Supernovae
S
tars have long lives. Even cosmic searchlights, such as
Rigel and Canopus, go on pouring out their energy for
tens of millions of years before disaster overtakes them.
Yet sometimes we can witness a stellar death – the literal
destruction of a star. We call it a supernova.
Supernovae are not merely very brilliant novae; they
come into an entirely different category, and are of two
distinct classes. A Type Ia supernova is believed to be a
binary system; Ib and Ic result from the collapse of single
stars. In a Type Ia binary, one component (A) is initially
more massive than its companion (B) and therefore evolves
more quickly into thered giant stage. Material from it is
pulled over to B, so that B grows in mass while A
declines; eventually B becomes the more massive of the
two, while A has become a white dwarf made up mainly
of carbon. The situation then goes into reverse. B evolves
to become a giant, and starts to lose material back to the
shrunken A, with the result that the white dwarf builds up
a gaseous layer made up mainly of hydrogen which it has
stolen from B. However, there is a limit, once the mass of
the white dwarf becomes greater than 1.4 times that of the
Sun (a value known as the Chandrasekhar limit, after the
Indian astronomer who first worked it out), the carbon det-
onates, and in a matter of a few seconds the white dwarf
blows itself to pieces. There can be no return to the old
state; the star has been completely destroyed. The energy
released is incredible, and the luminosity may peak at at
least 400,000 million times that of the Sun, greater than
the combined luminosity of all the stars in an average
galaxy. For some time afterwards wisps of material may
be left, and can be detected because they send out radio
radiation, but that is all. It is believed that each of these
events has the same luminosity and so they may be used as
cosmological standard candles.
A Type II supernova is very different, and is the result
of the sudden collapse of a very massive supergiant – at
least eight times as massive as the Sun – which has used
up its nuclear fuel, and has produced a nickel-iron core
which will not ‘burn’. The structure of the star has been
compared with that of an onion. Outside the iron-rich coreis a zone of silicon and sulphur; next comes a layer of
neon and magnesium; then a layer of carbon, neon and
oxygen; then a layer of helium, and finally an outer-region
of hydrogen. When all energy production stops, the outer
layers crash down on to the core, which collapses; the
protons and electrons are forced together to make up
neutrons, and a flood of neutrinos is released, travelling
right through the star and escaping into space. The tem-
perature is now 100,000 million degrees C, and there is
a rebound so violent that most of the star’s materialATLAS OF THE UNIVERSE
The Crab Nebula,
the remnants of the 1054
supernova. The Nebula
itself was discovered by
John Bevis in 1731, and
independently by Messier
in 1758. It is 6000 light-years
away and radiates at almost
all wavelengths, from the
long radio waves down
to the ultra-short X-rays
and gamma-rays. Its
‘power-house’, the pulsar
or neutron star in the centre,
was detected optically as
a very faint, flashing object
in 1969 by observers in
Arizona, at the Steward
Observatory. It flashes
30 times per second, and
is the quickest-spinning
of the ‘normal’ pulsars. This
image, showing the central
region of the Nebula, was
obtained by the Hubble
Space Telescope.E152-191 UNIVERSE UK 2003mb 7/4/03 5:49 pm Page 182