Jethalal S. Zaveri
Prof. Muni Mahendra Kumar
Evolution of a Star [1]
| [1]A Brief History of Time by Stephen Hawking, pp. 82-84. |
In the 'hot big bang model' as the universe expands; any matter or radiation in it gets cooler. At very high temperatures, particles would be moving around so fast that they could escape any attraction toward each other due to nuclear or electromagnetic forces, but as they cooled off, they attract each other and start to clump together. A star is formed when a large clump of gas (mostly hydrogen) starts to collapse in on itself due to its gravitational attraction. As it contracts, the atoms of the gas collide with each other more and more frequently and at greater and greater speeds - the gas heats up. Eventually, the gas will be so hot that when the hydrogen atoms collide, they no longer bounce off each other, but instead coalesce to form helium. The heat released in this reaction, which is like a controlled hydrogen bomb explosion, is what makes the star shine. This additional heat also increases the pressure of the gas until it is sufficient to balance the gravitational attraction, and the gas stops contracting. It is somewhat like a balloon - there is a balance between the pressure of the air inside, which is trying to make the balloon expand, and the tension in the rubber, which is trying to make the balloon smaller. Stars will remain stable like this for a long time, with heat from the nuclear reactions balancing the gravitational attraction. Eventually, however, the star will run out of its hydrogen and other nuclear fuels. Paradoxically, the more fuel a star starts off with, the sooner it runs out. This is because the more massive the star is, the hotter it needs to be to balance its gravitational attraction. And the hotter it is, the faster it will use up its fuel. Our sun has probably got enough fuel for another five thousand million years or so, but more massive stars can use up their fuel in as little as one hundred million years, much less than the age of the universe. When a star runs out of fuel, it starts to cool off and so to contract.
| [2]According to Pauli's exclusion principle. |
There is, however, a limit to the repulsion that the exclusion principle can provide. The theory of Relativity limits the maximum difference in the velocities of the matter particles in the star to the speed of light. This means that when the star got sufficiently dense, the repulsion caused by the exclusion principle would be less, than the attraction of gravity. A massive star would not be able to support itself against its own gravity.
There are three possible final states of a star. If a star is not very massive, it can settle down to a possible final state as a 'white dwarf with a radius of a few thousand miles and a density of hundreds of tons per cubic inch. A 'white dwarf’ is supported by the exclusion principle repulsion between the electrons in its matter. We observe a large number of these 'white dwarf’ stars. One of the first to be discovered is a star that is orbiting around Sirius, the brightest star in the night sky.
There is another possible final state for a star, also with a limiting mass of about one or two times the mass of the sun but much smaller even than a 'white dwarf.' These stars would be supported by the exclusion principle repulsion between neutrons and protons, rather than between electrons. They were, therefore, called "neutron stars". They would have a radius of only ten miles or so and a density of hundreds of million of tons per cubic inch. At the time they were first predicted, there was no way that 'neutron stars' could be observed. They were not actually detected until much later.
For the very massive star, however, the final state is the third alternative, viz, the 'black hole.' [See later, Chapter III, Section II.]
