There are fusion reactions in the interior of a star converting more and more hydrogen into helium. After some time only helium is left behind at the core of the star and fusion reactions would stop.
Consequently, the pressure at the core of the star decreases and the star under its own force of gravity begins to contract. But in the outer shell, hydrogen continues to fuse and liberate energy. The shell therefore, expands, increases its surface area and reduces the intensity of radiated energy.
Now the star appears red and this phase is called Red Giant Phase. The fate of the star depends upon its initial mass. If its mass is similar to that of the sun it would lose its expanding shell. The core which is left behind will gradually condense into an extremely dense ball of matter. Now the temperature of the interior increases and it is not as it was as a result of which the helium core starts converting into higher elements like carbon. The core glows as a white dwarf star, as long as the helium lasts and ultimately fades into a dense lump. The matter is highly compressed inside the white dwarf.
If the mass of the star is greater than that of the sun, then in red giant phase the core of helium goes on contracting and the temperature goes on increasing. At the time of contraction due to energy, the outer shell explodes with a brilliant flash. Such an exploding star is known as supernova.
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After supernova explosion, core part further contracts and finally forms the neutron star and is finally lost in the universe.
In some much heavier stars, the ‘Neutron star’ finally becomes the ‘Black hole’ after more and more contraction and is lost as highly dense mass in the sky.