SUPERNOVAS,NEUTRON STARS, PULSARS, QUASARS, AND BLACK HOLES

The largest stars of all do not die quetly. Their higher gravity increases the temperature of central core to prodigious levels, so carbon is consumed to produce a succession of different element, and finnaly iron atoms, at its core. By this time, the internal reactions are consuming more energy than they produce. The star collapses and then explodes as a Supernova, increasing 
its brightness up to a billion times, and hurling more than half its matter out into space, procucing a nebua that will eventually help to form news stars and planets.

The most recent known supernova was seen on February 23, 1987, in the large Magellanic Cloud, a neightboring galaxy to the Milky Way. Left behind is the dense central core, shrinking from the blast of the explosion and its own internal gravity. So fierce a these compressive forces, that atoms are crushed into neutrons and star once larger than the Sun will collapse into a neutron star only a few mile across, and so dense that 
a spoonful of its matter would weigh hundreds of millions of tons.

Tough these neutron stars were predicted as long ago as the 1930s, they were not located until 1967, when radio astronomers observed a series of rapidly pulsing radio sources, which they called "pulsar." These pules were so regular and so rapid they could only be emmited from a small and fast - spinning object. Since the only bodies in the universe known to be small enough were neutron stars, it was almost certain that these were the misterious pulsars.

Brilliant objects called quasars, from quasi - stellar radio sources, appear like bright stars when photographed through optical telescopes, but also send out intense radio signals.
Consequently, they can be detected over thousands of 
millions of light years, and are now being seen as they were when the universe was young.
                           
Perhaps they are new galaxies in the procces of formation, or an early stage in theformation of the universe itself.

LIFE CYCLE OF A STAR

Stars are born from huge clouds of dust and gas within the galaxies. As a cloud shrinks under the gravitational forces of the particles from which it is made, some areas will become denser than others. These collapse into globules of matter that form the future stars, and as the particles collide with one another at an increasing rate, the resulting friction causes this matter to glow hotter and hotter. Eventually, after perhaps millions of years, the temperature rises to the point where nuclear reactions begin.
                                                             
Most of the atoms of matter in the growing star are hydrogen atoms, and the enormous pressures and temperatures convert these to helium, in a similar reaction to the explosion of a hydrogen bomb. Once this reaction reaches a stable level, the star can go on burning as a thermonuclear furnace for thousands of millions years. How long this stable stage last depends on the size of the star, as larger stars burn more fiercely and more quickly than smaller ones.

The smallest stars never achieve a fierce enough reaction to burn with the brightness of the Sun, and their small size and dull glow earns the name of red dwarfs, which continue burning for millions of millions of years. The life of larger stars is limited when the reserves of unburned hydrogen at the core begin to run out. The fiercest reactions move outward from the center to the outer layersof the star, where hydrogen reserves still exits.

The star grows hotter and larger, to become a red giant. When the Sun reaches this stage, in 5,000 million years, it will expand to a hundred times its present size, and its output of heat and light energy will increase by a thousandfold. Some smaller stars will the fade away as their final reserves of hydrogen burn out.                                                             
                                                                                        
Other are large enough for the outer shell of hydrogen gas to continue heating the central core, which is now made up almost entirely of helium. When the temperature and pressure are high enough, another nuclear reaction breaks down the helium atoms, releasing still more and converting there core into carbon. This reaction then spreads outward in another red giant stage as the helium is used up. More and more of its matter spreads out into space as the central core cools, turning the star into a white dwarf. Finally the star dies down into a black dwarfs of intensely dense ash.