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Transcript
ASTR 1120: Stars & Galaxies Etched Hourglass Nebula Prof. Juri Toomre TA: Ben Brown Lecture 19 Wed 23 Feb 05 Today • Continue with life of a lowlow-mass star (like the Sun) after exhausting H in core -- post MS • 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 • Discuss white dwarfs formed at end of evolution of lowlow-mass stars • Planetarium #2 this FRIDAY `Birth of Stars’ -- go there directly -- read 17.2 Star Birth carefully – bring clickers • HW Set # 4 returned + answers + Evolution OV zeus.colorado.edu/astr1120zeus.colorado.edu/astr1120-toomre Reading Clicker -- life tracks • What can we find out about a star from its life track on the HH-R diagram? B. • A. When the star was born • B. The surface temperature and luminosity of the star at each stage of its life • C. The star’s current stage of life • D. Where the star is located Clicker – Stellar Evolution • Which is correct order for some stages of life in a lowlow-mass star? A. • A. protostar, protostar, mainmain-sequence star, red giant, planetary nebula, white dwarf • B. protostar, protostar, mainmain-sequence star, red giant, supernova, neutron star • C. mainmain-sequence star, white dwarf, red giant, planetary nebula, protostar • D. protostar, protostar, mainmain-sequence star, planetary nebula, red giant Clusters: test lifetimes on main sequence Life track in H-R diagram of solarsolar-mass star REMINDER: blow lovely shell RG II ? Many meanders, meanders, but MS phase longest, longest, red giant phase(s) phase(s) shorter, shorter, finally white dwarf left to cool slowly All stars at about same distance All formed at about same time Red Giant I protostar white dwarf ZAMS Range of different mass stars ! Stars “peel off” MS as core H exhausted Æ red giants --Young --Young cluster -- Oldest cluster 1 Overview of what will happen: 1: LowLow-Mass Star on MS MS Æ Red Giant I Æ Horiz Branch Æ Red Giant II (or Supergiant) Supergiant) H burning in core Longest phase: phase: 10,000 MY = 10 BY if solar mass RG II RG I MS 2: Subgiant to MS Æ subgiant Æ red giant Red Giant (first visit) H burning in shell, makes much more energy Vast expansion, RG phase lasts ~ 500 MY Huge convective envelope MS Æ subgiant Æ red giant Complex aside: “Degeneracy” pressure MS red giant Contracting core in red giant gradually becomes “electron degenerate” -- what does that mean? 2 Oops! Thermostat is missing in degenerate gas 3: Helium Flash He core burning -removes electron degeneracy Æ He core burning with thermostat Could get exciting! 4: Horizontal branch star He core burning, H shell burning Æ “horizontal branch star” Helium flash Æ He fusion to C in core (horizontal branch) Short phase, lasts ~50 MY TripleTriple-alpha fusion: fusion: 3 He Æ C 5. Red Supergiant DoubleDouble-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 6. Planetary Nebula Outer shells of red supergiant “puffed off” Great pictures! “Naked” white dwarf emerges 3