Oswalt has written over scientific articles and edits the I. Communications, an international journal for advanced amateurs, students, teachers and professionals who collaborate on research and educational projects in astronomy. His principal research interests are the study of hot white dwarf stars and the surrounding interstellar medium.
Recently he was a member of the Space Telescope Users' Committee helping to develop the plans for the final servicing mission, which took take place in May He is continues to fly sounding rockets, be involved in research with Hubble as well as leading the Leicester contribution to the ESA Gaia mission.
During the past few years, Dr. Barstow has played an increasingly important role in scientific funding and advisory structures, become a member of Science and Technology Facilities Council in This is volume 4 of Planets, Stars and Stellar Systems, a six-volume compendium of modern astronomical research, covering subjects of key interest to the main fields of contemporary astronomy. All chapters of the handbook were written by practicing professionals.
They include sufficient background material and references to the current literature to allow readers to learn enough about a specialty within astronomy, astrophysics and cosmology to get started on their own practical research projects. In the spirit of the series Stars and Stellar Systems published by Chicago University Press in the s and s, each chapter of Planets, Stars and Stellar Systems can stand on its own as a fundamental review of its respective sub-discipline, and each volume can be used as a textbook or recommended reference work for advanced undergraduate or postgraduate courses.
The effective Chandrasekhar mass for an iron core varies from about 1. Once this mass is reached, electrons begin to be captured into the iron-peak nuclei and the core becomes unable to support itself. The core collapses and the star is destroyed, either in a supernova or direct collapse to a black hole. When the core of a massive stars collapses, it will form a neutron star , or in the case of cores that exceed the Tolman-Oppenheimer-Volkoff limit , a black hole. Through a process that is not completely understood, some of the gravitational potential energy released by this core collapse is converted into a Type Ib, Type Ic, or Type II supernova.
It is known that the core collapse produces a massive surge of neutrinos , as observed with supernova SN A. The extremely energetic neutrinos fragment some nuclei; some of their energy is consumed in releasing nucleons , including neutrons , and some of their energy is transformed into heat and kinetic energy , thus augmenting the shock wave started by rebound of some of the infalling material from the collapse of the core.
Electron capture in very dense parts of the infalling matter may produce additional neutrons. Because some of the rebounding matter is bombarded by the neutrons, some of its nuclei capture them, creating a spectrum of heavier-than-iron material including the radioactive elements up to and likely beyond uranium. Neither abundance alone matches that found in the Solar System , so both supernovae and ejection of elements from red giants are required to explain the observed abundance of heavy elements and isotopes thereof. The energy transferred from collapse of the core to rebounding material not only generates heavy elements, but provides for their acceleration well beyond escape velocity , thus causing a Type Ib, Type Ic, or Type II supernova.
Current understanding of this energy transfer is still not satisfactory; although current computer models of Type Ib, Type Ic, and Type II supernovae account for part of the energy transfer, they are not able to account for enough energy transfer to produce the observed ejection of material. Some evidence gained from analysis of the mass and orbital parameters of binary neutron stars which require two such supernovae hints that the collapse of an oxygen-neon-magnesium core may produce a supernova that differs observably in ways other than size from a supernova produced by the collapse of an iron core.
The most massive stars that exist today may be completely destroyed by a supernova with an energy greatly exceeding its gravitational binding energy. This rare event, caused by pair-instability , leaves behind no black hole remnant. After a star has burned out its fuel supply, its remnants can take one of three forms, depending on the mass during its lifetime. White dwarfs are stable because the inward pull of gravity is balanced by the degeneracy pressure of the star's electrons, a consequence of the Pauli exclusion principle. Electron degeneracy pressure provides a rather soft limit against further compression; therefore, for a given chemical composition, white dwarfs of higher mass have a smaller volume.
With no fuel left to burn, the star radiates its remaining heat into space for billions of years. A white dwarf is very hot when it first forms, more than , K at the surface and even hotter in its interior. It is so hot that a lot of its energy is lost in the form of neutrinos for the first 10 million years of its existence, but will have lost most of its energy after a billion years.
The chemical composition of the white dwarf depends upon its mass. A star of a few solar masses will ignite carbon fusion to form magnesium, neon, and smaller amounts of other elements, resulting in a white dwarf composed chiefly of oxygen, neon, and magnesium, provided that it can lose enough mass to get below the Chandrasekhar limit see below , and provided that the ignition of carbon is not so violent as to blow the star apart in a supernova.
A star of less than about half the mass of the Sun will be unable to ignite helium fusion as noted earlier , and will produce a white dwarf composed chiefly of helium. In the end, all that remains is a cold dark mass sometimes called a black dwarf. However, the universe is not old enough for any black dwarfs to exist yet.
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If the white dwarf's mass increases above the Chandrasekhar limit , which is 1. Depending upon the chemical composition and pre-collapse temperature in the center, this will lead either to collapse into a neutron star or runaway ignition of carbon and oxygen. Heavier elements favor continued core collapse, because they require a higher temperature to ignite, because electron capture onto these elements and their fusion products is easier; higher core temperatures favor runaway nuclear reaction, which halts core collapse and leads to a Type Ia supernova.
This instability to collapse means that no white dwarf more massive than approximately 1. Mass transfer in a binary system may cause an initially stable white dwarf to surpass the Chandrasekhar limit. If a white dwarf forms a close binary system with another star, hydrogen from the larger companion may accrete around and onto a white dwarf until it gets hot enough to fuse in a runaway reaction at its surface, although the white dwarf remains below the Chandrasekhar limit.
Such an explosion is termed a nova. Ordinarily, atoms are mostly electron clouds by volume, with very compact nuclei at the center proportionally, if atoms were the size of a football stadium, their nuclei would be the size of dust mites. When a stellar core collapses, the pressure causes electrons and protons to fuse by electron capture.
Without electrons, which keep nuclei apart, the neutrons collapse into a dense ball in some ways like a giant atomic nucleus , with a thin overlying layer of degenerate matter chiefly iron unless matter of different composition is added later. The neutrons resist further compression by the Pauli Exclusion Principle, in a way analogous to electron degeneracy pressure, but stronger.
Their period of rotation shortens dramatically as the stars shrink due to conservation of angular momentum ; observed rotational periods of neutron stars range from about 1. Such neutron stars are called pulsars , and were the first neutron stars to be discovered. Though electromagnetic radiation detected from pulsars is most often in the form of radio waves, pulsars have also been detected at visible, X-ray, and gamma ray wavelengths. If the mass of the stellar remnant is high enough, the neutron degeneracy pressure will be insufficient to prevent collapse below the Schwarzschild radius.
The stellar remnant thus becomes a black hole.
Black holes are predicted by the theory of general relativity. According to classical general relativity, no matter or information can flow from the interior of a black hole to an outside observer, although quantum effects may allow deviations from this strict rule. The existence of black holes in the universe is well supported, both theoretically and by astronomical observation. Because the core-collapse mechanism of a supernova is, at present, only partially understood, it is still not known whether it is possible for a star to collapse directly to a black hole without producing a visible supernova, or whether some supernovae initially form unstable neutron stars which then collapse into black holes; the exact relation between the initial mass of the star and the final remnant is also not completely certain.
Stellar Structure and Evolution
Resolution of these uncertainties requires the analysis of more supernovae and supernova remnants. A stellar evolutionary model is a mathematical model that can be used to compute the evolutionary phases of a star from its formation until it becomes a remnant.
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The mass and chemical composition of the star are used as the inputs, and the luminosity and surface temperature are the only constraints. The model formulae are based upon the physical understanding of the star, usually under the assumption of hydrostatic equilibrium. Extensive computer calculations are then run to determine the changing state of the star over time, yielding a table of data that can be used to determine the evolutionary track of the star across the Hertzsprung—Russell diagram , along with other evolving properties.
From Wikipedia, the free encyclopedia. Changes to a star over its lifespan. Play media. Main article: Star formation. Main article: Protostar. Main article: Brown dwarf.
Main article: Main sequence. The evolutionary tracks of stars with different initial masses on the Hertzsprung—Russell diagram. Main article: Subgiant. Main article: Red giant branch. Main articles: Horizontal branch and Red clump. Main article: Asymptotic giant branch. Main article: Post-AGB. Main article: Supergiant. Main article: Supernova. Main articles: White dwarf and Black dwarf. Main article: Neutron star. Main article: Black hole. Nuclei in the Cosmos.
World Scientific. The Astrophysical Journal. His principal research interests are the study of hot white dwarf stars and the surrounding interstellar medium. He is continues to fly sounding rockets, be involved in research with Hubble as well as leading the Leicester contribution to the ESA Gaia mission. During the past few years, Dr.
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