Stars are one of the interesting objects of the sky. Stars are big balls of gas in which nuclear reactions are going on producing tremendous amount of energy and light. The life of all the stars starts in the same way from the nebula but death of stars occurs in different ways, some undergo peaceful death while others undergo violent explosions. The stars are the source of different type of chemical elements from which we, objects observed on the Earth and planets are formed. Thus the stellar evolution which involves stages of life of star is an important process.

Nebula

The space is not empty in fact it is filled with the gas and dust called interstellar medium. The gas is mainly consists of hydrogen gas whose molecules are about a centimetre apart and the dust is mainly microscopic grains and atoms like carbon and silicon. The interstellar medium between two stars has negligible mass as compared to the star. At some places the density of gas and dust is more than average. These clouds of gas and dust are known as nebula.

Protostar

The cloud of gas in the nebula experiences inward force due to its own gravity. Due to gravity the cloud of gas starts contracting resulting in increasing pressure and density. The temperature of gas increases due to high pressure. The gas becomes hotter and hotter with the collapse and ultimately it become so hot that it starts glowing. This glowing cloud of gas is called protostar. The protostar can be observed if the gaseous cloud surrounding it is thin.

Main sequence star

The protostar keeps on collapsing until it reaches at its highest temperature. If the mass of protostar is less than 8 % of the solar mass than this temperature is not high enough to start the thermonuclear reaction of hydrogen. This type of protostar does not convert into star and are called brown star. The brown stars are very difficult to detect.

If the mass of protostar is more than the 8 % of solar mass than the temperature become high enough to start the thermonuclear reaction of hydrogen. In this reaction the hydrogen starts converting into the helium and liberate huge amount of energy. This energy applies an outward pressure against the gravity and an equilibrium situation is attained. Now the collapse stops and the nuclear reaction produces tremendous amount of energy which makes the star glow brightly. This is the starting of main sequence star. The star spends 90% of its time as main sequence star.

Lifetime of star

The lifetime of star is decided by the amount of matter present in the initial nebula. The smaller star burns its hydrogen at slow rate and heavy star burn its hydrogen at faster rate. Thus the smaller star gets larger lifetime as compared to the heavy star.

Low mass star

During the main sequence , the hydrogen in the core converts into helium and radiates energy but the size of star remains nearly same. Ultimately the hydrogen of core gets exhausted and the nuclear reaction stops which results in no outward gas pressure. Thus the star starts collapsing due to gravity in the absence of gas pressure. As the core collapses the temperature increases, the layer just above the core also gets heat up, this layer still has unburned hydrogen. Once the temperature of this layer become high enough to start the thermonuclear reaction, the hydrogen of this layer ignites. This layer is called the hydrogen burning shell. The rate of burning of hydrogen in this shell is faster than the rate of hydrogen burning in core during main sequence. The energy produced by the hydrogen burning shell applies an outward pressure on the layers above it and the collapse stops. In fact the outer layer start moving outward, the size of star starts increasing and the star becomes red giant star. It takes millions of years for hydrogen of hydrogen burning shell to get finished.

When the hydrogen of this layer gets exhausted the nuclear reaction stops and the layers start collapsing. The collapsing core results in high enough temperature to start helium thermonuclear reaction in which the helium start converting into the carbon. For smaller stars this helium reaction is nearly instant and occurs in few seconds. The energy produced in the helium reaction is not high enough to be seen from outside. But the energy released in the helium reaction pushes the outer layer further out and increases the size of red giant star second time.

In slight heavy stars the helium reaction occurs smoothly. Finally helium fuel also gets exhausted and the star starts collapsing again. The collapsed star reaches to a limit when all the atoms are tightly packed as allowed by the fermion physics. The electron degeneracy pressure applies in the outward direction which balances the gravity and the collapse stops. Now the star is called the white dwarf. The density of white dwarf is very high. The continuous outward shock waves push the outer layers outward and outer layers spread into the interstellar medium resulting in the planetary nebula.

The electron degeneracy pressure can support maximum mass equal to 1.4 times the solar mass which is also called chandrasekhar Limit.

Medium mass star

For medium mass stars the main sequence is same as for low size stars but the burning is faster and they are brighter with lesser lifetime. The medium mass stars also have hydrogen burning shell and the core with nuclear reaction of helium. These stars also grow up as super red giant star. When the helium of the core gets exhausted the collapse starts again and increased temperature in the first layer ignites the helium and it becomes helium burning shell. The layer above helium burning shell becomes hydrogen burning shell. The increased temperature ignites the carbon present in the core and the carbon converts into the oxygen.

Thus the chain of reaction continues in which carbon converts into oxygen, which converts into neon, then magnesium then silicon … and finally iron. The energy liberated in these reactions is lesser then the energy liberated in the preceding reactions. The reaction stops at iron because iron is the most stable element and the conversion of iron into any other element does not liberate energy instead it requires energy. During the last stages of life, the star becomes about 100 times the size of sun like super red giant starBetelgeuse.

As the core of these stars have large fraction of mass of star, the core is heavier than 1.4 times the mass of sun. So the electron degeneracy pressure is unable to stop the collapse of core. The core keeps on collapsing till the electrons revolving around their nucleus get squeezed into the nucleus. When the electrons enter into the nucleus the proton is converted into the neutron and finally the core has only neutrons. The neutron pressure stops the collapse. The gravitation potential energy is get decreased due to collapse, this energy is liberated outward and all the layers get pushed outward with tremendous pressure that that layers just blow away. The energy is so huge that the unburned hydrogen in the upper layer undergo all the nuclear reactions in chain and converts into not only iron but also elements heavier than iron like copper, silver gold, lead and even uranium etc. This tremendous explosion is called type II supernova. The elements synthesized in this supernova get scattered into the interstellar medium and later they will be used by the other stars and planets.

The heavier star has same main sequence star as above and even the nuclear reactions till iron are also same. When the nuclear reaction stops at the iron, the outward pressure stops and the star start collapsing due to gravity. When the mass of core is more than the 3 times of the solar mass, the neutron pressure cannot stop the collapse. No force is known which can stop the collapse and the star keeps on collapsing. The core and layer above it collapses to point called singularity. No matter can escape this collapse not even light. This is called the black hole.

Conclusion

The life of star starts form the nebula and depending on the amount of matter present in the nebula, the lifetime and the fate of star is decided. The elements synthesized during the main sequenceand the supernova explosion form the raw material for the formation of planetary systems. Thus we can say that all the things that we see around us are formed in the stellar evolution.

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