AST 101 Lecture Notes - Lecture 16: Horizontal Branch, Soot, Photosphere
Document Summary
As the core of a star heats up during the m-s phase, it can actually ignite hydrogen burning in a shell surrounding the core. When the core has used up its hydrogen, the only energy generation occurring in the star is in this hydrogen burning shell. Now, as the core isn"t generating energy anymore, it is no longer supported against gravitational collapse. As it collapses, it"s density and pressure increase, so the core gets hotter. Thus, it actually releases more energy than it did when it was burning hydrogen. But it is gravitational energy, rather than nuclear energy. This increased energy output from the collapsing core plus the h-burning shell causes the envelope of the star to expand. The star becomes cooler and more luminous than it was on the main sequence. Red giant atmospheres are much less dense than those of main sequence stars, and are much more tenuously bound to the star.
