Ian Howarthhttp://www.star.ucl.ac.uk/~idh/

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

The Hertzsprung-Russell Diagram: Stars Struggle Against GravityThe 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 (1898-1974)

(coined Supernova)

SN 1937ANGC 4157

Tom Boles

M51

~1x107K

Nuclear ‘burning’: HHe

~3x107K

Helium burning:

The continuing struggle against gravity...

Carbon burning:

~108K

~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 photo-disintegrationγ+56Fe↔134He+4nand electron capturep++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 thanoxygen (and quite a lot of the lighter ones): “we are stardust”

http://www.star.ucl.ac.uk/~idh/