Stars spend their lifetimes trying to stay in
hydrostatic equilibrium and
thermal equilibrium.
When stars don't generate enough energy, they first fall out of thermal
equilibrium when then forces them out of hydrostatic equilibrium,
and so they start to evolve.
The story of
stellar evolution is simply the story of a star in its eternal
struggle against gravity.
Reminder: 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.)
The distinction between high mass 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
Type II supernova explosions are high mass stars
(M* > 8-12 MSun).
Stars that end their lives quietly through the
planetary nebula phase are
considered low mass stars (0.08 MSun <
M* < 8-12 MSun).
The range between 8 and 12
MSun is uncertain.
