together to form an atomic nucleus of a new heavier element, gamma ray photons are released from the nuclear fusion reaction. This energy is converted to other forms of electromagnetic energy, including visible light, by the time it reaches the star’s outer layers.
The color of a star, as determined by the peak frequency of the visible light, depends on the temperature of the star’s outer layers, including its photosphere.[83] Besides visible light, stars also emit forms of electromagnetic radiation that are invisible to the human eye. In fact, stellar electromagnetic radiation spans the entire electromagnetic spectrum, from the longest wavelengths of radio waves and infrared to the shortest wavelengths of ultraviolet, X-rays, and gamma rays. All components of stellar electromagnetic radiation, both visible and invisible, are typically significant.
Using the stellar spectrum, astronomers can also determine the surface temperature, surface gravity, metallicity and rotational velocity of a star. If the distance of the star is known, such as by measuring the parallax, then the luminosity of the star can be derived. The mass, radius, surface gravity, and rotation period can then be estimated based on stellar models. (Mass can be measured directly for stars in binary systems. The technique of gravitational microlensing will also yield the mass of a star.[84]) With these parameters, astronomers can also estimate the age of the star.[85]
[edit] Luminosity
In astronomy, luminosity is the amount of light, and other forms of radiant energy, a star radiates per unit of time. The luminosity of a star is determined by the radius and the surface temperature.
Surface patches with a lower temperature and luminosity than average are known as starspots. Small, dwarf stars such as the Sun generally have essentially featureless disks with only small starspots. Larger, giant stars have much bigger, much more obvious starspots,[86] and they also exhibit strong stellar limb darkening. That is, the brightness decreases towards the edge of the stellar disk.[87] Red dwarf flare stars such as UV Ceti may also possess prominent starspot features.[88]
[edit] Magnitude
Main articles: Apparent magnitude and Absolute magnitude
The apparent brightness of a star is measured by its apparent magnitude, which is the brightness of a star with respect to the star’s luminosity, distance from Earth, and the altering of the star’s light as it passes through Earth’s atmosphere.
Number of stars brighter than magnitude
Apparentmagnitude
Number of Stars[89]
0
4
1
15
2
48
3
171
4
513
5
1,602
6
4,800
7
14,000
Intrinsic or absolute magnitude is what the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs (32.6 light-years), and it is directly related to a star’s luminosity.
Both the apparent and absolute magnitude scales are logarithmic units: one whole number difference in magnitude is equal to a brightness variation of about 2.5 times[90] (the 5th root of 100 or approximately 2.512). This means that a first magnitude (+1.00) star is about 2.5 times brighter than a second magnitude (+2.00) star, and approximately 100 times brighter than a sixth magnitude (+6.00) star. The faintest stars visible to the naked eye under good seeing conditions are about magnitude +6.
On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; the larger the magnitude number, the fainter. The brightest stars, on either scale, have negative magnitude numbers. The variation in brightness between two stars is calculated by subtracting the magnitude number of the brighter star (mb) from the magnitude number of the fainter star (mf), then using the difference as an exponent for the base number 2.512; that is to say:
Δm = mf − mb
2.512Δm = variation in brightness
Relative to both luminosity and distance from Earth, absolute magnitude (M) and apparent magnitude (m) are not equivalent for an individual star;[90] for example, the bright star Sirius has an apparent magnitude of −1.44, but it has an absolute magnitude of +1.41.
The Sun has an apparent magnitude of −26.7, but its absolute magnitude is only +4.83. Sirius, the brightest star in the night sky as seen from Earth, is approximately 23 times more luminous than the Sun, while Canopus, the second brightest star in the night sky with an absolute magnitude of −5.53, is approximately 14,000 times more luminous than the Sun. Despite Canopus being vastly more luminous than Sirius, however, Sirius appears brighter than Canopus. This is because Sirius is merely 8.6 light-years from the Earth, while Canopus is much farther away at a distance of 310 light-years.
As of 2006, the star with the highest known absolute magnitude is LBV 1806-20, with a magnitude of −14.2. This star is at least 5,000,000 times more luminous than the Sun.[91] The least luminous stars that are currently known are located in the NGC 6397 cluster. The faintest red dwarfs in the cluster were magnitude 26, while a 28th magnitude white dwarf was also discovered. These faint stars are so dim that their light is as bright as a birthday candle on the Moon when viewed from the Earth.[92]
[edit] Classification
Surface Temperature Ranges forDifferent Stellar Classes[93]
Class
Temperature
Sample star
O
33,000 K or more
Zeta Ophiuchi
B
10,500–30,000 K
Rigel
A
7,500–10,000 K
Altair
F
6,000–7,200 K
Procyon A
G
5,500–6,000 K
Sun
K
4,000–5,250 K
Epsilon Indi
M
2,600–3,850 K
Proxima Centauri
Main article: Stellar classification
There are different classifications of stars according to their spectra ranging from type O, which are very hot, to M, which are so cool that molecules may form in their atmospheres. The main classifications in order of decreasing surface temperature are O, B, A, F, G, K, and M. A variety of rare spectral types have special classifications. The most common of these are types L and T, which classify the coldest low-mass stars and brown dwarfs.
Each letter has 10 sub-classifications numbered (hottest to coldest) from 0 to 9. This system matches closely with temperature, but breaks down at the extreme hottest end; class O0 and O1 stars may not exist.[94]
In addition, stars may be classified by the luminosity effects found in their spectral lines, which correspond to their spatial size and is determined by the surface gravity. These range from 0 (hypergiants) through III (giants) to V (main sequence dwarfs) and VII (white dwarfs). Most stars belong to the main sequence, which consists of ordinary hydrogen-burning stars. These fall along a narrow band when graphed according to their absolute magnitude and spectral type.[94] Our Sun is a main sequence G2V (yellow dwarf), being of intermediate temperature and ordinary size.
Additional nomenclature, in the form of lower-case letters, can follow the spectral type to indicate peculiar features of the spectrum. For example, an "e" can indicate the presence of emission lines; "m" represents unusually strong levels of metals, and "var" can mean variations in the spectral type.[94]
White dwarf stars have their own class that begins with the letter D. This is further sub-divided into the classes DA, DB, DC, DO, DZ, and DQ, depending on the types of prominent lines found in the spectrum. This is followed by a numerical value that indicates the temperature index.[95]
[edit] Variable stars
Main article: Variable star
The asymmetrical appearance of Mira, an oscillating variable star. NASA HST image
Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three principal groups.
During their stellar evolution, some stars pass through phases where they can become pulsating variables. Pulsating variable stars vary in radius and luminosity over time, expanding and contracting with periods ranging from minutes to years, depending on the size of the star. This category includes Cepheid and cepheid-like stars, and long-period variables such as Mira.[96]
Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.[96] This group includes protostars, Wolf-Rayet stars, and Flare stars, as well as giant and supergiant stars.
Cataclysmic or explosive variables undergo a dramatic change in their properties. This group includes novae and supernovae. A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions, including the nova and a Type 1a supernova.[4] The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion.[97] Some novae are also recurrent, having periodic outbursts of moderate amplitude.[96]
Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.[96] A notable example of an eclipsing binary is Algol, which regularly varies in magnitude from 2.3 to 3.5 over a period of 2.87 days.
[edit] Structure
Main article: Stellar structure
The interior of a stable, main sequence star is in a state of equilibrium in which the forces in any small volume almost exactly counterbalance each other. The balancing forces consist of inward directed gravitational force and the opposing pressure from the thermal energy of the plasma gas. For these forces to balance out, the temperature at the core of a typical star has to be on the order of 107 K or higher. The resulting temperature and pressure at the hydrogen-burning core of a main sequence star are sufficient for nuclear fusion to occur, and for sufficient energy to be produced to prevent further collapse of the star.[98]
As atomic nuclei are fused in the core, they emit energy in the form of gamma rays. These photons interact with the surrounding plasma, adding to the thermal energy at the core. Stars on the main sequence convert hydrogen into helium, creating a slowly but steadily increasing proportion of helium in the core. Eventually the helium content becomes predominant and energy production ceases at the core. Instead, for stars of more than 0.4 solar masses, fusion occurs in a slowly expanding shell around the degenerate helium core.[99]
In addition to hydrostatic equilibrium, the interior of a stable star will also maintain an energy balance of thermal equilibrium. There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior. The outgoing flux of energy leaving any layer within the star will exactly match the incoming flux from below.
This diagram shows a cross-section of a solar-type star. NASA image
The radiation zone is the region within the stellar interior where radiative transfer is sufficiently efficient to maintain the flux of energy. In this region the plasma will not be perturbed and any mass motions will die out. If this is not the case, however, then the plasma becomes unstable and convection will occur, forming a convection zone. This can occur, for example, in regions where very high energy fluxes occur, such as near the core or in areas with high opacity as in the outer envelope.[98]
The occurrence of convection in the outer envelope of a main sequence star depends on the spectral type. Stars with several times the mass of the Sun have a convection zone deep within the interior and a radiative zone in the outer layers. Smaller stars such as the Sun are just the opposite, with the convective zone located in the outer layers.[100] Red dwarf stars with less than 0.4 solar masses are convective throughout, which prevents the accumulation of a helium core.[2] For most stars the convective zones will also vary over time as the star ages and the constitution of the interior is modified.[98]
The portion of a main sequence star that is visible to an observer is called the photosphere. This is the layer at which the plasma gas of the star becomes transparent to photons of light. From here, the energy generated at the core becomes free to propagate out into space. It is within the photosphere that sun spots, or regions of lower than average temperature, appear.
Above the level of the photosphere is the stellar atmosphere. In a main sequence star such as the Sun, the lowest level of the atmosphere is the thin chromosphere region, where spicules appear and stellar flares begin. This is surrounded by a transition region, where the temperature rapidly increases within a distance of only 100 km. Beyond this is the corona, a volume of super-heated plasma that can extend outward to several million kilometres.[101] The existence of a corona appears to be dependent on a convective zone in the outer layers of the star.[100] Despite its high temperature, the corona emits very little light. The corona region of the Sun is normally only visible during a solar eclipse.
From the corona, a stellar wind of plasma particles expands outward from the star, propagating until it interacts with the interstellar medium. For the Sun, the influence of its solar wind extends throughout the bubble-shaped region of the heliosphere.[102]
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