Short explanation: A supernova is the
violent explosion of a star ending its life (click on image to witness
one, and turn up your volume).
Stars shine by burning hydrogen to helium. The energy thus produced keeps the
star from collapsing under its own gravity. When there is no more fuel left,
after a relatively short period of burning heavier elements, the star
collapses.
While the core of a light star, i.e. weighing less than approximately 8
Solar masses, contracts to a
white dwarf, for a more massive star nothing can
prevent the catastrophe: The extreme pressure forces the electrons to be
squeezed into the protons, making the inner core collapse even further to a
neutron star. While this star has a radius of but
10 km, it contains as much mass, or even more, as our Sun.
The remaining mass of the original star rebounces on the compact surface of
the neutron star and is ejected in a tremendous explosion, causing it to
brighten up enormously and then gradually fade. At peak light output,
supernova explosions can outshine a galaxy.
The outer layers of the exploding star are blasted out in a radioactive cloud.
This expanding cloud, visible long after the initial explosion fades from
view, forms a "supernova remnant".
If the neutron core is more massive than 2 – 3 Solar masses, not even the
pressure of the neutrons can hold it up, and it collapses to a
black hole.
Historically, supernovae were referred to as "novae" or "new stars", while the
Chinese referred to them as "guest stars". Important supernova in the
past were:
in 134 B.C., observed by Hipparchus, which led him to make a catalog of
stellar positions;
in 1054 A.D. in the constellation of Taurus, leaving behind the supernova
remnant now known as the
Crab Nebula.
It was observed for several days in the daytime by the Chinese and
Pueblo Indians in New Mexico, though not a single European recording exists;
Clicking
on the image shows an animation of this supernova
(3.8 Mb, credit: ESA/Hubble, M. Kornmesser & L. L. Christensen).
in 1572, observed by
Tycho Brahe who found the object had to be at a distance comparable to the
stars, thus demonstrating that the starry sky was not unchangable.
