The tiny specks of light twinkling at us in the night sky are definitely more than they seem to be. They are mostly as big and sometimes even bigger than the star that sustains life — the Sun. These ginormous spheres of gases have a life of their own — they are born, live and then they die.
A nebula is a cloud of gases (like hydrogen and helium) and dust. This stellar nursery is where stars are born. Some nebulae that exist are ‘glowing’ while others are ‘dark’. The stars that are born in these clouds are dense balls of gases which, because of nuclear reaction, start emitting light.
The star grows up and leaves its nursery. Some can be as big as the sun (which itself is probably middle aged in star years). The sun is an average sized star. Others are massive, 1.5-3 times the size of the sun. These giant stars have different stages in their life from those of average ones. They happily twinkle away for us from great distances while from up close they are burning from inside out, slowly but steadily exhausting all the gases in them. The stars burn like this for about nine billion years till the hydrogen in the core of the star is slowly and completely replaced by helium. The hydrogen is then fused in a shell around the core. This process makes the ‘atmosphere’ of the star bigger and bigger till it converts into a red giant.
The red giant keeps growing because the gravitational pull stops being as effective on the outer layers. It expands until its core collapses because internal pressure of the core is insufficient to balance gravity. Once the pressure of gravity ends, the remnants of the stars are scattered into a planetary nebula. This nebula is again a cloud of dust and gas and in the centre of this smattering of the dead star is the white dwarf.
The white dwarf is the corpse of the star. A white dwarf is very hot when it first forms — more than 100,000 K at the surface and even hotter in its interior. The white dwarf stage lasts for a billion years during which the star is cooling down. Once it does, it turns into a dark cool black dwarf. Since our universe is not that old, we don’t have any black dwarfs yet.
Our universe is estimated to be 13.77 billion years old. The oldest star in the universe is HE 1523-0901 which is 13.2 billion years old and is still in the red giant phase. Our Sun is about 4.5 billion years old right now.
A massive star usually explodes into a supernova instead of a planetary nebula. The supernova is more scattered and haphazard than the planetary nebula. It has a much bigger gravitational pull which ultimately causes the stellar material to condense into a black hole from which even light cannot escape because of the gravity. Contrary to popular belief, black holes are not like huge vacuum cleaners that suck in everything that comes in their way. They are like a giant room with black walls that absorbs all light that goes in it and doesn’t let it escape.
Songs and movies often talk about ‘millions of stars in the sky’ even though there are innumerable stars, only 2,000 or so are visible at once anywhere on earth.
Our eyes are actually seeing 19 quadrillion miles away so we can see one of the closest stars in the night sky, Deneb, which is visible from anywhere in the northern hemisphere. Even Eta Carina, which is 44 quadrillion miles away is faintly visible.
These are the stages in the life of a star and some random facts about them. These beautiful phenomena have been intriguing mankind since their inception. We use stars to tell direction; astrology studies the effect of stars on our lives and even predicts the future.