The great cosmic discovery - VI

Prof H Nandakumar Sarma *

The discovery of microwave background radiation, neutron star, pulsar; trying to understand the cosmic forces, the discovery of anisotropy of cosmic microwave background radiation, the discovery of Higgs boson ( God’s particle ), the detection of gravitational wave and black hole etc are some of the exciting moments in the understanding of the universe.

In this series, we trace some of the great cosmic discovery made from the beginning of the 20th century to the present day.

The different phases from the birth to the death of stars were studied by Prof. Subrahmanyan Chandrasekhar, one of the most distinguished theoretical astrophysicists of the 20th century. Chandrasekhar was the joint winner of 1983 Nobel Prize in physics with Prof. W. Fowler.

He was awarded Nobel Prize mostly for his work accomplished during his voyage to England and at Cambridge University. Chandrasekhar was born at Lahore on 19th October 1910. He received his master degree in 1930 from the Presidency College, Madras.

In the summer 1930, the young Indian sailed for England to begin his graduate work at Trinity College, Cambridge. At Cambridge he worked with Prof. R.H. Fowler. By the time Chandrasekhar started his research in astronomy there was much discussion about a new class of star, white dwarf discovered a decade or so earlier.

The masses of white dwarfs are of the order of our sun and their sizes are of planetary size. This implied that the density is about a million times the density of water. These objects were the subject of much discussion and investigation when Chandrasekhar entered the field.

A white dwarf represents the last stage in the life of stars having about the solar mass. Before we go further let us see something about the birth and death of stars. A star begins its life as a huge cloud of gas.

Under favourable circumstances, the cloud starts contracting due to gravitation and it is then called a proto star. The internal temperature of the proto star rises when the central temperature reaches more than a million degree kelvin, nuclear fusion reaction is trigger off and we say a star is born!

In the beginning, a star bums its hydrogen to helium by the process of nuclear fusion. This phase of the star’s life is the longest stage in the star’s life. When the hydrogen in the inner core is exhausted the nuclear reaction is stopped for the time being and so the core starts contracting.

The core becomes very hot and the trigger off the next series of nuclear reactions. The go-stop-go process of nuclear fusion reaction goes on until the iron group nuclei are built up by the fusion process in the central core of the star.

For massive stars say about six times the mass of the sun, when the star is ripe, explosive condition developed within the star and become a supernova. Depending on the initial mass of the star, the remnant core of a supernova explosion may end up as a neutron star or a black hole. Many of the heavy elements contain in our body are the by products of supernova explosion, so the products of stars are in us!

For less massive star say about the mass of our sun, the stars have less dramatic future. They settle down to become white dwarfs. Chandrasekhar found an equation, which relates inversely the radius of the white dwarf to the cube root of the mass of the star.

At high temperatures found inside the white dwarf stars, matter exists in the ionised state and velocities of the electron comparable to the velocity of the light - degenerate electrons.

Chandrasekhar’s great achievement was to show that the radius of a degenerated star decreases as its mass increases - in contrast to ordinary star. Chandrasekhar showed that there is an upper limit to the mass of the star that can be supported by electronic pressure of a star.

The limiting mass is of 1.4 times the mass of our sun and is known as Chandrasekhar mass limit. A large number of white dwarf stars are known today and all have masses below the Chandrasekhar mass limit.

Chandrasekhar presented his findings at the meeting of the Royal Astronomical Society on January 11, 1935. The great astrophysicists Eddington realised that the existence of this upper limit to the mass of the white dwarfs implied that unless accident intervene to save the star, black hole, as we now called them must form.

After presentation of Chandrasekhar’s paper, Eddington stated “The star has to go on radiating, and contracting and contracting until I suppose, it gets down to a few kilometre radius, when gravity becomes strong enough to hold and star can at least find peace”.

But Eddington added “ ... I felt driven to the conclusion that this was an almost reduction and absurd of the relativistic degeneracy formula, various accident may intervene to save the star, but I want more protection than that. I think there should be a law of nature to prevent a star from behaving in this absurd way!”

Because of Eddington’s supreme authority, the astronomical community did not accept immediately Chandrasekhar’s finding.

Chandrasekhar got Ph.D. Degree from Cambridge University in 1933 and D.Sc in 1942. From 1933 to 1937, he was Fellow of Trinity College. In 1937, he became Research Associate at Yerk Observatory of the University of Chicago.

While at Chicago, Chandra took special class for two students. Afterwards the whole of Chandra’s class got Nobel Prize in Physics for the discovery of parity violation in weak nuclear interaction. They are non other than Lee and Yang.

In 1947, Chandra was made the Distinguished Service Professor of Theoretical Astrophysics at the University of Chicago. Many honours were conferred to Chandrasekhar. In 1944, he was elected Fellow of Royal Society and in 1962 conferred the Royal medal. He was a recipient of Padma Vibhushan, the highest honour, the India government confer on a person who is not a citizen of India.