Subject: biography, astronomy
Indian-born US astrophysicist who was particularly concerned with the structure and evolution of stars. He is well-known for his studies of white dwarfs and the radiation of stellar energy.
Chandrasekhar was born on 19 October 1910 in Lahore, India (now in Pakistan). He went to Presidency College, University of Madras, from which he graduated with a BA in 1930. He continued his studies at Trinity College, Cambridge, gaining a PhD in 1933. There he studied under the physicist Paul Dirac. He left Trinity College in 1936 to take up a position on the staff of the University of Chicago, working in the Yerkes Laboratory. In 1938 he became assistant professor of astrophysics there and in 1952 was promoted to distinguished service professor. The following year he became a US citizen. He died on 21 August 1995.
Chandrasekhar's greatest contribution to astronomy was his explanation of the evolution of white dwarf stars, as laid out in his Introduction to the Study of Stellar Structure (1939). These stellar objects, which were first discovered in 1915 by Walter Sydney Adams, are similar in size to the Earth. They have a very high density and are therefore very much more massive than the Earth. This enormous density is explained in terms of degeneracy - a consequence of the Pauli exclusion principle in which electrons become so tightly packed that their normal behaviour is suppressed; as stars evolve, they ‘burn’ their hydrogen, which is converted to helium and, eventually, heavier elements. During his work at Cambridge, Chandrasekhar suggested that when a star had burned nearly all its hydrogen, it would not be able to produce the pressure against its own gravitational field to sustain its size and would then contract. As its density increased during the contraction the star would build up sufficient internal energy to collapse its atomic structure into the degenerate state.
Not all stars, however, become white dwarfs. Chandrasekhar believed that - up to a certain point - the greater the mass of a star, the smaller would be the radius of the eventual white dwarf. But he also stated that beyond this point a large stellar mass would not be able to equalize the pressure involved and would explode. He calculated that stellar masses below 1.44 times that of the Sun would form stable white dwarfs, but those above this limit would not evolve into white dwarfs. This limit - known as the Chandrasekhar limit - was based on calculations involving the complete degeneracy of the stellar matter; the limit is now believed to be about 1.2 solar masses.
Stars with masses above the Chandrasekhar limit are likely to become supernovae and rid themselves of their excess matter in a spectacular explosion. The remaining mass may form a white dwarf if the conditions of mass and pressure are suitable, but it is more likely to form a neutron star. Neutron stars were first identified by J Robert Oppenheimer and his co-workers in 1938. These stars are even more dense than white dwarfs, with an average radius of approximately 15 km/9 mi.
With the Polish astrophysicist Erich Schönberg, Chandrasekhar determined the Chandrasekhar-Schönberg limit of the mass of a star's helium core; if it is more than 10-15% of that of the entire star, the core rapidly contracts, often collapsing. Chandrasekhar also investigated the transfer of energy in stellar atmospheres by radiation and convection and the polarization of light emitted from particular stars.
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