According to the rule of nature that nothing is permanent. So, stars after completing their life period become dead ultimately.
Stellar evolution basically describes the life cycle of a star. According to stellar evolution, stars born and exist in the universe for certain years and at last they died.
Birth of a star: Lot of empty space is present between the stars. This space contains some gases and dust particles. From the experiments it is checked that the hydrogen gas is present in large amount. Approximately 75% of all gases is hydrogen. The remaining part is covered with helium and some other heavier elements.
Now you will be thinking that how these dust particles and gases will help in the birth of a star. Due to the influence of the gravitational force of attraction between the stars these dust particles and gases came nearer to each other. When they came nearer to each other a cloud is formed. The cloud becomes denser and denser as the molecules came closer. The gravitational forces of attraction also increases as the particles and gas molecules came near to each other.
Due to this process the cloud formed is transformed into a heavy mass which shrinks very rapidly to become as hard as possible.
This transformation will happen only if the cloud formed has a mass of nearly one thousand solar masses. If the shrinking will be continuous then both the temperature and pressure present inside the cloud will goes on increasing. At a certain point the cloud will split into smaller fragments similar to that of stellar size.
After splitting the shrinking of each fragment continuous to rise. When the temperature of the fragments will reach to millions of degrees then the nuclear fusion reaction takes place. At that time the emission of light from the fragments will start. All these individual fragments will become stars. Hence, a cluster of stars is formed.
Death of a star: Let’s start the discussion from the shining of star. Then we will move towards its death. The reason behind the shining of stars is the nuclear fusion reactions taking place. In a fusion reaction large amount of hydrogen is converted into helium and a large amount of energy is produced during this process. Stars shine by using this enormous amount of energy produced.
During these reactions when the amount of hydrogen decreases then the shrinking of the core starts. On the other hand the outer portion of star begins to expand. Due to this process the temperature of the surface goes on decreasing. This fall in temperature results in the formation of a ‘supergiant’ or a ‘giant’ star. This ‘supergiant’ star lives for millions of years. When the stars reach at the stage of the ‘supergiant’ star then energy formation is at a rapid rate. Due to this excess energy the explosion in the star takes place. After explosion it forms a ‘supernova’ or ‘nova’. This process results in loss of most of the mass of the star. This mass is scattered in space. This is the end of stars life or we can say that the death of the star. After explosion the ending of the remaining parts can happen in three ways:
1. If the mass of star is 2 to 5 times the mass of sun, then due to the explosion the compression of the core will starts. As a result high pressure will be created in the core. The electrons present into the star will forcibly enter into the nucleus. After combining with the protons they will form neutrons. This will results in the formation of a neutron star.
2. If the value of the mass is two times less than that of sun, then the compression taking place in the core of the star due to the gravitational pull will result in the formation of the electron gas by the combination of electrons and protons. The electron gas produced inside the core will provide an equal and opposite thrust to cancel the effect of the gravitational force. Hence it results in the equilibrium condition. This equilibrium stage of star is known as white dwarf. It has luminous power of its own. It becomes cool after sometime. Then it changes to a black dwarf.
3. If the solar mass is 5 times less than the mass of star then the recoiling will be forceful. Due to the effect of recoiling the core transforms into an object having very high gravity as well as density. So, when outside particles like photon will come inside this object then there will be no chance for the particle to escape out. The name assigned to this object is Black Hole.
A black hole is formed if the star contracts up to a particular radius. RBH = 2GM / C2
In the above equation M is the mass of the star.