Stars are huge celestial body and represent the most fundamental building block of the galaxy. Most of the part form of hydrogen and helium which produces light and heat in them. First stage of life of a star is birth.
A star is born when clouds of dust and gases condense. And at this stage a star is called nebulae. Over thousands of year, due to gravity, these gases get condescend and causes to nebulae collapsed under its weight and this contracting gases is called Protostar.
Over millions of years when core temperature of protostar rises up to 15 million degree , at that second stage of star life begins and that stage is called main sequence. In this stage hydrogen atom infuse to form helium and generate excessive amount of heat .the excessive generated energy in this stage keeps the star bright and shining. It is the longest stage of Star's life ( nearly 90% of life of star spend in this stage) and lasted long up billions of years.
As the stars proceed in its last stages of life, much of its hydrogen would be converted in helium in core and fusion of hydrogen atom stopped and core begins to collapsed and become much hotter.
Well, fusion of hydrogen atom continues at shells surrounding the core. Hot core pushes the outer layer of the star outwards causing them to cool and transform star into red giant. And this is the beginning of the end of the star. From here a star may come out of different from. It could become a white dwarf
or neutron star or a black hole. It's depend upon the size of the original star.
A smaller star live longer. Smaller the star longer the span of life . A star of size of sun lives around 10 billions of years. And a star 100 times larger than sun live a few million of years. A star, size of sun, once the nuclear fuel is exhasuted, its core form a white dwarf and its outer layer expelled as planetary nebula. Fate of stars depend upon their masses. A star, with mass between 1.4 to 3 times of solar mass ( 1.4 times of solar mass is called Chandrasekhar limit) collapse untill its all electron and proton form neutron and this leads to it become a neutron star. Neutron star is very very dense and a size of teaspoon neutron star carries billions tons.
If the star is of mass of more than 3 solar mass, then it collapse to form black hole. Black holes are the object in the space with huge gravity, in such a extent that even light can not escape from it.
Good knowledge about star's life span....
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