As we know that stars can be born. they live and die too. We already have seen types of stars. Now we will discuss their birth process.
Only energy was there when the universe was born through the big bang. the temperature of the universe was enormously high and it was invisible. eventually, it cooled down and the matter was formed in the form of quarks. Some of these quarks came together to form protons and neutrons and big clouds of the simplest element hydrogen were everywhere. We call them Giant molecular clouds or nebulae.
These Giant molecular clouds or nebulae are bigger than our solar system. sometimes they are of the size of roughly a hundred light-years. they have their own gravity big enough that they start revolving around their own axis, gains angular velocity and form a protoplanetary disc. After some thousand years this disc collapses into itself and forms a Proto-star which has a spherical shape. this is the last stage before the actual star forms.
If the mass of these Proto-stars is at least 13 times the mass of Jupiter then nuclear reactions get kicked because of gravity and it becomes a proper star. If the mass is less than 13 Jupiter masses then gravity applies pressure insufficiently and nuclear reactions do not start and it fails to become a star, therefore, called a failed star or a Brown Dwarf.
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| Credit- NASA |
If the mass is sufficiently high, and now the star has formed. In this case, the hydrogen in the star starts converting into deuteron and then deuteron into helium and so on. These nuclear reactions are highly exothermic. Emitted energy makes the star glow.
The mass of the helium is so higher than the mass of the proton and hens gravity pulls helium into the centre and the external layer of the star starts expanding because of heat. As the layer expands, it cools down and glows in red colour and this stage of the star is called a red giant. And stars are of the main sequence in the HR diagram.
In the same manner, now helium gets converted into carbon, carbon into oxygen and so on. The nucleus of the oxygen is more stable than the smaller nuclei and to proceed next reaction star needs more energy and temperature. These stars don't have this much energy so reactions do not continue. the external layer of the star keeps expanding and busts at some time. this blast is called the Planetary Nebula. Misunderstanding in observation led to this name. After the explosion small white coloured dead star remains at the centre and is known as White Dwarf.
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| Credit- NASA |
The process up to White dwarf takes billions of years. As white dwarfs do not have sufficient temperature and gravity, elements greater than oxygen cannot form over here. If the mass of a star is up to 1.4 solar masses, the star dies as a white dwarf. This limit was given by Indian scientist S. Chandrashekhar and is known as the "Chandrashekhar limit"
A star with a mass more than this limit lives differently, that part we will see next.
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