Star Formation. gas cloud protostar Star equilibrium PHYS 162 1

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1 Star Formation gas cloud protostar Star equilibrium PHYS 162 1

2 Star Formation protostar main sequence star. Happens faster if larger mass PHYS 162 2

3 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

4 SUN: Main Sequence Red Giant don t need to know PHYS 162 4

5 Helium Fusion Red Giant PHYS 162 5

6 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 3-Body Reaction He+He+He He+He OR two 2-Body Reactions C m( m( m( 4 8 Be(4p,4n) He) Be) 12 C) u u u He+Be C PHYS 162 6

7 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 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

8 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

9 Helium Fusion Red Giant Change radius and surface temperature in late stages of stars life cycles PHYS 162 9

10 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

11 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

12 Open Star Clusters - Pleiades Seven Sisters chased by Orion the hunter (Greek) Subaru cluster (Japan) only 6 really visible to unaided eye Subaru Telescope Hawaii PHYS

13 Open Star Clusters - Pleiades 7 daughters of Atlas and Pleinoe: Alcyone, Merope, Electra, Caleano, Taygeta, Maia (mother of Hermes), Sterope PHYS

14 Globular Star Clusters fuzzy cotton ball by eye or with modest telescope usually dim red stars dense with 100,000 stars in 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

15 Very Young Star Cluster moving to main sequence Note many more low mass stars Some with high surface temp (25,000) PHYS

16 100 million year old Star Cluster PLEIADES largest stars moving off main sequence to become giants Modest highest temp (10,000) PHYS

17 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

18 Fate of Stars INITIAL MASS Final State relative to Sun s mass M < 0.01 planet.01 < M <.08 Brown dwarf (not true star) 0.08 < M < 0.25 not Red Giant White Dwarf 0.25 < M < 12 Red Giant White Dwarf 12 < M < 40 Supernova: neutron star M > 40 Supernova: black hole PHYS

19 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

20 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

21 Planetary Nebula NOT planets (historic term) material ejected by pulsating Red Giant. Can lose over half the star s mass Helix nebula PHYS

22 Red Giant White Dwarves white dwarf at center of planetary nebula heats up surrounding gass PHYS

23 REMINDER Hertzprung- Russell Diagram Plot Luminosity versus surface temperature PHYS

24 Hertzprung- Russell Diagram Stars with larger sizes are brighter then a smaller star with the same surface temperature PHYS

25 Sun: Main Sequence Red Giant White Dwarves white dwarf PHYS

26 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

27 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

28 Supernova Explosions 1 billion times brighter then the Sun for a few months PHYS

29 Supernovas 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

30 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

31 Degenerate electrons Being close depends on electrons energy. Easier for lower energy to be close (wavelength = h/p) PHYS

32 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 B0- M10, add on below them the surface temperature with: B0= 24,000 A0=11,000 F0=7000 G0=6000 K0=5000 M0=4000 PHYS

Star Clusters. stars are usually near other stars - CLUSTER

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