Making (Stellar) Waves

1931: Subrahmanyan Chandrasekhar

For most people, an ocean voyage is a time for a little R&R. But for the young Indian physicist Subrahmanyan Chandrasekhar, it was a time for C&C — contemplation and calculation. He pondered the fates of stars, and realized that they won't all end their lives in the same way. His work, for which he eventually received the Nobel Prize, helped lead to the understanding that some stars will collapse to form black holes.

A young Chandrasekhar [AIP]

Chandrasekhar was born in 1910 in British India. After earning his degree in physics, he was invited to pursue a doctorate at Cambridge University. It was during the two-and-a-half week trip to England, in 1930, that he contemplated the fates of stars.

Stars like the Sun die by casting their outer layers into space, leaving only their cores, known as white dwarfs. Most of the mass of the original star is packed into a ball the size of Earth, so a teaspoon of white dwarf material would weigh several tons.

At such great density, gravity tries mightily to make the white dwarf collapse to an even smaller size. But under extreme pressure, the electrons in atoms jump to higher and higher energy levels, exerting an outward pressure that balances theinward pull of gravity and preventing further collapse.

As Chandra, as he's best known, began his ocean voyage, astronomers thought that all stars, no matter how big or massive, were fated to become white dwarfs.

Yet Chandra discovered otherwise. He calculated that there is a weight limit for white dwarfs: around 1.4 times the mass of the Sun. Beyond that mass, known today as the Chandrasekhar limit, the electrons cannot exert enough outward pressure to resist the inward pull of gravity. Anything heavier must either collapse further or explode.

But Arthur Eddington, one of England's leading astronomers, considered the idea absurd. He turned most other astronomers against Chandra's ideas. So the young physicist left England and took a job at the University of Chicago, where he remained for the rest of his life.

Before long, however, others confirmed Chandra's calculations. If a stellar core is more than about 1.4 times the mass of the Sun, it forms a neutron star, which compresses the mass of several suns into a sphere no bigger around than a city. Its matter is squeezed so tightly that all of the space between atoms is crushed out of existence, and their particles are fused together to form a ball of solid neutrons.

Later, others would calculate that for the heaviest stellar cores, with masses three times greater than the Sun, even that is not the ultimate fate. Gravity eventually overcomes all other forces of nature and crushes the core into a single point, forming a black hole.

Chandrasekhar pursued many areas of research during his career, including black holes. But in 1983, he received the Nobel Prize for his work on the evolution of stars — work that began with a boat ride more than five decades earlier.