11-1 Protostars and Pre-Main-Sequence Stars Stars form out of enormous volumes of gas and dust . Radio telescopes detect CO at 2.6 mm, calculate 1e4 H2 molecules per CO. Giant molecular clouds + perturbation -> dense cores -> propostars
11-2 Supernova explosions in cold, dark nebulae trigger the birth of stars. Gravitational contractin -> heating (1e5 yrs)
EYES ON ... Nebulae
11-3 When a protostar ceases to accumulate mass, it becomes a pre-main-sequence star, which contracts more slowly (1e7 yrs for sun) till ignition
11-4 The evolutionary track of a pre main-sequence star depends on its mass. Gravity in balances gas pressure out -> hydrostatic equilibrium. Large stars burn rapidly, less efficient (race cars). Small stars are more common and more efficient, burning slowly (economy models)
11-5 Young star clusters are found in H II regions. EX: Orion Nebula. OB winds + UV -> shock wave -> stellar formation
11-6 Plotting a star cluster on an H-R diagram reveals its age. T-Tauri are PMS, like sputtering campfire (not quite hot enough for MS)
11-7 Stars spend most of their lives on the main sequence. H Fusion + HSE -> MS
11-8 When core hydrogen fusion ceases, a main sequence star becomes a giant. shell H fusion puffs uter layers -> mass lss
11-9 Helium fusion begins at the center of a giant . 3 He -> C (and C + He -> O)
11 -10 As stars evolve, they move on the H-R diagram . Ex Sun: Multiple He flashes -> hot and dimmer
11-11 Globular clusters are bound groups of old stars. He flash -> horizontal branch. Turnoff point (+ mass) -> age of culser
RR Lyrae = low-M post HE flash
11 - 12 A Cepheid pulsates because it is alternately expanding and contracting. Henrietta Leavitt 1912: period ~ size~ luminosity -> distance! (used by Hubble)
II - 13 Cepheids enable astronomers to estimate vast distances. Apparent magnitude + absolute magnitude -> distance.
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Discussion Questions from Ch.11: RQ# 5, 9, 12 (p.268)
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12-1 Low-mass stars expand into the giant phase twice before becoming planetary nebulae.
shell He fusion -> AGB. 300,000 yrs between He flashes
12-2 The burned-out core of a low-mass star becomes a white dwarf. Can be hot or cold
12-3 A series of different types of fusion leads to the creation of supergiants .
C -> ... Silicon ... Fe ...
12-4 High-mass stars die violently by blowing themselves apart in supernovae . Electron degeneracy supports against collapse until core exceeds Chandrasekhar limit (1.4 Ms). Collapse in a few seconds, ... gammas break down nuclei ... bounce off neutron core ... shock waves + neutrinos -> heavy metals
Expansion for days
12-5 Supernova SN 1987A gave us a close-up look at the death of a massive star
12-6 Accreting white dwarfs in close binary systems can also become supernovae
12-7 The remnants of a supernova explosion can be detected for centuries afterward
12-8 The cores of many Type II supernovae become neutron stars . Predicted by Zwicky ad Baade (1932), discovered by Jocelyn Bell 1967. 1054 SN -> Crab Nebula
12-9 A rotating magnetic field explains the pulses from a neutron star
12-10 Pulsating x-ray sources are neutron stars in close binary systems . Electrons in B fild -> bremsstrahlung -> x rays
12- 11 Other neutron stars in binary systems emit powerful jets of gas
accretion onto WD -> nova: (H fusion in acc. layer)
accretion onto NS -> XRB (He fusion in acc. ayer)
collision of NS or NS int BH -> GRB (last week)
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Discussion Questions from Ch.12: DQ 23, 24; RQ 3, 11 (p.290)
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Maintained by E.J. Zita