S. Chandrasekhar: The Physicist Who Was Right All Along

Subrahmanyan Chandrasekhar was an Indian-born astrophysicist. Friends called him Chandra. He studied how stars are built and how they die.

Where did he come from?

• He was born in Lahore on 19 October 1910. Lahore was then part of British India.
• His family moved to Madras in 1918.
• He took a BSc with honours in physics at Presidency College in June 1930.
• In July 1930 he won a Government of India scholarship to study at Cambridge.
• He took his PhD at Cambridge in the summer of 1933. His supervisor was R. H. Fowler.
• C. V. Raman, the 1930 Physics Nobel laureate, was his paternal uncle.

When did he die?

Chandrasekhar died in Chicago on 21 August 1995.

What is the memory hook?

Think “1910 Lahore, 1930 ship, 1933 PhD”. Raman was his uncle, not his teacher.

The Chandrasekhar limit is the largest mass a white dwarf can have. It is about 1.4 times the mass of the Sun. A heavier white dwarf collapses.

What is a white dwarf?

A white dwarf is the small, dense core left by a Sun-like star. Pressure from crowded electrons holds it up against gravity. Physicists call this electron degeneracy pressure.

Where did he find it?

• He sailed from Bombay on 31 July 1930. He was 19.
• On the ship he worked out the physics of the electron gas inside white dwarfs.
• He saw that the fastest electrons must be treated with relativity.
• That meant the pressure cannot grow fast enough to stop a heavy star collapsing.

What number did he get?

• His first value was 0.91 times the Sun’s mass.
• The estimate of the star’s makeup was later revised.
• That changed the value to 1.44 times the Sun’s mass.
• Today the limit is quoted as about 1.4.

What happens above the limit?

• A slightly heavier star ends as a neutron star.
• A star above about 2 to 3 Sun masses ends as a black hole.
• A white dwarf pushed over the limit by a companion star explodes as a supernova.

What is the memory hook?

Think “1.4 Suns, then collapse”. The ship voyage gave the idea. Cambridge gave the paper.

Arthur Eddington thought the result was physically absurd. He was a famous astronomer. He attacked the result in public in January 1935.

What happened at the meeting?

• Chandrasekhar presented an exact solution to the Royal Astronomical Society in London in January 1935.
• Eddington arranged to speak straight after him.
• Eddington attacked the physical meaning of the result, not the mathematics.
• He said a star should not behave “in this absurd way”.
• His own words were: “I think there should be a law of Nature to prevent a star from behaving in this absurd way!”

Why did people side with Eddington?

Eddington was famous and persuasive. Chandrasekhar’s result also went against Eddington’s own model of stars, which did not allow for stars that fail to end as white dwarfs.

Did Eddington repeat the attack?

Yes. He repeated it in Paris in July 1935. The meeting was chaired by Henry Norris Russell. Russell did not allow Chandrasekhar to reply.

What did it cost Chandrasekhar?

The dispute closed doors in England. Otto Struve then invited him to Chicago.

What is the memory hook?

Think “right but silenced”. Eddington won the room. Chandrasekhar won the physics.

Later physics and astronomy confirmed his limit. The Nobel Prize for it came in 1983. That was 48 years after the Eddington attack.

What did the Nobel Prize say?

• The 1983 Nobel Prize in Physics was split into two halves.
• One half went to Chandrasekhar for studies of how stars are built and how they evolve.
• The other half went to William A. Fowler for studies of nuclear reactions in the formation of the elements.
• The Nobel committee called his white dwarf work his possibly best-known achievement.
• He did that work in his twenties.

What did the limit explain?

• It explains why very heavy stars cannot end as white dwarfs.
• It explains why some white dwarfs explode as supernovae.
• It is now taught as a clear case of quantum physics shaping a whole star.

Did he only work on white dwarfs?

No. He moved on to many other problems. They included how stars move, how radiation flows through stars, and black holes.

What is the memory hook?

Think “1935 attack, 1983 prize”. Two halves of the prize: Chandrasekhar and Fowler.

Chandrasekhar spent the rest of his life at the University of Chicago. He joined its faculty in January 1937. He stayed there until his death in 1995.

How did he get there?

• He visited Harvard in early 1936.
• Otto Struve and President Robert Hutchins then offered him a post in Chicago.
• He married Lalitha Doraiswamy in September 1936.
• He joined the Chicago faculty in January 1937.

Where did he work?

At first he worked at Yerkes Observatory in Wisconsin. Later he moved to the Chicago campus.

What was the class of two?

• In the 1940s he drove more than 100 miles round trip each week from Yerkes.
• He taught a class of just two registered students.
• Those students were T. D. Lee and C. N. Yang.
• They won the Nobel Prize in Physics in 1957.

When did he become an American?

He and his wife became naturalised US citizens in 1953. NASA calls him an Indian-American Nobel laureate. Exams often call him Indian-born.

What is the memory hook?

Think “Jan 1937 Chicago, 1953 citizen”. Two students, two Nobel Prizes.

The Chandra X-ray Observatory is NASA’s flagship X-ray telescope. It carries his nickname. It was launched on 23 July 1999.

How did it get his name?

• NASA first called it the Advanced X-ray Astrophysics Facility.
• In December 1998 it was renamed to honour the late Nobel laureate.
• A public naming contest drew more than six thousand entries.
• In Sanskrit, “chandra” means “moon” or “luminous”.

How was it launched?

The space shuttle Columbia carried it on mission STS-93. It was the largest satellite Columbia ever launched.

Why does it fit his work?

X-ray telescopes study hot, extreme places. These include exploding stars and the edges of black holes. Those are the ideas his theory opened up.

What is the memory hook?

Think “Chandra the man, Chandra the moon”. The name fits both.

Four mix-ups come up again and again. Learn each pair.

What are the traps?

• Which Nobel year? Raman won in 1930. Chandrasekhar won in 1983.
• Whose nationality? Chandrasekhar took US citizenship in 1953. He won the 1983 prize as a US citizen.
• Which number? The limit is about 1.4 solar masses. Do not mix it with the 2 to 3 for black holes.
• Which year? The idea came in 1930. The Eddington attack came in 1935. The Nobel came in 1983.

How do you remember them?

Think “Raman 1930, Chandra 1983”. Uncle first, nephew later.