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STAR LIFE & DEATH
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Life on the Main Sequence
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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
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Main Sequence structure depends on mass . . .
pgs. 276
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luminosity increases with age
Low-mass stars: luminosity increases with age zero-age main sequence Luminosity Sun Temperature
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Change in composition of
1 solar mass star. Fusion ceases when core converted to helium – star now leaves main sequence.
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Sun: ~ 10 billion years
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Star Death I: Low Mass Stars (M < 8M)
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Surface cools, core contracts & heats, radius expands. ‘Evolutionary tracks’ Red Giant p. 277
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Sun as a red giant Vigorous H He fusion in shell drives envelope outward. Inert helium core (shrinking) p. 277
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Red Giant: Aldebaran T = 3500 K L = 370 L R = 50 R M 3 M
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* Core temp 100 million K: Helium fusion begins
Another Helium Beryllium Gamma Ray Helium Carbon Gamma Ray
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In addition . . . 12C + 4He 16O + gamma ray
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On the HR diagram . . . Supergiant Core He exhaustion He ignition Horizontal branch
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Helium-burning, Horizontal Branch star p. 279
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Supergiant Star Helium-fusing shell Contracting carbon-helium core Hydrogen-burning shell
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* Supergiants lose mass:
> Stellar winds > ‘Flashes’ in helium-burning shell Ring Nebula Old stellar core Planetary Nebula Ejected stellar envelope p. 281
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p. 281
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Hourglass Nebula Old stellar core shrinking to White Dwarf state.
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The whole story . . . p. 280
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Star Death II: High Mass Stars (M > 8M)
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High temp., rapid fusion on CNO Cycle Again hydrogen fusion ceases when core converted to helium – star now leaves main sequence.
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Multiple core fusion stages are possible. p. 283 core re-ignition
core exhaustion p. 283
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Core Fusion Core Temp Duration
For a 25 M star: Core Fusion Core Temp Duration H fusion 40 million K million yr He fusion 200 million K ,000 yr Carbon fusion 600 million K yr Neon fusion 1.2 billion K yr Oxygen fusion 1.5 billion K mos Silicon fusion 2.7 billion K day results in Iron
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As fusion ceases . . . ‘Onion Skin’ p. 283
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Fusion ceases when iron is produced . . .
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SUPERNOVA! 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!
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Elements heavier than iron created in blast.
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Before Supernova 1987A After
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SN 1987A in 1999 SN ejecta Stuff ejected before SN.
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SN blast wave reaches inner ring
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proton + electron neutron + neutrino
SN 1987A (deep underground) Neutrino arrival
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SN probably occur ~ once per 100 yrs in our galaxy.
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Crab Nebula Supernova Remnant 600 mi/s (Exploded 1054 AD) Pulsar (rotating neutron star) Visible in broad daylight for 23 days in July, 1054!
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".. In the 1st year of the period Chih-ho, the 5th moon, the day chi-ch'ou, a guest star appeared south-east of Tien-Kuan [Zeta Tauri]. After more than a year, it gradually became invisible .."
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Supernova recorded at Chaco Canyon, NM?
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Cygnus Loop ~13,000 BC
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Vela Supernova Remnant
(~10,000 BC) Interstellar medium ‘seeded’ with heavy elements.
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Cassiopeia A Supernova Remnant
Neutron star? Black hole? X-ray
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Iron Cassiopeia A Supernova Remnant Silicon
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