Fate of Stars. relative to Sun s mass

Transcription

1 INITIAL MASS relative to Sun s mass M < 0.01 Fate of Stars Final State 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 2017: Some very massive stars go directly to a Black Hole without a supernova. Postulated for a while as the number of massive stars seems greater that the number of SN remnants. A BH has been detected without a SN remnant, which seems to confirm 1 this. Some WD can become SN by acquiring mass from companion star

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 2

3 Helium Fusion Red Giant 3

4 Red Giants White Dwarves for light stars (less than 10 times the mass of the Sun) burning of He to Carbon (Oxygen) is final fusion stage and an inert C-O core (no fusion) is built up inside the Red Giant 4

5 Red Giants White Dwarves II oscillate and 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 5

6 Planetary Nebula NOT planets (historic term) the material ejected by pulsating Red Giant can emit light as heated up by star at center Can lose over half the star s mass. Red Giant moving towards WD Blinking eye nebula. In Cygnus LY away 6

7 Planetary Nebula Ring Nebula Ring nebula in Lyra M57. 2,300 LY away. Can only see with a telescope. Emission lines of Hydrogen and Oxygen are prominent 7

8 Planetary Nebula Hourglass and Helix Nebula Hourglass nebula in constellation Musca. 8,00 LY away. Helium, oxygen, nitrogen, carbon shells give different colors Helix nebula. In Aquarius, 700 LY away Sometimes called Eye of God or Eye of Sauron 8

9 Red Giant White Dwarves Star becomes smaller but hotter as inner layers being exposed. Note very high surface temperatures 9

10 Hertzprung-Russell Diagram Reminder Plot Luminosity versus surface temperature. Star size is diagonal line large stars in upper right hand corner. WD radius 0.01 Sun = size of Earth 10

11 Sun: Main Sequence Red Giant White Dwarves white dwarf 11

12 White Dwarves Mass vs Radius In WD, gravity is balanced by pressure due to degenerate electrons White Dwarves are about the size of Earth A heavier WD will have smaller radius. Counter-intuitive but easily calculated using senior level physics. First determined by an Indian graduate student S. Chandrashekar in 1930s 12

13 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, white dwarves electrons are forced to higher energy states and so exert more pressure than normal 13

14 Degenerate electrons Being close depends on electrons energy. Easier for lower energy to be close (wavelength = h/p) DON T NEED TO KNOW 14

15 White Dwarves Mass vs Radius S. Chandrashekar worked out in 1930 on boat from India to England prior to grad school. Later became professor at Chicago. Nobel prize Could not get faculty position in UK due to racial discrimination. Earth radius Radius 0 15

16 White Dwarves Mass vs 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 mass limit then the white dwarf will collapse and have radius go almost to 0 Type I Supernova and (usually) a Neutron Star with ~20 km radius 16

17 Stellar Supernova Explosions Two Types 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. TODAY 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). TODAY and NEXT LECTURE leaves either Neutron Star or Black Hole. Produces most heavy elements 17

18 Supernova Explosions >1 billion times brighter then the Sun for a month. Decay in intensity over years (decay time due to lifetime of radioactive nuclei) 18

19 Supernova Explosions 1 billion times brighter then the Sun for a few months. Can outshine all other stars in a galaxy. Before on left and during supernova on right. Note the position and brightness of the other stars which do not change between photos 19

20 Supernovas supernovas occur every1000 years in a galaxy the size of the Milky Way (~400 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 Can be observable during the day 20

21 1572 (Tycho Brahe) and 1604 (Kepler) In Milky Way both probably Type Ia. Can now see and study remnants for a few thousand years. Example, tell what atoms are present from spectrum, measure velocity of ejected material 1572: in Cassiopeia about 9,000 LY away. Has companion star similar to Sun 1604: in Ophiuchus about 20,000 LY away 21

22 Supernova 2014j Jan 2014 In M82 Galaxy (Ursa Major). Type Ia. Closest of this type observed in modern times million LY away. Discovered at undergrad session Univ Coll London (SN1972 e was 11 MLY but pre modern ) 22

23 Supernova PTF 11kly Sept 2011 In Pinwheel Galaxy. Type Ia. 2nd closest Ia observed in modern times. 21 million LY away 23

24 Type II Supernova 1987a (in movie) Large Magellanic Cloud 180,000 LY away PHYS

25 Type II Supernovas and Core Collapse massive stars have fusion to heavier nuclei (Neon, Silicon, Sulpher, etc) end up with core of Iron nuclei plus 26 unbound free electrons for every Fe electrons are degenerate as so close together. This causes them to provide most of the pressure resisting gravity enormous stress. If electrons give way leaves hole in center of star PHYS

26 Supergiant Iron Core PHYS

27 core collapses into mostly neutrons very hot outer layers rush into hole smashing into shock wave from core happens when mass of core about > 1.4 Mass Sun. similar to Chandrashekar limit Type II Core Collapse PHYS

28 During Type II Supernova core collapse gives 200 billion degrees energetic photons breaks up many nuclei Fe 26p + 31n O 8p + 8n new nuclei form photons, n, and p strike shell around core. How Universe makes heavy elements p + e n + neutrino 1. Burst of neutrinos times more energy than from light (photons) 2. Leftover neutron star PHYS