Transcript Science Centre Talk

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’: HHe
~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/