Nuclear Fission and Fusion - Part VII

8. How sun produces its energy, production of heavy elements in the stars
Hydrogen is the most abundant element in the universe. It was formed during early universe when the temperature of the universe was more than million million degrees; the elementary particles protons and electrons came close together and combined to form the hydrogen atom. Very little of higher elements, including isotopes of hydrogen was formed in the early universe. From the basic hydrogen atom, other elements were formed in the interior of the stars by the process of nuclear fusion. The mass of stars is very high, this makes the matter fuse under gravitational pull; the gravitational interaction leads to high temperatures, which initiates fusion reactions. The fusion reactions themselves give off energy, which increases the temperature of the interiors of the stars. So the cycle continues. It has to be remembered that for a fusion reaction to take place, the nuclei have to overcome their electrostatic repulsion. Nuclei are all positively charged because of the presence of protons (neutrons are electrically neutral). The electrostatic repulsion is also called the nuclear potential barrier. The interacting nuclei can overcome the barrier only when they are brought close together, within a distance of a few fermis (1f = 1 x 10^-15m). In the laboratory, a nuclear fusion can take place on a small scale when nuclei are accelerated and bombarded onto each other.

To re-create nuclear fusion conditions similar to the interior of the sun or the stars in the laboratory, giant magnets are employed to confine hydrogen nuclei in a small region. Then the temperature of the system is raised to more than a million °C by lasers. Nuclear fusion reactions similar to that in the sun, have been successful, but only on a very limited scale. No large scale thermonuclear plants have yet been made which are producing commercially useful electricity, similar to nuclear power plants based on fission reactions. Another application of fusion reaction is the hydrogen bomb. Similar to the bombs based on uranium-plutonium uncontrolled fission reaction, an hydrogen bomb is an uncontrolled fusion reaction where deuterium atoms fuse to become helium atoms releasing enormous amounts of energy.

The basic fusion reactions in the stars are with two hydrogen nuclei (protons) fusing together to form a helium nuclei. This is an exothermic reaction and releases a lot of heat energy. It is because of this the interiors of stars or the sun is said to have a thermonuclear reactor. There are two basic cycles is which such fusion reactions can take place; one is called the proton-proton cycle and the second process is called the carbon-carbon cycle (or the CNO cycle). In a proton-proton cycle, protons collide with each other under high temperature and gravitational fields, leading to helium. In the carbon-carbon cycle, a series of steps occur where a carbon nucleus is formed. From the carbon nuclei all the nuclei of other heavier elements are synthesized.

There are many ways to represent the proton-proton cycle. One such is shown below.  

The formation of deuteron (nucleus of a deuterium atom) is a direct result of fusion of two protons. The deuteron may fuse either with another deuteron to form ⁴He or may fuse another proton to form ³He.

The final step of the proton-proton cycle is when a ⁴He is formed and two protons are returned to the cycle.

The mass difference, which appears as energy, in the entire cycle, is about 25 MeV. This is a very large amount of energy.

                The proton-proton cycle

The carbon-carbon cycle proceeds in the following way :

Helium fusion reaction takes place to produce ¹²₆C.  

Once the star has exhausted all its hydrogen fuel, the helium fusion starts to take place.  From ¹²₆C other nuclei are produced by fusion. This is the reason why this cycle is known as carbon-carbon cycle or CNO cycle.  

The outcome of the carbon-carbon cycle is same as the proton-proton cycle, but at times it tends to produce higher elements also. Carbon-carbon cycle does not depend on the production of deuterium and hence can proceed faster than the proton-proton cycle.  

                                             The carbon-carbon cycle

After hydrogen and helium, carbon is the most abundant element in the universe. It undergoes nuclear fusion reactions with various other nuclei to give higher elements. Some examples are given below.  

These processes need extremely high temperatures. The presence of higher elements can give information about the age of the star or its generation. In the first generation stars hydrogen is converting to helium. In the second-generation stars helium is burning up to produce carbon and so on and so forth. Our sun is observed to be a fourth or fifth generation star. It is presumed that to produce heavy elements, very high temperatures have to be reached; these can be obtained only during super nova explosions.

First stars were formed from only hydrogen as their fuel. When such a star finished its fuel it first cooled and then collapsed under its own gravitational field. Then it exploded to release its mass. This is called a super nova explosion. The debris from the explosion scatters in space. When a second generation star forms from coalescence of this matter, it will have hydrogen and helium as its fuel. The cycle continues. The presence of higher heavier elements indicates the generation of the star. In our solar system, the sun, the planets have heavier elements in them; this shows that our sun was born out of debris of a super nova explosion that has gone through four to five cycles.

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