At the time of supernova,the light of the star becomes much more than all other stars of the galaxy.Great shells of gases fly off the star.Only the tiny core of the star remains left.This core contains only neutrons so it is called a neutron star .It is extremely dense.Some times after the supernova explosion the massive star becomes a black hole.
A star begins its life as a cloud of dust and gas (mainly hydrogen) known as a nebula. A protostar is formed when gravity causes the dust and gas of a nebula to clump together in a process called accretion. As gravity continues to pull ever more matter inward towards the core, its temperature, pressure and density increases. If a critical temperature in the core of a protostar is reached, then nuclear fusion begins and a star is born. If the critical temperature is not reached, however, it ends up as a brown dwarf, or dead star, and never attains star status.
A typical star like our own Sun (technically a yellow dwarf star), then, is fuelled by nuclear fusion, the conversion of hydrogen (the simplest atom, with a nucleus consisting of just one proton) into helium (the second simplest, with two protons and two neutrons in its nucleus). The nucleus of a helium atom actually weighs only 99.3% as much as the two protons and two neutrons that go to make it up, the remaining 0.7% being released as heat and light energy. This 0.7% coefficient, which is essentially due to the extent to which the strong nuclear force is able to overcome the electrical repulsion in the atoms, turns out to be a critical one in determining the life-cycle of stars and the development of the variety of atoms we see in the universe around us.
The Sun's own gravity traps and squeezes this ultra-hot gas into a confined space, thus generating enough heat for the fusion reaction to take place. The process remains in equilibrium as long as it retains enough fuel to create this heat- and light-producing outward energy which counteracts the inward pressure of its gravity (known as hydrostatic equilibrium). This is the period known as the main sequence of the star.
Already about 4.5 - 5 billion years old, when the Sun's hydrogen fuel starts to run out (in an estimated further 5 billion years or so), its main sequence comes to an end, and it starts to cool down and collapse under its own gravity. However, energy from the collapse then heats up the core even more, until it is hot enough to start burning helium and, under the extra heat of the helium burning, its outer layers expand briefly (for a "mere" 100 million years) into a massive red giant star.
Eventually, the outer layers blow off completely and the core settles down into a white dwarf star, a small cinder about the size of the Earth composed mainly of carbon and oxygen. Over a very long stretch of time, white dwarfs will eventually fade into black dwarfs, and this is the ultimate fate of about 97% of stars in our galaxy. The matter which makes up white and black dwarfs is largely composed of, and supported by, electron-degenerate matter, in which the atoms making up the star are prevented from further collapse by the effective pressure of their electrons, due to the Pauli Exclusion Principle (which states that no two electrons can occupy identical states, even under the pressure of a collapsing star of several solar masses).
However, a star significantly larger than our Sun is hotter and burns up its fuel more quickly and generally has a shorter but more dramatic life. A star of ten solar masses, for example, would burn fuel at about a thousand times the rate of the Sun, and would exhaust its hydrogen fuel in less than 100 million years (compared to the Sun's 10 billion year lifetime). A star 20 times the mass of our Sun would burn its fuel 36,000 times faster than the Sun, and might live only a few million years in total.
Larger stars are much hotter and the higher temperatures within such a star are sufficient to fuse even helium. The helium then becomes the star's raw fuel, and it goes on to release ever higher levels of energy as the helium is fused into carbon and oxygen, while the outer layer of hydrogen actually cools and expands significantly in the star's red giant phase.
Even larger stars continue in further rounds of nuclear fusion, each of successively increased violence and shorter duration, as carbon fuses into neon, neon into magnesium and oxygen, then to silicon and finally iron. So, although a star the size of our own Sun does not progress very far along this path, a larger star continues through a chain of transmutations to progressively heavier nuclei. Eventually, a star of sufficient initial mass becomes a red supergiant, which has a core layered like an onion, with a broad shell of hydrogen on the outside, surrounding a shell of helium, and then successively denser shells of carbon, then neon, then oxygen, then silicon, and finally a core of white-hot iron.