What
is a Supernova?
Supernovae
are vast explosions in which a
whole star is blown up. They are
mostly seen in distant galaxies
as `new' stars appearing close
to the galaxy of which they are
members. They are extremely bright,
rivalling, for a few days, the
combined light output of all the
rest of the stars in the galaxy.
As most observed supernovae occur
in very distant galaxies they
are too faint even for the largest
telescopes to be able to study
them in great detail. Occasionally
they occur in nearby galaxies
and then a detailed study in many
different wavebands is possible.
The last supernova to be seen
in our galaxy, the Milky Way system,
was seen in 1604 by the famous
astronomer Kepler. The brightest
since then was supernova 1987A
in the Large Magellanic Cloud,
a small satellite galaxy to the
Milky Way. The brightest supernova
in the northern sky for 20 years
is supernova 1993J in the galaxy
M81 which was first seen on March
26 1993.
Supernovae fall into two different
types whose evolutionary history
is different.
Type
I supernovae result
from mass transfer inside a binary
system consisting of a white dwarf
star and an evolving giant star.
Type II supernovae are, in general,
single massive stars which come
to the end of their lives in a
very spectacular fashion.
We first discuss Type II supernovae
and then, briefly those of Type
I.
Type
II Supernovae
The structure of all stars is
determined by the battle between
gravity and radiation pressure
arising from internal energy generation.
In the early stages of a star's
evolution the energy generation
in its centre comes from the conversion
of hydrogen into helium. For stars
with masses of about 10 times
that of the Sun this continues
for about ten million years.
After this time all the hydrogen
in the centre of such a star is
exhausted and hydrogen `burning'
can only continue in a shell around
the helium core. The core contracts
under gravity until its temperature
is high enough for helium `burning',
into carbon and oxygen, to occur.
The helium `burning' phase also
lasts about a million years but
eventually the helium at the star's
centre is exhausted and it continues,
like the hydrogen `burning', in
a shell. The core again contracts
until it is hot enough for the
conversion of carbon into neon,
sodium and magnesium. This lasts
for about 10 thousand years.
This pattern of core exhaustion,
contraction and shell `burning'
is repeated as neon is converted
into oxygen and magnesium (lasting
about 12 years), oxygen goes to
silicon and sulphur (about 4 years)
and finally silicon goes to iron,
taking about a week.
No further energy can be obtained
by fusion once the core has reached
iron and so there is now no radiation
pressure to balance the force
of gravity. The crunch comes when
the mass of iron reaches 1.4 solar
masses. Gravitational compression
heats the core to a point where
it endothermically decays into
neutrons. The core collapses from
half the Earth's diameter to about
100 kilometres in a few tenths
of a second and in about one second
becomes a 10 kilometre diameter
neutron star. This releases an
enormous amount of potential energy
primarily in the form of neutrinos
which carry 99% of the energy.
A shock wave is produced which
passes, in 2 hours, through the
outer layers of the star causing
fusion reactions to occur. These
form the heavy elements. In particular
the silicon and sulphur, formed
shortly before the collapse, combine
to give radioactive nickel and
cobalt which are responsible for
the shape of the light curve after
the first two weeks.
When the shock reaches the star's
surface the temperature reaches
200 thousand degrees and the star
explodes at about 15000 kilometres/sec.
This rapidly expanding envelope
is seen as the initial rapid rise
in brightness. It is rather like
a huge fireball which rapidly
expands and thins allowing radiation
from deeper in towards the centre
of the original star to be seen.
Subsequently most of the light
comes from energy released by
the radioactive decay of cobalt
and nickel produced in the explosion.
Type I Supernovae
Type I supernovae are even brighter
objects than those of type II.
Although the explosion mechanism
is somewhat similar the cause
is rather different.
The origin of a Type I supernova
is an old, evolved binary system
in which at least one component
is a white dwarf star. White dwarf
stars are very small compact stars
which have collapsed to a size
about one tenth that of the Sun.
They represent the final evolutionary
stage of all low-mass stars. The
electrons in a white dwarf are
subject to quantum mechanical
constraints (the matter is called
degenerate) and this state can
only be maintained for star masses
less than about 1.4 times that
of the Sun.
The pair of stars loses angular
momentum until they are so close
together that the matter in the
companion star is transferred
into a thick disc around the white
dwarf and is gradually accreted
by the white dwarf. The mass transferred
from the giant star increases
the mass of the white dwarf to
a value significantly higher than
the critical value whereupon the
whole star collapses and the nuclear
burning of the carbon and oxygen
to nickel yields sufficient energy
to blow the star to bits. The
subsequent energy released is,
as in the Type II case from the
radioactive decay of the nickel
through cobalt to iron.
After the Explosion
The evolution of the supernova
after the explosion is one in
which the ejected material continues
to expand in a shell around the
progenitor site while, in Type
II supernovae, the central neutron
star remains. The ejecta continue
to expand for thousands of years
until they impinge on gas and
dust clouds in the surrounding
interstellar space. There the
ejected gas will mix with the
interstellar material and eventually
may be incorporated into a new
generation of stars.
The
Origin of the Elements
Theories of the Big Bang have
successfully predicted the abundances
of the light the elements. The
first stars were composed of hydrogen,
helium and a very small amount
of lithium and beryllium and almost
nothing else. Some of these stars
became supernovae and distributed
the `heavy' elements, made in
their interiors, into the interstellar
environment. Subsequent generations
of stars have further increased
the proportion of `heavy' elements
such as carbon, oxygen, phosphorus
and iron.
It is a sobering thought that
all the heavy elements that we
encounter were formed in this
way in the centres of stars and
that without such violent explosions
we would not exist.
