Objective 4: Existence of neutron stars and black holes

First a brief summary of the evolution of a star!

Stars are born when gigantic clouds of dust and other stellar particles, stretching over 1-5 light years, collapsed under the force of gravity. Thus this interstellar dust is gradually, for millions of years, warmed up to about 15 million degrees. Now here a nuclear fusion begins in its core and energy in the form of heat and light travell out from the centre of this new star. As long as there is still material within the star, the nuclear fusion process continues for trillions of years with hydrogen being fused to form helium.

Now for stars with different masses and thus size the end is different:

SUN SIZED STARS (medium/small):

After millions of years as stars as big as the sun has used up its core hydrogen fuel, the star starts to fuse helium and eventually other elements. Now the star will expand into a red giant or super-giant, becoming tens or even hundreds of times larger. Eventually, no more fuel will be available and the star will collapse under gravity to form a white dwarf star only a few thousand kilometres in diameter.

MASSIVE STARS (large):

For large stars, the end is more dramatic.Everytime one type of the nuclear fuels is consumed, the core contracts a little bit, the temperature rises and a new area in the star (where the new fuel is located is ignited). After a series of such steps with the star producing increasingly heavier elements the core is turned into iron (as it is the end product of a nucelar fusion, a stable element). At this point the core will not be able to support itself and will collapses violently with the outer layers of the star being torn apart with a supernova explosion.

The core now keeps on shrinking due to the tremondous amount of gravity increase which is no longer retaliated by the nuclear fusion process collapses to a neutron star which is only a few tens of kilometres wide but with its material so tightly packed that a teaspoon of it would have a mass of about a billion tonnes.

SUPERMASSIVE STARS (Humongous):

Stars having an initial mass of more than 8 times that of the Sun is to fall so far under gravity that they turn into black holes, from which nothing, not even light can escape. This is as the big body produces tremendous amount of gravity pull which had been before opposed by the fusion process, and thus after fusion ceases the star shrinking into a black hole, the densest matter in the universe.