Supernova Remnants (Clouds ,Nebulae Star Births And Deaths)
The explosion of a supernova
SUPERNOVA EXPLOSIONS are the most spectacular events that occur in our Galaxy. During the first week of its supernova phase, a star can reach an absolute magnitude of —20.5, which is 10 billion times more luminous than the Sun, and as bright as many complete galaxies. Supernova explosions are rare events — only five are known to have taken place in our Galaxy in the past thousand years – but the debris from explosions such as these remain visible to astronomers for hundreds of thousands of years. These supernova remnants (sometimes abbreviated to SNRs) are among the brightest X-ray, gamma ray and radio sources in the sky, and “are therefore a very important constituent of the interstellar medium. SNRs are particularly important in a galactic context since they are a major (and possibly the only) source of cosmic rays and of many of the heavy elements.
Most of what we Know about supernova explosions is derived from the study of galaxies exterior to the Milky Way system .These studies have shown that supernova explosions can be divided into two classes, Type I and Type II, depending on their optical spectrum. Type I supernovae are all very similar to each other and are probably all derived from the same type of star. We do not know just what sort of star this is, but a plausible guess is that it is a comparatively small object, of around 1.4M0 or maybe some sort of binary. Type II supernovae are more of a mixture; they are found only in galaxies containing Population I objects, so are generally believed to have been produced by the collapse of massive stars, larger than perhaps 4M0. The details of what happens during supernova explosions are very uncertain, but we know that they occur when the inner part of a star suffers severe gravitational collapse to something like a black hole or a neutron star. The large amounts of energy liberated by this collapse are imparted to the outer layers of the star, which are thereby ejected into space with a velocity of 10-20 000 km s-1. It is the radiation from this hot, fast-moving gas and its subsequent interaction with the nearby interstellar medium that make the supernova remnants visible. The collapsed object at the centre is much more difficult to detect, but in some cases it may manifest itself as a pulsar.