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UCL Science Centre ‘Science Lectures for Schools’ 2010 Nov 26 Ian Howarth http://www.star.ucl.ac.uk/~idh/ The Hertzsprung-Russell Diagram: Stars Struggle Against Gravity What’s this got to do with supernovae? Normal stars are in a state of equilibrium between gas pressure pushing outwards and gravity pulling inwards (just like our atmosphere). However, to maintain the gas pressure we need a heat source. When that source is exhausted, gas pressure is removed, and the star will collapse. A big star will undergo a big collapse: a supernova SN 1994D in NGC 4526 RCW 86: remnant of “Guest Star” from 185 1054, Crab Nebula SN 1006: brightest star ever seen “Tycho’s Star” (1572) De nova [et nullius aevi memoria prius visa] stella Kepler’s Star (1604) SN 1885 in M31 Fritz Zwicky (1898-1974) (coined Supernova) SN 1937A NGC 4157 Tom Boles M51 Nuclear ‘burning’: HHe ~1x107K ~3x107K Helium burning: ~108K The continuing struggle against gravity... Carbon burning: ~109K Then what...? Gravity’s victory! Lifetimes (yrs) Burning Stage Sun 9M☼ 25M☼ H burning 1010 2x107 7x106 He burning 108 2x106 7x105 C burning 380 160 Ne burning 1.1 1.0 Si burning 0.004 0.003 Collapse!! Timescale ~1s Velocities ~1/4 c Cooling by photodisintegration γ+56Fe↔134He+4n and electron capture p++e-→n+νe Most energy comes out in neutrinos Shock wave propagates out over a day or so observed SN SN 1987A (Feb 23) 25 neutrinos = all extragalactic neutrino astronomy...confirms core-collapse model (and limits neutrino mass) To recap: Stellar evolution is the struggle of pressure against gravity. Gravity always defeats gas pressure, eventually For solar-type stars, the last defence is electron degeneracy pressure (the sun will end its life as a white dwarf). For more massive stars, the final fate is a neutron star, or a black hole, formed in a supernova explosion On the way, massive stars make pretty much all the elements heavier than oxygen (and quite a lot of the lighter ones): “we are stardust” http://www.star.ucl.ac.uk/~idh/