What are the main physical processes which determine the structure of stars?
Stars are held together by the force of gravitation, the attraction
exerted on each part of the star by all other parts. This gravitational
force is resisted by an internal pressure gradient due to the
thermal motion of the particles which make up the stellar material.
These two forces, gravitational attraction and thermal pressure,
play the principal role in determining the structure of stars. They
must be almost in balance if the stars are not to change their properties
more rapidly than is observed. In addition to these forces,
we must also consider the thermal properties of stars. The stars are
continually radiating energy into space. If stars are not to change their
thermal properties more rapidly than is observed, energy must be continually
supplied to make good this loss. The origin
of this energy and the way it is transported to the surface of the star
must be incorporated into any theories of stellar structure.
In this section we will use our knowledge of the physical processes
described above to formulate the equations of stellar structure.
In doing so, we will make two fundamental assumptions about the structure
of stars. First, we will assume that, although stars do
evolve, their properties change so slowly that it is a good approximation
to neglect the rate of change of these properties with time. Second,
we will assume that all stars are spherical and symmetric about their
centres. If these two assumptions are made, the structure of a star is
governed by a set of equations in which all the physical quantities
depend on the distance from the centre of the star alone. We will
start by making these two assumptions and later consider how valid they
©Vik Dhillon, 27th September 2010