The simplest commonly used model of stellar structure is the spherically symmetric quasi-static model, which assumes that a star is very close to an equilibrium state, and that it is spherically symmetric. It contains four basic first-order differential equations: two represent how matter and pressure vary with radius; two represent how temperature and luminosity vary with radius.
For conductive luminosity transport, the matter-pressure (or hydromechanical) equations, in Eulerian coordinates are:
and the temperature-luminosity equations are:
where r is the distance from the star centre, m(r) is the cumulative mass inside of a sphere of radius r centred at the star centre, P(r) is the total pressure (matter plus radiation), ρ(r) is the matter density, l(r) is the luminosity (photons) at r, k is the thermal conductivity, T(r) is the temperature, assumed identical for matter and photons, ε(r) is the luminosity produced (from nuclear reactions) per unit mass, εν is the luminosity produced by neutrinos per unit mass, and G is the newtonian gravitational constant.
Similar equations for the case of radiative luminosity transport are obtained by replacing k.
The case of convective luminosity transport is usually modelled by the more ad hoc mixing length theory.
External links
Variational Principles for Stellar Structure (http://arxiv.org/abs/astro-ph/9610099), Dallas C. Kennedy, Sidney A. Bludman, 1996
Thus, stellar evolution is a necessary consequence of the physical theory of stellarstructure, which requires that the luminosity, temperature, and size of a star must change as its chemical composition changes because of thermonuclear reactions.
The initial phase of stellar evolution is contraction of the protostar from the interstellar gas, which consists of mostly hydrogen, some helium, and traces of heavier elements.
Because the middle age of a star is the longest period in stellar evolution, one would expect most of the observed stars to be at this stage and to show a strong correlation of luminosity with color (color is a measure of stellartemperature).
In astronomy, stellar evolution is the sequence of changes that a star undergoes during its lifetime, the millions or billions of years during which it emits light and heat.
Stellar evolution is not studied by observing the life cycle of a single star—most stellar changes occur too slowly to be detected even over many centuries.
Current understanding of stellar collapse is not good enough to tell whether it is possible to collapse directly to a fl hole without a supernova, if there are supernovae which then form fl holes, or what the exact relationship is between the initial mass of the star and the final object that remains.
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