stellar structure

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]

(astrophysics)

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.