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STAR LIFE & DEATH Life on the Main Sequence Life on the Main Sequence • Stable fusion: hydrogen helium • Accumulation of helium in core Steady increase in luminosity • 90% of star’s life spent on main sequence • More mass shorter MS lifetime Main Sequence structure depends on mass . . . pgs. 276 Low-mass stars: luminosity increases with age zero-age main sequence Sun Temperature Change in composition of 1 solar mass star. Fusion ceases when core converted to helium – star now leaves main sequence. Sun: ~ 10 billion years Star Death I: Low Mass Stars (M < 8M) ‘Evolutionary tracks’ Surface cools, core contracts & heats, radius expands. Red Giant p. 277 Sun as a red giant Vigorous H He fusion in shell drives envelope outward. Inert helium core (shrinking) p. 277 Red Giant: Aldebaran T = 3500 K L = 370 L R = 50 R M3M * Core temp 100 million K: Helium fusion begins Another Helium Beryllium Gamma Ray Helium Carbon Gamma Ray In addition . . . 12C + 4He 16O + gamma ray On the HR diagram . . . He ignition Core He exhaustion Horizontal branch Supergiant Helium-burning, Horizontal Branch star p. 279 Supergiant Star Helium-fusing shell Hydrogen-burning shell Contracting carbon-helium core * Supergiants lose mass: > Stellar winds > ‘Flashes’ in helium-burning shell Old stellar core Planetary Nebula Ejected stellar envelope Ring Nebula p. 281 p. 281 Hourglass Nebula Old stellar core shrinking to White Dwarf state. The whole story . . . p. 280 Star Death II: High Mass Stars (M > 8M) High temp., rapid fusion on CNO Cycle Again . . . hydrogen fusion ceases when core converted to helium – star now leaves main sequence. Multiple core fusion stages are possible. core re-ignition core exhaustion p. 283 For a 25 M star: Core Fusion Core Temp Duration H fusion 40 million K 7 million yr He fusion 200 million K 500,000 yr Carbon fusion 600 million K 600 yr Neon fusion 1.2 billion K 1 yr Oxygen fusion 1.5 billion K 6 mos Silicon fusion 2.7 billion K 1 day results in Iron As fusion ceases . . . ‘Onion Skin’ p. 283 Fusion ceases when iron is produced . . . p. 284 Iron core contracts, heats Nuclei disintegrate Protons absorb electrons: proton + electron neutron + neutrino Core stiffens, bounces back slightly Core bounce + neutrino flow ejects envelope: SUPERNOVA! Elements heavier than iron created in blast. Supernova 1987A Before After SN 1987A in 1999 SN ejecta Stuff ejected before SN. SN blast wave reaches inner ring SN 1987A proton + electron neutron + neutrino (deep underground) Neutrino arrival SN probably occur ~ once per 100 yrs in our galaxy. 600 mi/s Crab Nebula Supernova Remnant (Exploded 1054 AD) Pulsar (rotating neutron star) Visible in broad daylight for 23 days in July, 1054! ".. In the 1st year of the period Chih-ho, the 5th moon, the day chi-ch'ou, a guest star appeared south-east of TienKuan [Zeta Tauri]. After more than a year, it gradually became invisible .." Supernova recorded at Chaco Canyon, NM? Cygnus Loop ~13,000 BC Vela Supernova Remnant (~10,000 BC) Interstellar medium ‘seeded’ with heavy elements. Cassiopeia A Supernova Remnant Neutron star? Black hole? X-ray Iron Cassiopeia A Supernova Remnant Silicon
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