Every time when you look up in the night sky you definitely see lots of stars. But question here is "what is a star?" In a scientific sense, a star is a massive ball of hydrogen, helium and enough mass so that it can originate nuclear fusion in its core for a long time. The star we can clearly see in the sky is much larger than all the others or we can say that it looks much larger than others to us and that star is for sure is our Sun. As our Sun is a star so why not other stars can be seen like Sun, the reason is they are too far from us to be seen clearly. Stars come in different sizes and colors.
So let’s start learning what is a star? We know that 75% of the all the matter in the Universe is hydrogen and 23% is helium reason behind this fact is that this amount of hydrogen and helium is left over from the Big Bang. These elements mostly exist in universe in the form of large stable clouds of cold molecular gas. At some point a gravitational disturbance caused by some explosion, like a supernova explosion or a galaxy collision will cause a cloud of gas to collapse from this point of collapse the process of star formation begins.
During the main-sequence phase of star formation, a star fuses all the hydrogen present in its surroundings in its core to helium. When some of the hydrogen fuel is used by the fusion reaction on star, the core of the star starts to contract and more heat is produced. The shell of the star, where some hydrogen is still somehow burning starts forming helium, and star starts to expand to dissipate the heat and the star becomes cooler and redder. This stage of a star is called the red giant phase. Red Giant stars are almost between ten and a few thousand times more luminous than our sun. Super giants on the other hand are even brighter than Red Giants, Red Giants luminosity is more than 10,000 times the Sun's (sometimes even going into the million times). Red super giants have a radius of about 200 to 800 times that of the radius of Sun with a mass of about ten times the Sun’s mass.
Sometimes very massive stars produce enough heat to transforms to blue super giants. Blue Super Giants can be from 10 to 100 times the mass of our Sun. The least massive stars in star family are red dwarf stars. Astronomers have calculated that there are some red dwarf stars that could live 10 trillion years of life. They put out a fraction of the energy released by the Sun. The largest super giant stars, on the other hand, have very short lives as compared to Red Giants. It is very interesting fact that a star like Eta Carinae, with 150 times the mass of our Sun is emitting more than 1 million times as much energy as our Sun is now producing. It has probably only lasted a few million years and will soon detonates itself as a powerful supernova and destroys itself completely leaving gas and dust behind.
Most of the known stars are in the main sequence phase of their lives, where they are burning hydrogen gas in fusion reaction and producing a huge amount of energy. Once this hydrogen runs out in a star, and only helium is left in the core of star, than the stars have to burn something else. The largest stars can continue fusing heavier and heavier elements after burning hydrogen gas until they can’t sustain fusion any more in their core. On the other hand the smallest stars eject their outer layers after burning out all of their hydrogen gas and become white dwarfs, while the more massive stars have much more violent ends and they become neutron stars and even black holes.