AST 341 Lecture Notes - Lecture 15: Red Giant, White Dwarf, Subgiant
25 Dec 2016
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Document Summary
The details depend on whether we are talking about low-mass stars or high-mass stars, but the central issue is similar for both. The central issue is the exhaustion of successive rounds of nuclear fuel as the star ghts the pull of self-gravity. Once hydrogen is exhausted in the core, the core is too cool to burn helium, hence it contracts due to self-gravity. As the core contracts, it heats up, up to a point where helium burning can then start. From this point on, the star starts its post-main-sequence evolution. The dominant observational signature of this post-main-sequence phase is a rapid expansion of the envelope to form a red-giant. This means that, during the contraction, not much thermal energy is lost, and therefore energy is conserved. Then, we have + u = const. (energy conserved). From the virial theorem, we have = 1. Putting these two equations together we deduce that = const. and u = const.
