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1 ASTR 1040 Accel Astro: Stars & Galaxies Prof. Juri Toomre TA: Kyle Augustson Lecture 15 Tues 4 Mar 08 zeus.colorado.edu/astr1040-toomre toomre MID-POINT LECTURE Etched Hourglass Nebula Today + Discuss how a star may be born: getting to the MS Look at post-ms evolution of a low-mass star Consider red giant (RG I) phase, with H shell burning Helium flash goes off in shrinking degenerate core: horizontal branch star with He core burning Double shell burning (H and He) yields red supergiant (RG II), blows off planetary nebula Read 18.1 about white dwarfs formed at end of evolution of low-mass stars Homework Set # 6 due today, new HW # 7 out Become a Learning Assistant! Info Session (Tomorrow) Wed March 5, 6-8pm6 MCD Biology Interactive Classroom If you ve ever considered teaching,, this is a great way to gain experience or try it out. You ll work closely with faculty preparing for classes each week. $10/hr for 10 hrs/week = $1500/semester All majors welcome, many science/math classes across the campus are using LAs Clicker starbirth: : what has stuck? The vast majority of stars in a newly formed star cluster are? A. A. less massive than the Sun B. very high-mass, type O and B stars C. red giants D. about the same mass as Sun Grand design spiral M51 Whirlpool Beauty and richness of the ISM Disk stars/gas rotate through traffic jams Bright O & B stars mark the spiral pattern: regions of star birth Emission nebulae around star birth regions -- in dark clouds of obscuring dust and gas 1

2 Gravitational collapse of molecular clouds Battle between (1) gravity pulling inwards building clumps, (2) pressure of heated gas pushing outwards to resist further collapse Need big clouds & cooling Simulation of giant cloud collapse and birth of stars Stages in building the PROTOSTAR Too much angular momentum jet it away! Actual edge-on disk and jet (Hubble ST) Many jets from young stellar objects Often protostar is still hidden in cocoon of dust and jets are episodic (Herbig-HaroHaro objects) More massive protostars get to MS faster! 2

3 Collapse of molecular cloud makes many small stars, fewer massive O & B stars Star Birth Cauldron NGC 604 in M33 Triangulum What happens to nuclear fusion when the hydrogen in a star s s core runs low? E. A. It stops B. It shifts from the core to a shell around the core C. Other elements start to fuse D. The star goes out of balance and becomes a red giant E. B and D Overview of what will happen: MS Red Giant I Horiz Branch Red Giant II (or Supergiant) RG I MS RG II 1: Low-Mass Star on MS H burning in core Longest phase: 10,000 MY = 10 BY if solar mass 2: Subgiant to Red Giant (first visit) H burning in shell, makes much more energy Vast expansion, RG phase lasts ~ 500 MY Huge convective envelope 3

4 MS subgiant red giant MS subgiant red giant MS red giant Contracting core in red giant gradually becomes electron degenerate -- what does that mean? Oops! Thermostat is missing in degenerate gas Complex aside: Degeneracy pressure Could get exciting! Degeneracy pressure analogy 3: Helium Flash He core burning -- removes electron degeneracy Limited quantum mechanical states (exclusion principle) (see S4.4) He core burning with thermostat horizontal branch star 4

5 4: Horizontal branch star He core burning, H shell burning Helium flash He fusion to C in core (horizontal branch) Short phase, lasts ~50 MY Triple-alpha fusion: 3 He C H-R R diagram of globular cluster Discussion: What does it tell us? Why is it useful? 5. Red Supergiant Double-shell burning of H and He Phase could be very short if He burning is erratic (unstable) -- then lasts only a few MY, and blows off outer shells Sun in its far future ~5+ BY Life track in H-R R diagram of solar-mass star blow lovely shell? RG II Thermal pulses in red supergiant blow off outer shells Many meanders, but MS phase longest, red giant phase(s) ) shorter, finally white dwarf left to cool slowly white dwarf Red Giant I ZAMS protostar 5

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