Discuss the process of nuclear fusion and how energy is generated in stars.

Explanation

Nuclear Fusion :
When two or more light nuclei `(A lt 20)` combine to form a heavier nucleus, then it is called nuclear fusion.
In the nuclear fusion, the mass of the resultant nucleus is less than the sum of the masses or original light nuclei..The mass difference appears as energy .The nuclear fusion never occurs at room temperature unlike nuclear fission. It is because when two ligt nuclei come closer to combine, it is strongly repelled by the coulomb repulsive force.
To overcome this repulsion, the two light nuclei must have enough kinetic energy to move closer to each other such that the nuclear force becomes effective.This can be achieved if the temperature is very much greater than the vaue `10^(7)` K.When the surrounding temperature reaches around `10^(7)`K, lighter nuclei start fusing to form heavier nuclei and this resulting reaction is called thermonuclear fusion reaction.
Energy generation in stars:
The natural place where nuclear fusion occurs in the core of the stars, since its temperature is of the order of `10^(7)K`. In fact , the energy generation in every star is only through thermonuclear fusion.Most of the stars including our Sun fuse hydrogen into helium and some stars even fuse helium into heavier elements.
The early stage of a stars is in the form of cloud and dust.Due to their own gravitational pull, thses clouds fall inward.As a result, its gravitational potential energy is converted to kinetic energy and finally into heat.
When the temperature is high enough to initiate the thermonuclear fusion, they start to release enormous energy which tends to stabilize the starand prevents it from further collapse.
The sun.s interior temperature is around `1.5 xx 10^(7)K` .The sun is converting `6 xx 10^(11)Kg` hydrogen into helium every second and it has enough hydrogen such that these fusion lasts for another 5 billion years.
When the hydrogen is burnt out, the sun will enter inot new phases called red giant where helium will fuse to become carbon. During this stage, sun will expand greatly in size and all its planets will be engulfed in it.
According to Hans Bethe, the sun is powered by proton- proton cycle of fusion reaction.This cycle consists of three steps and the first two steps are as follows:
`""_(1)^(1)"H" + ""_(1)^(1)"H" to ""_(1)^(2)"H" + e^(+) + v` ....(1)
` ""_(1)^(1)"H" + ""_(1)^(2)"H" to ""_(2)^(3)"H" + gamma` ....(2)
A number of reactions are possible in the third step. But the dominant one is
`""_(2)^(3)"H" + ""_(2)^(3)"H" to ""_(2)^(4)"H" + ""_(1)^(1)"H" + ""_(1)^(1)"H"`
The overall energy production in the above reactions is about 27 MeV. The radiation energy we received from the sun is due to these fusion reactions.