stellar structure

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stellar structure,

physical properties of a starstar,
hot incandescent sphere of gas, held together by its own gravitation, and emitting light and other forms of electromagnetic radiation whose ultimate source is nuclear energy.
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 and the processes taking place within it. Except for that of the sun, astronomers must draw their conclusions regarding stellar structure on the basis of light and other radiation from stars that are light-years away; this light enables them to observe only the stars' surfaces. Knowledge of the processes taking place in a star and of conditions within its interior must be inferred from the laws of physics and chemistry. A star is a nearly spherical body of incandescent gas, mostly hydrogen and helium. Because it is observed to be stable, astronomers can conclude that the inward pressure of gravitation holding the star together is balanced by the outward pressure due to the energy generated by the star, and that the rate at which energy is radiated away from the star's surface is equal to the rate at which it is produced in the interior. The most important properties of a star are its size, mass, luminosityluminosity,
in astronomy, the rate at which energy of all types is radiated by an object in all directions. A star's luminosity depends on its size and its temperature, varying as the square of the radius and the fourth power of the absolute surface temperature.
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, chemical composition, and the temperaturetemperature,
measure of the relative warmth or coolness of an object. Temperature is measured by means of a thermometer or other instrument having a scale calibrated in units called degrees. The size of a degree depends on the particular temperature scale being used.
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, pressurepressure,
in mechanics, ratio of the force acting on a surface to the area of the surface; it is thus distinct from the total force acting on a surface. A force can be applied to and sustained by a single point on a solid.
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, and densitydensity,
ratio of the mass of a substance to its volume, expressed, for example, in units of grams per cubic centimeter or pounds per cubic foot. The density of a pure substance varies little from sample to sample and is often considered a characteristic property of the
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 at all distances from its center to its surface. These last three properties are related by the gas lawsgas laws,
physical laws describing the behavior of a gas under various conditions of pressure, volume, and temperature. Experimental results indicate that all real gases behave in approximately the same manner, having their volume reduced by about the same proportion of the
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; their values decrease with distance from the star's center. Stars vary widely in size and luminosity but have masses only within the range from about 0.08 to 100 times the mass of the sun, with few exceptions; less massive bodies cannot support the energy-producing processes of a star (see brown dwarfbrown dwarf,
in astronomy, celestial body that is larger than a planet but does not have sufficient mass to convert hydrogen into helium via nuclear fusion as stars do. Also called "failed stars," brown dwarfs form in the same way as true stars (by the contraction of a swirling
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), while more massive bodies are generally unstable. An ordinary star has a surface temperature of thousands of degrees, implying central temperatures of millions of degrees. The central pressure and density are also extremely high, but the temperature is such that the material will still remain in the gaseous state. At these temperatures, energy is produced by thermonuclear fusion (see nuclear energynuclear energy,
the energy stored in the nucleus of an atom and released through fission, fusion, or radioactivity. In these processes a small amount of mass is converted to energy according to the relationship E = mc2, where E is energy, m
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), in which two or more nuclei are fused to form a single heavier nucleus. As such fusion processes proceed within the star, its chemical composition necessarily changes, with heavier elements increasing at the expense of lighter elements (see nucleosynthesisnucleosynthesis
or nucleogenesis,
in astronomy, production of all the chemical elements from the simplest element, hydrogen, by thermonuclear reactions within stars, supernovas, and in the big bang at the beginning of the universe (see nucleus; nuclear energy).
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). The mass and chemical composition of the star together determine all of its other properties, e.g., size, luminosity, and temperature. Astronomers can determine the temperature and chemical composition of the star's surface from analysis of the spectrumspectrum,
arrangement or display of light or other form of radiation separated according to wavelength, frequency, energy, or some other property. Beams of charged particles can be separated into a spectrum according to mass in a mass spectrometer (see mass spectrograph).
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 of light from the star. Such a spectrum consists of a continuous blackbodyblackbody,
in physics, an ideal black substance that absorbs all and reflects none of the radiant energy falling on it. Lampblack, or powdered carbon, which reflects less than 2% of the radiation falling on it, crudely approximates an ideal blackbody; a material consisting of a
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 spectrum produced by complex conditions within the star superimposed on which is a series of dark lines due to absorption of energy by the cooler stellar atmosphere. From such observations much is learned about the other properties and conditions within the star and thus about its stage of stellar evolutionstellar evolution,
life history of a star, beginning with its condensation out of the interstellar gas (see interstellar matter) and ending, sometimes catastrophically, when the star has exhausted its nuclear fuel or can no longer adjust itself to a stable configuration.
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stellar structure

The interior constitution of a star, defining the run of temperature, pressure, density, chemical composition, energy flow, and energy production from the center to the surface. The structure of main-sequence stars, initially of uniform composition, is relatively simple: only very near the center are the temperature and pressure high enough for nuclear reactions to occur. In stars more than about twice the Sun's mass, convection currents in the core enlarge this zone and keep it mixed; less massive stars in contrast have a convective layer near the surface and an unmixed core.

Changes in structure as stars evolve can be calculated by following changes in chemical composition resulting from nuclear reactions, and recalculating the structure for the new composition. After the star has passed the Schönberg–Chandrasekhar limit, the structure changes to that of a giant, with an inert helium core surrounded by a hydrogen fusion shell and an extended envelope. Further core reactions in a very massive star will give it an onionlike shell structure, culminating in an iron core surrounded by successive shells of silicon, neon and oxygen, carbon, helium, and outermost, the hydrogen-rich envelope.

In principle, from an assumed composition, structure, and total mass, the other parameters of a stellar interior are derived by solving four differential equations: (1)  dP /dr = –GMρ/r 2 (2)  dM /dr = 4πr 2ρ (3)  dL /dr = 4πr 2ρ∊ (4)  dT /dr = 3κL ρ/16πacr2T 3

Equation 1 is that of hydrostatic equilibrium, 2 is that of continuity of mass, 3 is that of energy generation, and 4 is that of radiative transport (see energy transport). Accurate solutions require a large computer since the pressure (P ), opacity (κ), and energy generation rate (∊), also depend on the density (ρ), the temperature (T ), and the chemical composition; in addition in some parts of the star energy may be transported by convection rather than by radiation. Of the other symbols, r is the radius, M the mass within that radius, G the gravitational constant, L the luminosity at radius r , a the radiation density constant, and c the speed of light.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006

stellar structure

[′stel·ər ¦strək·chər]
The mathematical study of a rotating, chemically homogeneous mass of gas held together by its own gravitation; a representative model of the observable properties of a star; thermonuclear reactions are postulated to be the main source of stellar energy.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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