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Star Formation gas cloud protostar Star PHYS 162 equilibrium 1 Star Formation protostar main sequence star. Happens faster if larger mass PHYS 162 2 Stellar Evolution • • • • • • 90% of its lifetime: star converts Hydrogen to Helium p-p cycle Main Sequence Helium builds up in the core, but not yet burning gravity compresses which increases temperature helium starts burning, more energy produced Different equilibrium, less stable NOT on main sequence where on HR diagram is complicated (you don’t need to know) simplistically Red Giants=He burning PHYS 162 3 SUN: Main Sequence Red Giant don’t need to know PHYS 162 4 Helium Fusion Red Giant PHYS 162 5 Helium Fusion I As mass Carbon12 (6p,6n) is less than the mass of 3 He4 (2p,2n) then combining 3 He into C releases energy m( 4He) 4.0026u 3-Body Reaction m(8Be ) 8.0053u 12 m ( C ) 12.000u C He+He+He OR two 2-Body Reactions He+He Be(4p,4n) He+Be C PHYS 162 6 Helium Fusion II • • • • Helium to Carbon burning is suppressed 3-body reactions are always suppressed 2-body Beryllium(8) is unstable. (It decays into 2 He nuclei in 10-16 seconds). An “accident” of Nature. Need to have Be+He reaction occur before the Be decays slows up reaction Larger electric repulsion than p-p as larger electric charge (2 for He and 4 for Be). Therefore need about 100,000,000 degrees K for He burning Stars like our Sun remain main sequence longer due to this PHYS 162 7 Our Sun Red Giant in ~5 billion years, our Sun will expand to about the size of 1 AU = Earth’s orbit PHYS 162 8 Helium Fusion Red Giant Change radius and surface temperature in late stages of stars’ life cycles PHYS 162 9 Stellar Evolution • test out model of stellar evolution using Star Clusters • HR diagram of a cluster gives “snapshot” of stars with the same age but different masses • Birth Main Sequence Red Giant “live+die” faster if higher mass • tell age of cluster by most massive star still on Main Sequence PHYS 162 10 Star Clusters stars are usually near other stars - CLUSTER • formed at the same time • similar chemical composition • about the same distance from us Can classify by appearance and use to: • study stellar lifetimes • measure distances (earlier: spectroscopic parallax) PHYS 162 11 Open Star Clusters - Pleiades “Seven Sisters” chased by Orion the hunter (Greek) Subaru cluster (Japan) only 6 really visible to unaided eye PHYS 162 Subaru Telescope12Hawaii Open Star Clusters - Pleiades 7 daughters of Atlas and Pleinoe: Alcyone, Merope, Electra, Caleano, Taygeta, Maia (mother of Hermes), Sterope 13 PHYS 162 Globular Star Clusters “fuzzy cotton ball” by eye or with modest telescope • usually dim red stars • dense with 100,000 stars in 50-300 LY region with less than LY separating stars • no heavy elements. Just Hydrogen and Helium • often outside plane of galaxy Understood as group of old stars formed in early history of the galaxy 3-12 billion years old PHYS 162 14 Very Young Star Cluster “moving” to main sequence Note many more low mass stars Some with high surface temp (25,000) PHYS 162 15 100 million year old Star Cluster PLEIADES largest stars “moving” off main sequence to become giants Modest highest temp (10,000) PHYS 162 16 5 billion year old Star Cluster largest stars are gone stars little more massive the Sun have become giants Highest temp on main sequence now 6000 PHYS 162 17 Fate of Stars INITIAL MASS relative to Sun’s mass M < 0.01 .01 < M < .08 0.08 < M < 0.25 0.25 < M < 12 12 < M < 40 M > 40 Final State planet Brown dwarf (not true star) not Red Giant White Dwarf Red GiantWhite Dwarf Supernova: neutron star Supernova: black hole PHYS 162 18 White Dwarves • for “light” stars (less than 10 times the mass of the Sun) burning of He to Carbon (Oxygen) is final fusion stage • electrons’ pressure resisting gravity • Outer Layers keep expanding (or oscillating) losing matter. See as planetary nebula inside layers of star become surface. PHYS 162 19 White Dwarves II • loss of over 1/2 star’s mass during Red Giant phase Mass being lost core At some point all that is left is the hot, dense, inert (no fusion) C/O core about size of Earth: WHITE DWARF Will slowly cool down over time PHYS 162 20 Planetary Nebula • NOT planets (historic term) material ejected by “pulsating” Red Giant. • Can lose over half the star’s mass Helix nebula PHYS 162 21 Red GiantWhite Dwarves white dwarf at center of “planetary” nebula heats up surrounding gass PHYS 162 22 REMINDER HertzprungRussell Diagram Plot Luminosity versus surface temperature PHYS 162 23 HertzprungRussell Diagram Stars with larger sizes are brighter then a smaller star with the same surface temperature PHYS 162 24 Sun: Main SequenceRed GiantWhite Dwarves white dwarf PHYS 162 25 White Dwarves Mass vs Radius • In WD, gravity is balanced by pressure due to degenerate electrons • A heavier WD will have smaller radius • if Mass(WD) > 1.4 M(Sun) degenerate electrons can not resist gravity called Chandrasekhar limit and no WD has a mass greater than this • If WD can acquire mass from companion star and goes over this limit Supernova and (usually) a Neutron Star PHYS 162 26 Stellar Supernova Explosions For heavy white dwarves with a companion star • acquire mass, if becomes > 1.4 M(Sun) SUPERNOVA (Ia). p + e n + neutrino • Usually leaves neutron star For high mass stars • fusion continues beyond C,O • core of degenerate electrons builds up - opposes gravity • if Mass(core) > 1.4 M(Sun) core collapses in SUPERNOVA (II) • leaves either Neutron Star or Black Hole PHYS 162 27 Supernova Explosions 1 billion times brighter then the Sun for a few months PHYS 162 28 Supernovas • 10-20 supernovas occur every1000 years in a galaxy the size of the Milky Way (~200 billion stars) with ~15% being type Ia • 8 observed in last 2000 years (185, 386, 393, 1006, 1054, 1181, 1572, 1604) • Hard to observe if on “opposite” side of Milky Way all recent observed SN are in other galaxies PHYS 162 29 Degenerate electrons not on tests same as Pauli exclusion - two electrons which are “close” to each other can not occupy the same quantum state. Causes: • Periodic table and different chemical properties for different atoms • why metals conduct electricity • interior of stars and planets electrons are forced to higher energy states and so exert more pressure than normal PHYS 162 30 Degenerate electrons Being “close” depends on electrons energy. Easier for lower energy to be close (wavelength = h/p) PHYS 162 31 HR Digram Worksheet • For Sirius B use spectral class B1 • Vertical axis is Luminosity and a log scale so do .00001, .0001, .001, .01, .1, 1, 10, 100, 1000, 10,000, 100,000, 1,000,000 with 2,3 or 4 lines separating each • After labeling horizontal axes with spectral class B0M10, add on below them the surface temperature with: B0= 24,000 A0=11,000 F0=7000 G0=6000 K0=5000 M0=4000 PHYS 162 32