In 1930, a young Indian physicist named Subrahmanyan Chandrasekhar boarded a ship from India to England. Just 19 years old, he spent much of the long voyage working out the mathematics of what happens when a star collapses under its own gravity.
What he found was shocking: stars above a certain mass couldn’t stop their collapse — no matter how much pressure built up inside them. Instead, they would shrink endlessly, forming objects so dense that nothing, not even light, could escape. Today we call these black holes. But when Chandrasekhar presented his idea in London, the great physicist Arthur Eddington publicly dismissed it. “There should be a law of nature,” he scoffed, “to prevent a star from behaving in this absurd way.”
This idea — that gravity could trap light — interestingly emerged first not from observation but from mathematics. Nearly 15 years prior to Chandrashekhar’s radical proposal, German physicist Karl Schwarzschild, working amid the chaos of World War I, found the first exact solution to Einstein’s then-newly published equations of general relativity. He showed that if enough mass were compressed into a sufficiently small sphere, it would form a boundary — now known as the Schwarzschild radius — beyond which not even light could escape. At that time, this was seen as a mathematical oddity. Even Einstein himself believed that nature would never allow such extreme objects to form.
Fast forward to today, a century later, black holes have moved from absurdity to centerpiece. They are now understood to be a natural, even common, endpoint in the life of massive stars — and a crucial player in shaping galaxies and the cosmos.
What is a black hole?
A black hole is a region of space where gravity is so strong that nothing can escape — not even light. Imagine compressing the entire mass of the Earth into a sphere the size of a marble. That’s roughly the kind of density we’re talking about.
At the heart of a black hole lies what’s called a “singularity” — a point where our laws of physics break down, and gravity becomes infinite.
But the defining edge of a black hole is the event horizon — the point of no return. Anything that crosses this boundary, whether a particle or a beam of light, is lost forever to the outside universe. We can’t see what’s inside, but we can detect black holes by their effects: the way they pull on nearby stars, or the radiation released as matter spirals in.
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Stellar-mass black holes: Death of a giant
The most common black holes are formed when massive stars — more than eight times the mass of the Sun — run out of fuel. As long as the star burns hydrogen in its core, it generates energy that pushes outward, balancing the inward pull of gravity. But when the fuel is exhausted, this balance tips. Gravity takes over. The core collapses in on itself.
If the remaining mass is large enough — typically more than about three times the mass of the Sun — not even neutron pressure (the last barrier) can halt the collapse. A black hole is born.
These are called stellar-mass black holes, and dozens have been found in our galaxy, often when they’re part of a binary system. As matter from a companion star falls in, it heats up and emits powerful X-rays — one of the key ways astronomers detect them.
Intermediate black holes: A missing link?
For years, astronomers believed black holes came only in two sizes: the stellar kind, and the supermassive monsters at galactic centers. But over the last two decades, signs have emerged of a middle class — intermediate-mass black holes, perhaps 100 to 10,000 times the Sun’s mass. They are harder to spot because they’re too big to come from a single star, but too small to sit at a galaxy’s core.
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One theory is that they form from the merger of many smaller black holes, or in dense star clusters where multiple massive stars collapse close together. Evidence remains limited, but recent gravitational wave detections — ripples in space-time from black hole collisions — have added weight to their existence.
Supermassive black holes: The hearts of galaxies
At the center of nearly every large galaxy, including our own Milky Way, lies a supermassive black hole — with masses ranging from millions to billions of Suns. These giants are not born from stars, but likely from a complex chain of events early in the universe: dense regions collapsing, merging, and feeding on gas over billions of years.
In 2019, the Event Horizon Telescope captured the first-ever image of such a black hole, in the galaxy M87 — a glowing ring of hot gas surrounding a dark silhouette. That image marked a turning point, not just in astronomy but in public imagination. It showed that black holes are not just theoretical oddities — they’re real, and we can observe them.
Our own galaxy’s black hole, Sagittarius A*, weighs about four million Suns. It’s quiet now, but evidence shows it was more active in the past, likely influencing star formation, gas flows, and the very structure of the Milky Way.
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Why black holes matter
Black holes are no longer just endpoints — they’re engines. When matter falls into them, it doesn’t vanish silently. It forms a swirling disk of superheated gas that can shine brighter than all the stars in a galaxy. These accretion disks can launch jets of particles at near light-speed, injecting energy into their surroundings and regulating galaxy growth.
Some black holes even collide, sending out gravitational waves — ripples in space-time that were first directly detected in 2015. These waves have opened a new window into the universe, letting us hear cosmic events that were once invisible.
A final thought
Subrahmanyan Chandrasekhar would go on to win the Nobel Prize ( 1983) for his work — long after being mocked for suggesting stars could collapse into darkness. Today, his prediction is a central pillar of astrophysics. Black holes, once dismissed as absurd, are now indispensable to our understanding of the universe. They teach us humility: that even the brightest stars can vanish into silence, and that the most powerful forces in nature may lie in the places we cannot see.
Shravan Hanasoge is an astrophysicist at the Tata Institute of Fundamental Research.
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