Click the above image for a larger and more detailed view.
A supernova is a massive star exploding! These stars begin their main sequence lives with more than about 10
times the Sun's mass. These stars go through all the stages of nuclear burning in their cores and become
supergiants with iron cores surrounded by shells or burning fronts which fuse the lighter elements in each
overlying shell into successively heavier elements which sink into the layer beneath the front. Once the central
core is made of iron, it can no longer generate heat energy, since iron can not be burned by nuclear fusion to
release energy, since it is the most stable atomic nucleus. This means that gravity is unopposed by thermal
(heat) energy and the core contracts until it becomes supported by an electron degenerate gas, similar to that
which supports white dwarf stars. A degenerate electron gas is extremely dense and is a plasma in which the
electrons, stripped from the atomic nuclei, are squeezed by the immense pressure into the lowest energy states
that quantum mechanics allows them to be in (in it is very unlike any material encountered in daily life on
Earth!). At this point the pressure simply can not squeeze the core any smaller.
However, as nuclear burning continues around the core, more and more iron is added to the core. Eventually
the iron core exceeds the critical mass known as the Chandrasekhar limit (about 1.3 times the Sun's mass for
iron, see the section on white dwarfs). Now the star is in big trouble!
When the core's mass exceeds the Chandrasekhar limit, the degenerate electron gas cannot resist the
tremendous pressures anymore. Gravity wins again and the core starts to collapse under its own weight, and it
does so very rapidly, in less than one second.
As the core rapidly contracts, the heavy nuclei in the core absorb many of the electrons (a process called
electron capture). Since the electrons were the only thing resisting the immense gravitational field up to this
point, their sudden depletion only causes the core to contract even more rapidly! As the core contracts,
gravitational potential energy is turned into heat and much of this heat is absorbed by the
photodisintegration of iron into helium and neutrons. Photodisintegration, as the name suggests, is the
disintegration of the iron by the intense light (electromagnetic) energy produced from the gravitational energy.
The core simply disintegrates into a plasma of helium and a gas of neutrons. The absorption of heat in this
process also means that this gas has insufficient heat energy to oppose gravity. Now, the core collapses in free
fall - it is imploding more rapidly than ever before!
More heat energy is released as photons (particles of light or electromagnetic energy) and these energetic
photons disintegrate the helium nuclei (imagine light so intense that atomic nuclei are disintegrated by it!)
turning the nuclei into their constituent subatomic particles, protons and neutrons. This process absorbs so
much heat that the ability of the material to resist compression is now reduced to a minimum and the core
collapse faster than ever! What can resist such an immense implosion?
As pressures increase, protons combine with remaining electrons to form neutrons and neutrinos, absorbing
more heat energy and accelerating the rate of core collapse still further! The core is now a particle gas,
consisting mostly of neutrons and collapses extremely fast until it reaches the critical pressure of 10^18 kg per
cubic metre (or 1 000 000 000 000 000 grams per millilitre! Thus a tea spoonful of this neutron gas would
weigh one billion tonnes!). At this point the gas suddenly becomes a degenerate neutron gas, this means
that the neutrons have been squeezed as tightly together as the laws of quantum mechanics will allow and the
core affectively crystallises in an instant. All the in-rushing gas suddenly meets what is in effect a solid surface
that can not be breeched and the outer material suddenly rebounds off the core. A tremendous shock wave
rips through the star, blasting off the bulk of the star, which is in the envelope surrounding the core. The
tremendous numbers of neutrinos produced by the conversion of protons and electrons into neutrons, slam
into the very dense inner envelope, heating it tremendously and blasting off the star's plasma into space at
tremendous speeds of about 10 000 kilometres per second!
Soon the shock wave blasts through the greatly distended outer layers of the supergiant star and the vast
amounts of energy produced in the explosion come pouring out into space. Initially this energy is mostly intense
ultraviolet light, but after a few hours the supernova has expanded and cooled enough to become visible as a
spectacularly bright star. The star suddenly brightens over a billion times its former brightness and outshines
an entire galaxy of one hundred billion stars! These bright stellar explosions shine for months or years, before
the remains of the star are scattered far into space, where they cool and form spectacular stellar remnants that
remain visible as nebulae for thousands of years, until the material blasted far into space cools and dims
What about the core?
The core remains as a hot neutron star, usually sent hurtling through space at tremendous speeds and
spinning rapidly as a pulsar, which will slowly spin down and cool down and fade. If, however, the star was a
very massive star, with at least 60 to 80 times the mass of the Sun, and if sufficient mass remained in the core
such that the core is heavier than about three times the Sun's mass (the exact critical mass is still uncertain)
then the pressure becomes too great for even a degenerate neutron gas to resist gravity and the core will have
contracted further. As the density of a stellar core increases, so does its escape velocity - the speed an object
would need to escape the immense gravitational field and leave the surface of the core. Eventually, as the core
collapses beyond the neutron star stage, its escape velocity inevitably exceeds the speed of light, so that not
even light can escape from the surface and the core becomes a black hole. Quite what happens inside a black
hole is not certain.
Some historic supernovae include the 1054 AD supernova that produced the Crab Nebula with its Crab
Pulsar core remnant. The Crab Nebula is in the Milky Way Galaxy. In 1987, a supernova, designated
SN1987A, was observed in the LMC (Large Magellanic Cloud) Galaxy. The neutrinos produced were detected
on Earth several hours before the star became visible in ordinary light. Detailed observations of the remnant
have supported theoretical computer models that reproduce the salient features of this supernova.
What happens next?
From the ashes a Phoenix may arise! The material blasted into space carries with it all the heavier elements
that the star manufactured within itself, from hydrogen and helium, by nuclear fusion. More heavy elements,
including elements heavier than iron are manufactured very rapidly by the intense temperatures generated in
the supernova explosion as the shock wave rips through the star. Some of these heavy elements are
unstable and radioactive and the radiation released as some of these radioactive elements decay, adds to
the glow of the supernova remnant. The early Universe contained only very light elements, the elements like
the carbon and iron in your bodies and the oxygen that you breath, were manufactured mostly inside stars -
you are made of star dust! The uranium that you find on Earth was probably manufactured in a supernova
explosion. As dying stars add material to the nebulae in space, so these nebulae may again collapse under
their own gravity, once they have become very cool and condense into new stars and planets (and possibly
when the shock wave of a nearby supernova or passing galaxy triggers this condensation). Generations of
stars have successively enriched the material in the Universe, adding all the elements essential for life as we
What is a neutrino?
A neutrino is a small and almost massless subatomic particle, similar in some way to a photon (the particle of
light) but also very different - neutrinos exist in three types that may interchange and neutrinos are extremely
penetrating as they hardly react with matter at all. The Kamiokande neutrino detector on Earth detected only
20 of the ten thousand billion neutrinos that passed through every square metre of the Earth's surface
having travelled 170 000 light years from the SN1987A supernova (which actually occurred 170 000 years
ago, but light and neutrinos took that long to reach the Earth) and most of these neutrino passed through the
earth as if it wasn't there! Billions of neutrinos from SN1987A probably passed through your body if you were
on Earth in 1987!
Click on the thumbnails above to see additional views of the supernova shown in the main image. Zoom in on
the images and notice the shock waves and gas fine structure, such as bellows, columns and filaments
beginning to form.
A typical supernova releases about ten times more energy in a year than the Sun would release if it shone at
its current luminosity for its whole main sequence lifetime of about ten billion years!
If all the Earth's electricity generators and power stations had been producing energy at their current rate
since the Earth formed about 5 billion years ago, then their total energy output would be but a tiny drop of
the total energy emitted by a supernova! Eventually, the supernova remnant will cool, become transparent
and eventually fade from view.