Stars spend their lifetimes trying to stay in
hydrostatic equilibrium and
thermal equilibrium.
When stars don't generate enough energy, they fall out of thermal
and hydrostatic equilibrium
and they evolve.
The story of
stellar evolution is simply the story of a star in its eternal
struggle with gravity.
Russell-Vogt Theorem:
If a star is in hydrostatic and thermal equilibrium and
it derives all of its energy from nuclear reactions, then its structure is
completely and uniquely determined by its total mass and by the distribution
of the various chemical elements throughout its interior. In other words,
the mass and composition, the properties with which a star are born,
determine its structure. (Notice that no mention of
rotation or magnetic fields was made--these are implicitly assumed to be
of no importance.)
High Mass versus Low (and Intermediate) Mass Stars
The distinction between high mass and intermediate and low mass stars is made based upon the
way in which stars end the normal phases of their lifetimes.
Stars that
die violently in core-collapse supernova explosions are high mass stars,
M* > 8-12 MSun
Stars that end their lives quietly through planetary nebula phase are
considered low/intgermediate
mass stars, M* < 8-12 MSun.
Low/intermediate mass stars do not ignite the higher
burning stages in their cores, in particular, carbon burning and above.
