Subrahmanyan Chandrasekharਸੁਬਰਾਮਨੀਅਨ ਚੰਦਰਸ਼ੇਖਰ

Subrahmanyan Chandrasekhar (1910 to 1995) was an astrophysicist born in Lahore, Punjab, who changed how science understands the death of stars. As a young man he calculated the Chandrasekhar limit, the maximum mass of a white dwarf star, and in 1983 he shared the Nobel Prize in Physics.

Early life in Lahore and Madras

Subrahmanyan Chandrasekhar was born on 19 October 1910 in Lahore, then part of British India and now in Pakistan. He was the third child and first son in a Tamil family settled far from its southern roots. His father, C. Subrahmanya Ayyar, was a government auditor posted in Lahore as Deputy Auditor General of the Northwestern Railways. His mother, Sita, was a woman of serious intellectual interests who translated Henrik Ibsen's play A Doll's House into Tamil. His uncle was C. V. Raman, who won the Nobel Prize in Physics in 1930, so the boy grew up in a family where science carried the highest prestige.

Chandrasekhar was taught at home until he was twelve. The family moved from Lahore to Allahabad in 1916 and settled permanently in Madras in 1918. He attended the Hindu High School in Triplicane from 1922 to 1925, then studied at Presidency College, Madras, from 1925 to 1930. He was publishing research papers while still a student: his first, on the Compton effect and the new quantum statistics, appeared in 1929 after a lecture by Arnold Sommerfeld fired his interest. He took his BSc in physics in June 1930 and won a Government of India scholarship to study at Cambridge.

What is the Chandrasekhar limit?

The Chandrasekhar limit is the maximum mass a white dwarf star can have, about 1.44 times the mass of the Sun. A white dwarf is the dense, burnt-out core left when a star like the Sun runs out of fuel. It no longer burns, and what holds it up against its own gravity is the pressure of its packed electrons. On the sea voyage from India to England in 1930, the nineteen-year-old Chandrasekhar combined quantum physics with Einstein's theory of relativity and showed that this electron pressure has a ceiling. Above about 1.44 solar masses, nothing can stop the collapse. Such a star keeps falling in on itself, blows off its outer layers in a supernova explosion, and ends as a neutron star. A still heavier star collapses all the way into a black hole. This single calculation, made on a ship, laid the ground for the modern understanding of supernovae, neutron stars and black holes.

Cambridge and the clash with Eddington

At Trinity College, Cambridge, Chandrasekhar worked as a research student under Ralph Fowler, the theorist whose work on white dwarfs his own voyage calculations had extended. On Paul Dirac's advice he spent a year at the institute in Copenhagen, in Niels Bohr's circle. He took his PhD in the summer of 1933 and was elected to a Prize Fellowship at Trinity for 1933 to 1936, only the second Indian to receive one, sixteen years after Srinivasa Ramanujan.

Then came the episode that shaped his early career. In January 1935 he presented his complete theory of white dwarfs at a meeting of the Royal Astronomical Society in London. Arthur Eddington, the most famous astrophysicist in Britain and until then a supporter, spoke immediately after him and publicly attacked the theory, dismissing the relativistic reasoning behind the limit. Leading physicists such as Bohr and Léon Rosenfeld privately found Eddington's arguments weak, but few would challenge him in public. Chandrasekhar finished the work anyway, setting it out in his 1939 book An Introduction to the Study of Stellar Structure, and then deliberately moved to other fields. His limit was resisted for decades before astronomers accepted it in full, and the Nobel Prize that recognised it came nearly half a century later.

The Chicago years

On a visit to Harvard in 1936, Chandrasekhar was offered a post by Otto Struve and President Robert Maynard Hutchins of the University of Chicago. He married Lalitha Doraiswamy, a friend from his Presidency College days, in India in September 1936, and joined the Chicago faculty in January 1937, working at the Yerkes Observatory in Wisconsin. He never left. He became a full professor in 1943, took American citizenship in 1953, and held the Morton D. Hull Distinguished Service Professorship from 1952 until he became emeritus in 1986. From 1952 to 1971 he also edited The Astrophysical Journal, the leading journal in his field, and was known for its exacting standards.

His way of working was distinctive. He would choose one field, master it completely over about a decade, publish the papers, sum it up in a book, and then start again in a new field as if from the beginning. In this way he moved from stellar structure to stellar dynamics, then to radiative transfer, set out in his 1950 book Radiative Transfer, then to hydrodynamic and hydromagnetic stability, then to the equilibrium of rotating masses, and finally to the mathematical theory of black holes, crowned by The Mathematical Theory of Black Holes in 1983. Along the way he wrote ten books in all. During the Second World War he also did ballistics work for the United States Army at the Aberdeen Proving Ground in Maryland.

Honours and later life

Recognition came steadily, if slowly. Chandrasekhar was elected a Fellow of the Royal Society in 1944. He received the Gold Medal of the Royal Astronomical Society in 1953, the Royal Medal of the Royal Society in 1962, the United States National Medal of Science in 1966, and the Padma Vibhushan, one of India's highest civilian honours, in 1968. In 1983 he shared the Nobel Prize in Physics with William A. Fowler, cited for his theoretical studies of the physical processes of importance to the structure and evolution of the stars. The Copley Medal of the Royal Society, its oldest and most prestigious award, followed in 1984. He worked to the end of his life: his last book, Newton's Principia for the Common Reader, appeared in 1995. He died in Chicago on 21 August 1995, aged 84.

Legacy

The Chandrasekhar limit is now a foundation of astrophysics: it explains why some stars die quietly as white dwarfs while others end in supernova explosions, and it pointed the way to neutron stars and black holes before either was observed. In 1999 NASA launched its Chandra X-ray Observatory, named in his honour; Chandra is also the Sanskrit word for the Moon, and the observatory has transformed the study of hot, violent regions of the universe. On this site he stands in a remarkable Punjab-born line of Nobel laureates in science, alongside Abdus Salam of Jhang and Har Gobind Khorana of Raipur, three scientists from one province whose work changed physics and biology. Chandrasekhar is remembered not only for the limit that bears his name but for the rigour, patience and intellectual honesty with which he pursued it, waiting fifty years for the recognition his shipboard calculation deserved.