Reading Assignment: Chapter 20 and 21
Stars spend their time trying to overcome the effects of gravity. When they are able to balance gravity, they exist in stable equilibrium states, such as the Main Sequence. However, stars are not always able to balance gravity (to maintain hydrostatic equilibrium). The reason stars may fall out of hydrostatic equilibrium is quite simple and based on an obvious fact; stars radiate energy to space, that is, they shine. This simple act means that stars continuously lose energy to space and therefore they must continuously generate energy to maintain thermal and hydrostatic equilibrium. When they don't generate enough energy, gravity wins and they go out of thermal and hydrostatic equilbrium and evolve. The story of stellar evolution is, simply put, the story of a star in its eternal struggle with gravity.
Russell-Vogt Theorem
Recall the 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 a star is born with are just the properties which determine its structure. (Notice that no mention of rotation or magnetic fields was made--these are implicitly assumed to be of no importance.)
General Comments on Stellar Evolution
Just as the mass determines the Main Sequence structure of a star, it also determines how the star evolves, how the star dies, and what it becomes after its death.