Subrahmanyan Chandrasekhar and the Evolution of Stars

Subrahmanyan Chandrasekhar (1910-1995)

On October 19, 1910, Indian-American astrophysicist and Nobel Laureate Subrahmanyan Chandrasekhar was born. He won the 1983 Nobel Prize for Physics together with William Alfred Fowler for key discoveries that led to the currently accepted theory on the later evolutionary stages of massive stars. If you are a frequent reader of this blog, you might have encountered articles about scientists, artists, or explorers you might have never heard of. This is, because we are trying to cover a wide range of topics within science, technology, and art. Now, all physicists may raise their hand, because the name Chandrasekhar is a rather familiar one for them. Actually, winning Nobel Prizes in physics must lie somehow in the family of Chandrasekhar, because his uncle Sir Chandrasekhara Venkata Raman, won the Nobel Prize for Physics already in 1930, too.

Where the mind is without fear and the head is held high;
Where knowledge is free;
Where words come out from the depth of truth;
Where tireless striving stretches its arms towards perfection;
Where the clear stream of reason has not lost its way into the dreary desert sand of dead habit;
into that haven of freedom, Let me awake.
( from Gitanjali, a poem by Rabindranath Tagore, cited by Chandrasekhar in his 1983 Nobel Prize banquet speech)

The important scientific contribution, for which Chandrasekhar received his Nobel Prize concerns a rather fundamental topic for our universe. How do stars evolve and what happens to stars, when they are rather massive, i.e. much more massive than our own Sun. You should know that the way, how a star is evolving in his later stage, depends on his mass. And this is, what Chandrasekhar did come up with: he showed that there is a maximum mass which can be supported against gravity by pressure made up of electrons and atomic nuclei. The value of this limit is about 1.44 times the mass of our Sun. This was already derived by Chandrasekhar in 1930, when he was a student. The so-called Chandrasekhar Limit plays a crucial role in understanding the stellar evolution. If the mass of a star exceeded this limit, the star would not become a white dwarf, i.e. a stellar remnant composed mostly of electron-degenerate matter. It would continue to collapse under the extreme pressure of the gravitational forces of its own mass.

The formulation of the Chandrasekhar Limit led to the discovery of neutron stars and black holes. For almost all the time of their live, stars are stable, that is they do not collapse because internal pressures balance gravity. The internal pressure in a star stems from the thermal motion of the atomic nuclei and electrons and also from the pressure of the radiation generated by nuclear reactions. However, for every star a time will come when nuclear reactions will cease and that means there will be no internal pressure to match the gravitational pull of its mass. Depending on the mass of the star, we know that there are three possible final stages of a star: white dwarf, neutron star and black hole.

Subrahmanyan Chandrasekhar was known throughout his life simply as Chandra. Born on October 19, 1910 to C Subrahmanyan Ayyar, an Indian government auditor whose job was to audit the Northwest Railways and Sitalaksmi Aiyar in a large family, having two older sisters, three younger brothers and four younger sisters. Although he was supposed to his father into government service, Chandra wanted to be a scientist and his mother encouraged him to follow this route. Actually his uncle Sir Chandrasekhara Venkata Raman was some kind of a role model for him and Chandra studied at Presidency College, University of Madras. In July 1930, Chandra was awarded a Government of India scholarship to pursue graduate studies at the University of Cambridge, where he was admitted to Trinity College.

In his first year at Cambridge, Chandra was already introduced to the monthly meetings of the Royal Astronomical Society. At the invitation of Max Born he spent the summer of 1931, at Born’s institute at Göttingen. On the advice of Paul A. M. Dirac, he spent his final year of graduate studies at the Institute for Theoretical Physics in Copenhagen, where he met Niels Bohr. He finished his PhD in 1933 and was elected to a Prize Fellowship at Trinity College for the period 1933–37. In 1937, Chandra was recruited to the University of Chicago, where he remained for his entire career until attaining emeritus status in 1985. In 1999, NASA named the third of its four “Great Observatories” after Chandrasekhar. American astronomer and science communicator Carl Sagan, who studied Mathematics under Chandrasekhar, at the University of Chicago, praised him in one of his books: “I discovered what true mathematical elegance is from Subrahmanyan Chandrasekhar.

At yovisto you can learn more about the late stage of evolution of stars in the lecture of UC Berkeley astrophysicist Alex Filippenko on ‘Exploding Stars – Celestial Fireworks‘.

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