How a star becomes a black hole

A star is a long tug of war. Gravity pulls all of its gas inward. Nuclear fusion in the core, the same process behind a hydrogen bomb, heats the gas and pushes outward. For most of a star's life the two sides balance. The Sun has held steady for about 4.6 billion years and has roughly 5 billion to go.

When the fuel runs out, the push stops. Gravity wins. What's left depends mostly on how heavy the star was to begin with.

Treat those cutoffs as rough. They depend on how much gas a star blows away during its life, what it's made of, and how fast it spins. Astronomers still argue about the exact lines.

The last second of a giant

A heavy star fuses lighter elements into heavier ones, layer by layer, until its core is iron. Fusing iron doesn't release energy, so the core suddenly has nothing holding it up. It collapses in a fraction of a second.

Often the outer layers blast away in a supernova, an explosion that can outshine a whole galaxy for weeks. What happens to the core depends on its weight. Up to roughly 2 or 3 times the Sun's mass, the core can stop as a neutron star, so dense that a teaspoon of it would weigh billions of tons. Heavier than that, and nothing we know of can hold it up. It keeps falling inward, past the size from the last lesson, and becomes a black hole.

Play or scrub the collapse. Follow the core as it shrinks through the marked radius; the figure leaves out the outer star and any supernova.

These stellar black holes usually weigh somewhere between 5 and a few dozen Suns. Astronomers estimate our galaxy holds millions of them, nearly all invisible because nothing is falling into them.

Our own Sun is far too light for any of this. In a few billion years it will swell into a red giant, shed its outer layers, and settle down as a white dwarf. It will never become a black hole.

The giants in the middle

There's a second kind, and it's much bigger. Supermassive black holes weigh millions to billions of Suns and sit at the centers of most large galaxies. Ours, called Sagittarius A*, is about 4 million Suns and roughly 26,000 light-years away. The one at the center of the galaxy M87 is about 6.5 billion Suns.

How they got so big is an open question. Swallowing gas and merging with other black holes for billions of years explains part of it. But telescopes keep finding giant ones that already existed less than a billion years after the Big Bang, which seems too early to grow that way. Maybe some started huge, from enormous gas clouds collapsing all at once. Nobody knows yet.