Energy Generation in Stars

Nuclear fusion is the method through which our sun generates energy from atomic nuclei. Nuclear fusion is a method of releasing energy by combining nuclei. The word ‘fusion’ should give you a hint that things are fusing or coming together. Do not mistake nuclear fusion with nuclear fission, which is the breakdown of atomic nuclei into tiny fragments. Nuclear power plants employ nuclear fission right here on Earth. The sun, on the other hand, employs nuclear fusion.

The reaction in which four hydrogen nuclei unite to form one lighter helium nucleus is the most crucial nuclear fusion reaction for humans to comprehend. A single proton exists in a hydrogen nucleus, whereas two protons and two neutrons exist in a helium nucleus. The helium nucleus has less mass than the four hydrogen nuclei when our sun fuses them together. This mass loss is transformed into energy.

This reaction may be summarised using the simple equation presented above. You can see how four hydrogen protons unite to form helium and energy there. Unfortunately, this is an oversimplification. The truth is significantly more complicated, and we’ll go through part of it today. I believe I just overheard a grumble.

What is Nuclear Fusion?

A process in which two very light nuclei (A≤8) combine to form a nucleus with a larger mass number along with the simultaneous release of a large amount of energy is called nuclear fusion.

For example:

• When two nuclei of deuterium ₁H² fuse together, the following products are formed:

₁H² + ₁H² ⇢₂He³ +₀n¹ +Q

• When four hydrogen (1H1)nuclei fuse together, the following products are formed:

4₁H¹ ⇢₂He⁴+ 2 +₁e⁰ + 2v + Q

 Energy Generation in Stars

It has been calculated that the sun radiates energy at the rate of about 10²⁶ J per second. The sun is radiating at this rate for several millions of years. The sources of energy of the sun cannot be the chemical reactions because the energy released in chemical reactions can not last so long. It has also been found that hydrogen and helium constitute about 90% of the mass of the sun and 10% are other elements. Since heavy elements present in the sun are very small in quantity, so the source of energy of the sun cannot be nuclear fission.

Fusion reactions are the source of energy in the sun and the stars, inside which the temperature is of the order of 10⁷ -10 ^8K. The basic energy-producing process in the sun is the fusion of hydrogen nuclei and the same is true for many other stars.

Hans Bethe in 1939 suggested that the source of Stellar energy is thermonuclear reactions. He proposed that the thermonuclear reactions taking place in the sun and other stars follow two different series of processes.

Proton-Proton Cycle

It is a thermonuclear reaction in which the direct collisions of protons result in the formation of heavy nuclei.

In Proton- Proton cycle fusion of four hydrogen nuclei (₁H¹) into Helium nucleus (₂He⁴) takes place in the following steps:

₁H¹ + ₁H¹      ⇢     ₁H² + ₁e⁰ + v + 0.42MeV

₁H² + ₁H¹      ⇢      ₂He³ + γ + 5.49 MeV

₂ He³ +₂He³     ⇢       ₂He⁴ + 2 ₁H¹ +12.86 Mev

The abovecombinations give 

4₁H¹  ⇢  ₂He⁴ +2₁e⁰ +2v +2γ + 24.68MeV

Thus, the net effect of the sequence of proton Proton cycle is that four protons(₁H¹) combine to form one helium nucleus(₂He⁴) plus two positrons(₁e⁰, i.e.β⁺) two gamma Rays(γ) and two neutrinos with a release of about 25 MeV of energy. The proton-proton cycle is dominant at temperatures ∼10 ⁷K.

Carbon Nitrogen Cycle

It is a thermo nuclear reaction in which carbon nuclei absorbs a succession of protons until they emit alpha particles to become carbon nuclei once more to repeat the cycle indefinitely.

Carbon -Nitrogen cycle starts that have hotter interiors carbon cycle predominates. The net result of this cycle again is the formation of a Helium nucleus, two positrons,  two γ rays and two neutrinos from 4 protons with the evaluation of 24 .68 MeV.

The sequence of nuclear reactions is as follows:

₆C¹² +₁H¹ ⇢ ₇N¹³ + Q₁(MeV)

₇N¹³⇢ ₆C¹³ +₁e⁰ +v

 ₆C¹³ +₁H¹ ⇢ ₇N¹⁴ + Q₂(MeV)

₇N¹⁴ +₁H¹⇢₈O¹⁵ +Q₃(MeV)

₈O¹⁵⇢₇N¹⁵ + ₁e⁰+v

₇N¹⁵ +₁H¹⇢₆C¹² + ₂He⁴

Total energy released in this cycle = 24.68MeV

The above combinations give:

4₁H¹⇢₂He⁴+ 2₁e⁰+ 2v +2γ +24.68MeV

The initial ₆C¹² acts as a kind of catalyst for the process since it reappears at its end. The above stated thermo-nuclear reactions take place in the sun and other stars and hence they are the source of energy in the solar system.

Thus, energy released by fusion is greater than the energy released by fission.

Sample Questions 

Question 1: Are all fusion reactions exoergic?

Answer:

Fusion reactions between sufficiently light nuclei are exoergic because the B.E/A increases. If the nuclei are too massive, however, B.E/A decreases is the fusion is endoergic. (I.e.,it takes in energy rather than releasing it).

Question 2: Why is it difficult to realise nuclear fusion terrestrially?

Answer:

• The extremely high temperature required to initiate nuclear fusion cannot be easily realised .
• The nuclei which fuse together in nuclear fusion are at very high temperatures and they cannot be contained as no container remains in solid state at this temperature.

Question 3: A fusion reaction is more energetic than a fission reaction. Comment.

Answer:

Since energy released by the fusion reaction is greater than the energy released by the fission reaction, therefore fusion reaction is more energetic than a fission reaction.

Question 4: A very high temperature is needed to initiate a nuclear fusion reaction. Explain, why?

Answer:

When two nuclei fuse together, the process is called a nuclear fusion. Two nuclei being positively charged repel each other as they come closer to each other. To overcome this force of repulsion for fusing together, these two nuclei should have large kinetic energy. This large kinetic energy can be attained at a very high temperature.

Nuclear fusion can be carried out at extremely high temperature(∼10⁷). Since this much temperature cannot be generated in any furnace, so nuclear fusion cannot be initiated. Moreover, if this amount of temperature is generated by atomic explosion, then it is difficult to contain the material used in fusion. Due to these difficulties, nuclear fusion cannot be carried out easily.

Question 5: Why is nuclear fusion not possible in the laboratory?

Answer:

Because of very high temperature (∼10⁷) needed for nuclear fusion cannot be attained by any known method in the laboratory.