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Welcome to the Paczynski Stellar Evolution Programs

This set of programs produces models of the interior of a star as the
composition changes with time due to nuclear reactions at the core.
The first program builds a Zero-Age Main Sequence (ZAMS) stellar model
with the mass and composition you have chosen for the outer boundary condition
calculations. The star has presumably contracted from an interstellar cloud
during the past few million years (these programs do not accurately portray
that physics), during which time convection has made the chemical composition
the same from center to surface.

The equations which this program solves take into account energy generated
by nuclear reactions near the center of the star, the transfer of that
energy by convection and radiation to the surface, the hydrostatic equation
(where gas and radiation pressure balance the pull of gravity), and a gas
equation of state for ideal gas, radiation, and partial degeneracy.

These equations are all simultaneously satisfied when four boundary
conditions are satisfied, which is what the program solves for: the central
temperature T_{c} and density rhoc, and the surface temperature *T*_{eff}
and luminosity *L*. In order to do this, you the user must guess reasonable
values for each of these quantities. Then the program solves the above
equations starting at the star's center, and again at the star's surface.

The solution of the equations will provide *T(m)* and rho(m) (the
temperature and density of the gas a distance from the star's center such
that mass *m* is contained inside), and *L(m)* and *r(m)*
(the luminosity produced and the distance from the center). These trial
solutions are plotted on the screen, each function in a different color.

You will notice that in the middle of the star, the solutions will not
agree, but rather, a jump in each quantity is seen. This is not physical,
and results from you having guessed the wrong conditions at the center
and surface. The program can improve your guesses, which it does, and tries
again when you call up the next picture. Now you will see that the jumps
in physical quantities get smaller with each subsequent improvement, until
there is a smooth flow of each across that fitting point.

When the program has "converged" a model, it now knows the correct values
of the four variables you guessed at the start. Thus the characteristics
of any star are determined, once the mass and composition of the model
are chosen.