Ian Howarth UCL Science Centre ‘Science Lectures for Schools’ 2010 Nov 26

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 SN 1006: brightest star ever seen 1054, Crab Nebula

“Tycho’s Star” (1572) De nova [et nullius aevi memoria prius visa] stella

Kepler’s Star (1604)

SN 1885 in M31

Fritz Zwicky ( ) (coined Supernova)

SN 1937A NGC 4157

Tom Boles

M51

~1x10 7 K Nuclear ‘burning’: H  He

~3x10 7 K

Helium burning: The continuing struggle against gravity... Carbon burning: ~10 8 K ~10 9 K

Then what...? Gravity’s victory!

Lifetimes (yrs) Burning StageSun9M ☼ 25M ☼ H burning10 2x10 7 7x10 6 He burning10 8 2x10 6 7x10 5 C burning Ne burning Si burning

Collapse!! Timescale ~1s Velocities ~ 1 / 4 c Cooling by photo- disintegration γ+ 56 Fe↔13 4 He+4n and electron capture p + +e - →n+ν e 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”