Up to this point, the inward force of gravity trying to make the star collapse has been balanced by an equal outward force caused by the intense heat of a nuclear activity in its core. Although fusion ceases ion the star's core, it continues in a shell outside it. More and more helium is dumped into the core until it cannot support its own weight and it collapses. The collapse itself generates heat which makes the outer layers expand. The surface expands so much that it cools to a dull red colour, giving the star its name - a red giant. When this happens to the sun, it will grow so large that it will swallow up Mecury, Venus and the Earth. Anything living on Earth at that time will be vaporised.
The outer layers of a dying star eventually expand out into space, leaving behind a small, cool star called the white dwarf. This cools down and finally disappears from view.
The demise of the heavyweights
Astronomers believe that quite a different fate awaits stars that are more than 12 times the mass of the sun. The core of a 'heavyweight' collapses suddenly. It can go from the size of the Earth to between 10 and 100 kilometres across in less than a second! The shock of this sudden collapse is so violent that it blows the outer layers of the star away in a giant explosion called a supernova.
The brightest supernova is clearly visible to the naked eye. Astronomers have estimated that they must be 10 billion times brighter than the sun. They do not stay this bright for very long. As the shell of material is blown away into space, it expands, cools, loses its brightness and fades away after a few months. The collapsed core is incredibly dense, so dense that elements and chemical compunds as we know them cannot exist. The dying star is composed almost entirely of particles called neutrons. It is therefore called a neutron star. Because so much matter is compressed into such a small volume, the force of gravity at the star's surface is enormous, perhaps 100,000 million times the force of gravity on Earth.
The collapse that forms a neutron star is so sudden that the neutron star spins very quickly - as fast as 1,000 times per second! Particles escaping from the star's surface are caught up in the intense magnetic field that surrounds it. They give out radio waves which are squeezed by the field into two beams. As the star spins radio beams sweep around the sky like beams of light from a lighthouse.
Astronomers can detect them on Earth. The rate at which the star blinks on and off at radio wavelengths shows how quickly it is spinning.
The particles shot into space by a supernova spread throughout
the galay. Many of them are swept up by other stars, sucked in by the star's
force of gravity. Others are trpped by clouds of matter and, after millions
of years, form new stars. We have reason to be grateful that some stars
explode in this way, because we, and everything around us in the solar
system, are composed of atoms made in stars that have blown themselves
to bits. We are literally made from dust of stars.
|Early astronomers in China saw supernovae in 1006, 10554, 1572 and
1604. They named them 'guest stars' beacause they arrived, stayed a while
and left again. However, supernovae are not extremely rare events. Many
have gone unnoticed or been hidden by clouds of gas and dust. Mordern astronomers
have scanned the sky for supernovae reamins. They estimated from their
studies that a supernova occurs in our galaxy more than once every 100