This sudden collapse of a massive star's core core into a volume over a million times smaller than its original volume is really bad news for the star. The outer layers of the star come raining down onto the core. Somehow this collapse changes into an explosion: a type II supernova. The process by which this happens is still being investigated, but evidently the core collapses to something below its equilibrium radius and then rebounds slightly. That bounce transfers an enormous amount of energy to the layers falling down from above. (To see this, try balancing a small ball, like a handball or tennis ball, on top of a larger ball, e.g. basketball, and carefully drop it straight down onto the ground. Just watch that smaller ball take off after they hit the ground!) A strong wave of energy--a shock wave--travels out through the envelope and heats the star so much that the outer layers are blown away. Another important effect is the huge numbers of neutrinos that are produced when the neutron star is formed. Ordinarily, neutrinos don't interact much with matter, but these neutrinos are so numerous and energetic that they help push the outer layers of the star away.
The total amount of energy released in a Type II supernova is
about 
ergs. About 99% of that energy is emitted as neutrinos, whereas
only 1% is
converted into the kinetic and heat energy of the ejecta (i.e.,
outer gas layers). Yet enough light
is emitted by a supernova to make it as bright as a billion Suns.
The most
famous historical Type II SN became visible on July 4, 1054 and
was noted
by astronomers in Imperial China. It was easily visible in broad
daylight
for weeks and did not disappear from nighttime skies until 2
years later.
At the position where the supernova was observed, we now see a
glowing cloud
of gas called the Crab nebula which is expanding at
thousands of km/s.
Near the center of the Crab is a strong source of radio waves and
X-rays
called the Crab pulsar. We'll discuss pulsars in a minute.
In the process of a supernova explosion, the temperatures are briefly so high (billions of K) that elements heavier than iron can be produced. Remember, we said we can't get energy by fusing an iron nucleus with another nucleus, but who's to say we can't provide enough energy to make this happen? That energy needed to make elements beyond iron is readily available during the supernova explosion. Unfortunately, many of these elements are hard to detect with spectroscopy so observational proof is still lacking. We do know, however, that many or most of the elements beyond iron had to have been created very rapidly, so supernovae are still the best bet.
But the significance of supernovae goes far beyond the production of rare elements. Even light nuclei like carbon and oxygen, which can be produced by low mass stars, would be locked up inside white dwarfs if it wasn't for supernovae. Supernovae enrich the gas between the stars with all kinds of chemical elements that are necessary for the production of planets and life. The pressure of a supernova blast may trigger the formation of stars and planets in an interstellar cloud of gas and dust. Supernovae are also thought to be the major source of high energy cosmic rays, which can affect the evolution of life by causing mutations.
The most important supernova that has happened in modern astronomical history is known as SN 1987A, and became visible, as the name suggests, on February 24, 1987. The explosion occurred in a nearby satellite galaxy of the Milky Way called the Large Magellanic Cloud, so named because it was not known to Europeans until Magellan voyaged south of the Equator. Because the LMC, as it's called, is over 100,000 light years away, the explosion actually occurred over 100,000 years ago. (Remember that the further out we look in space, the further back we are looking in time).
By studying the spectrum and the apparent brightness of SN 1987A,
astronomers confirmed many of the ideas for how Type II
supernovae
occur. They even had pictures of the star before it exploded.
It was
a blue supergiant star with a mass of around 20 
and a
luminosity of around 
. They found evidence of radioactive
Co in
the SN's spectrum. (This isotope of cobalt is radioactive with a
short half-life, indicating that it was freshly synthesized in
the star.) Experiments on
Earth, which look for neutrinos from the Sun, witnessed
a sudden burst of neutrinos just before the SN became visible,
supporting another theoretical prediction.
To summarize, we learned from SN 1987A that:
Start:Stars
