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Subrahmanyan Chandrasekhar explained what happens when humongous stars die

1.4 times the mass of our sun — why “Chandrasekhar’s limit” is essential for understanding why some stars form black holes.

X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA/JPL/Caltech; Radio: NSF/NRAO/VLA; Ultraviolet: ESA/XMM-Newton
Brian Resnick is Vox’s science and health editor, and is the co-creator of Unexplainable, Vox's podcast about unanswered questions in science. Previously, Brian was a reporter at Vox and at National Journal.

All things die, even stars. When stars run out of hydrogen — the fuel that sustains the nuclear fusion reactors at their cores — they become unstable and collapse in on themselves. But not all stars collapse in the same way. Some of the most massive ones explode into a supernova and then collapse down into neutron stars, or black holes. We know this because of the work of astrophysicist Subrahmanyan Chandrasekhar, who would be 107 Thursday and is honored with a Google Doodle.

Chandrasekhar — an Indian-born scientist who spent 50 years at the University of Chicago — is most famous for coming up with the theory that explains the death of the universe’s most massive stars.

Before Chandrasekhar, scientists assumed that all stars collapsed into white dwarfs when they died. He determined this isn’t so.

On a long sea voyage from India to England in 1930 at the age of 19, he worked it out. According to the science of quantum mechanics, there are forces within the very atoms of the white dwarf star that counteract the force of gravity. Chandrasekhar determined this force would be overwhelmed if the star were massive enough.

He determined that any star remnants 1.4 times more massive than our sun would be too massive to form a stable white dwarf. After the limit, the force of gravity would cause the white dwarf to collapse.

“This discovery is basic to much of modern astrophysics, since it shows that stars much more massive than the Sun must either explode or form black holes,” NASA explains.

(For a more technical description of Chandrasekhar’s methodology, check out this great PBS story.)

This figure — 1.4 times the mass of our sun — is now known as the “Chandrasekhar limit,” and it’s key to understanding the evolution of stars in our universe. Beyond this limit, stars at the end of their lives either explode into a supernova or explode and then collapse into a neutron star or even a black hole.

Neutron stars are some of the weirdest objects in the universe. They’re small — just 15 or so miles across — but contain a mass equal to the sun. A teaspoon of neutron star weighs around 10 million tons. They’re so dense that the only things that can exist inside of them are neutrons, which are protons and electrons fused together. (The LIGO gravitational wave observatory just witnessed two of them crashing into each other. Subsequent observations showed that this collision actually contained the right energy and conditions to create heavy elements like gold and platinum.)

At the time of his discovery in the 1930s, Chandrasekhar didn’t know what, exactly, these massive stars would turn into once they spent all their fuel.

Initially, his idea was met with ridicule.

Sir Arthur Eddington, a physicist whose experimental work was key in proving Einstein’s theory of general relativity, openly mocked Chandrasekhar’s theory at a meeting of the Royal Astronomical Society in 1935.

"The star has to go on radiating and radiating and contracting and contracting until, I suppose, it gets to a few kilometers' radius, when gravity becomes strong enough to hold the radiation and the star can at last have peace," Eddington said, inadvertently describing the very thing Chandrasekhar’s limit would explain: the creation of black holes. “I think there should be a law of Nature to prevent a star from behaving in this absurd way!” he added. (There isn’t.)

This incident was so embarrassing for the young Chandrasekhar that he almost quit the field, the New York Times explains. (Chandrasekhar and Eddington would eventually make amends.)

Of course, scientists would go on to find more and more evidence of the existence of black holes and neutron stars. And for his work, Chandrasekhar won half of the 1983 Nobel Prize in physics. His theory represents one of the very early, important steps in our understanding of black holes and neutron stars.

And today his name adorns one of NASA’s prized space telescopes: The Chandra X-ray Observatory, whose data has contributed to the most spectacular images we have of dying stars exploding into supernovas.

This is the Crab Nebula, the result of a supernova explosion 4,500 light-years away from Earth.

Chandrasekhar was a prolific writer — he published more than a dozen textbooks on a range of topics in physics — and a devoted teacher (having once regularly driven a 100-mile round trip just to teach a class with two students), the University of Chicago explains. He was also the editor of the prestigious Astrophysical Journal for two decades. He passed away in 1995.

When he accepted the Nobel Prize in 1983, Chandrasekhar chose to read aloud a poem he had memorized from his youth in India. It was by Rabindranath Tagore, an Indian who won the Nobel prize for literature in 1913. Its message: Freedom is knowledge.

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.

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