AST201_winter2010_mreid_slideset6_v2_Bb

AST201_winter2010_mreid_slideset6_v2_Bb - 2/5/2010 AST 201...

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2/5/2010 1 AST 201 Stars and Galaxies 1 AST 201, Winter 2010, University of Toronto, Dr. M. A. Reid The Lives of Stars AST 201, Winter 2010, University of Toronto, Dr. M. A. Reid 2 Most stars lead very quiet lives, engaged in a protracted but very uneventful battle with gravity. AST 201, Winter 2010, University of Toronto, Dr. M. A. Reid 3 To talk about what happens beyond this point, we must now introduce the distinctions among stars of different masses. AST 201, Winter 2010, University of Toronto, Dr. M. A. Reid 4
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2/5/2010 2 Loosely, we can group stars into low-mass stars and high- mass stars . For most purposes, the division occurs at about 8 times the mass of the Sun. AST 201, Winter 2010, University of Toronto, Dr. M. A. Reid 5 AST 201, Winter 2010, University of Toronto, Dr. M. A. Reid 6 Stars of different masses are broken into different layers, according to how each layer transfers energy (either by radiation or convection ). Star Death AST 201, Winter 2010, University of Toronto, Dr. M. A. Reid 7 How a star will die depends strongly on how massive it is. Let’s consider low -mass stars like the Sun first. AST 201, Winter 2010, University of Toronto, Dr. M. A. Reid 8
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2/5/2010 3 The Death of a Low-Mass Star AST 201, Winter 2010, University of Toronto, Dr. M. A. Reid 9 AST 201, Winter 2010, University of Toronto, Dr. M. A. Reid 10 Remember: pressure and gravity are battling it out in the Sun. If the heat source runs out, gravity wins. AST 201, Winter 2010, University of Toronto, Dr. M. A. Reid 11 As solar core fusion progresses, more and more of the Sun’s core becomes helium. The Sun is not currently hot enough to fuse helium nuclei. After 10 billion years on the main sequence, most of the Sun’s core will be helium ash .” The Sun’s 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. AST 201, Winter 2010, University of Toronto, Dr. M. A. Reid 12
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2/5/2010 4 AST 201, Winter 2010, University of Toronto, Dr. M. A. Reid 13 At this stage, hydrogen can only burn in a shall 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 of the Sun with more energy than they used to receive.
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AST201_winter2010_mreid_slideset6_v2_Bb - 2/5/2010 AST 201...

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