A SUPERNOVA accurs when a massive star has used up its nuclear fuel and the pressure drops in the central core. The matter in the core is crushed to higher and higher densities by gravity, and the temperatures reaches billions of degrees. Under these ext reme conditions it becomes nuclear matter, whose enormous repulsive force violently reverses the collapse. A thermonuclear shock wave is generated through the expanding debris, fusing lighter elements into heavier ones and producing a brilliant cosmic bo nfire. The shell of matter spewed out creates a bubble of multimillion-degree gas, called the supernova remnant. The hot gas will expand and produce X-rays for thousands of years. About every 50 years a star explodes in our galaxy. Cas A is a prime examp le.
The study of supernova remnants is essential to understand the origin of life on the earth. The heavier elements present in the clouds of gas and dust that collapsed to form the sun and other planets, such as carbon, nitrogen, oxygen, iron and so on, cam e from such catastrophic explosions, which spread them throughout the galaxy. Supernovae thus keep renewing the galaxy through star formation out of the spewed-out matter.
Quasars are peculiar, single, star-like objects that radiate as much energy per second as a thousand or more galaxies from a region that has a diameter about one millionth of that of the host galaxy. Supermassive black holes with the mass of millions of stars are thought to lie at the centre of most large galaxies, driving this enormous energy generation. The evidence comes from optical and radio observations which show a sharp rise in the velocities of stars or gas clouds orbiting the centres of galaxi es. High orbital velocities mean that something massive is creating a powerful gravitational field, which accelerates the stars. X-ray observations indicate that a large amount of energy is produced at the centres of many galaxies, presumably by the infa ll of matter into a black hole. In any event, the energy radiated by a supermassive black hole can be stupendous as it gobbles up large clouds of gas. This is apparently what is happening in quasars.
