f10_nov11 - Ast 307 - Nov. 11 , 2010 chs. 20 - 22: Stellar...

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Ast 307 - Nov. 11 , 2010 chs. 20 - 22: Stellar End-Games Life on the Main Sequence Main Sequence stars of different masses have different arrangements of radiation and convection zones in their interiors. How long do these stars stay on the MS? Until they burn up their fuel (H)!! Massive stars have more fuel, but they are also much more luminous, so they use up their fuel faster. Leaving the Main Sequence • The core begins to contract there; this is called a “ hydrogen-burning shell – the higher pressure in this hot region pushes outward, so - paradoxically - the outer layers of the star expand Evolution off the Main Sequence
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Perryman et al., Astron. Astrophys. 331, 81{120 (1998) Solar B-V = +0.65 Solar M V = +4.82 The Hyades color-magnitude diagram • At first, there are stars of all masses on the cluster’s Main Sequence • The most massive stars “die off” first • As time passes, lower- mass stars begin to move off the Main Sequence • Find this “turn-off” point: then the cluster’s age is equal to the Main Sequence lifetime of stars at the turn-off. Main-sequence turnoff Finding the Ages of Star Clusters Star clusters of different ages have different turn-off points; younger ones still have high-mass Main Sequence stars, for older clusters, more of the M.S. has “peeled off.” Finding the Ages of Star Clusters http://s94958815.onlinehome.us/angryastronomer/m92isochrones.gif http://www.dur.ac.uk/ian.smail/gcCm/gcCm_intro.html HR Diagrams of Old Star Clusters
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! Stellar Evolution and Cluster Ages Fig. 19-10, p. 509 ! Main Sequence phase (80-90% of life) Exhaustion of H in the core: “core H-burning” (core = central region) The star moves up and to the right in the HR diagram, into the “Red Giant” region Core contracts, releasing gravitational energy via Kelvin-Helmholtz contraction The outer layers (“envelope”) expand From Main Sequence to Red Giant The Main Sequence vs. Red Giant Sun Helium Burning The He core contracts until it heats up to 10 8 K He fusion begins (3 He " 1 C, + He " O) – sometimes called the “triple- # process” The star, now fusing He in its core, is again stable. gravity vs. pressure from He fusion reactions red giants create and release most of the Carbon from which organic molecules (and life) are made
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Core Helium-Burning Stage The star becomes smaller and hotter - moves onto the ‘horizontal branch,’ another stable, long-lived phase. !
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This note was uploaded on 04/30/2011 for the course AST 317 taught by Professor Dinerstein during the Fall '10 term at University of Texas at Austin.

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f10_nov11 - Ast 307 - Nov. 11 , 2010 chs. 20 - 22: Stellar...

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