Lecture 11 Notes

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Astronomy & Astrophysics
Stefan Mochnacki

Cliff Lau University of Toronto AST201 Lecture 11 (February 17, 2011) Most stars lead very quiet lives, engaged in a protracted but very uneventful battle with gravity Loosely, we can group stars into low-mass stars and high-mass stars; for more purposes, the division occurs at about 8 times the mass of the Sun Stars of different masses are broken into different layers, according to how each layer transfers energy (either by radiation or convection) How a star will die depends strongly on how massive it is Death of a low-mass star pressure and gravity are battling it out in the Sun (low-mass star); if the heat source runs out, gravity wins. As solar core fusion progresses, more and more of the Suns core becomes helium; the Sun is not currently hot enough to fuse helium nuclei; after 10b years on the main sequence, most of the Suns core will be helium ash. The Suns core is not currently hot enough to fuse helium, so once the core is mainly helium, fusion shuts off and the core starts to cool and contract; this releases a lot of energy formerly stored as gravitational potential energy. At this stage, hydrogen can only burn in a shell around the core, but the rate of burning INCREASES because of pre-heating from below; the Sun becomes a layered onion. Hydrogen shell burning is so intense that it floods the outer layers o
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