What Is A Black Hole? - National Geographic

In their final stages, enormous stars go out with a bang in massive explosions known as supernovae. Such a burst flings star matter out into space but leaves behind the stellar core. While the star was alive, nuclear fusion created a constant outward push that balanced the inward pull of gravity from the star's own mass. In the stellar remnants of a supernova, however, there are no longer forces to oppose that gravity, so the star core begins to collapse in on itself.

If its mass collapses into an infinitely small point, a black hole is born. Packing all of that bulk—many times the mass of our own sun—into such a tiny point gives black holes their powerful gravitational pull. Thousands of these stellar-mass black holes may lurk within our own Milky Way galaxy.

One black hole is not like the others

Supermassive black holes, predicted by Einstein's general theory of relativity, can have masses equal to billions of suns; these cosmic monsters likely hide at the centers of most galaxies. The Milky Way hosts its own supermassive black hole at its center known as Sagittarius A* (pronounced “ay star”) that is more than four million times as massive as our sun.

The tiniest members of the black hole family are, so far, theoretical. These small vortices of darkness may have swirled to life soon after the universe formed with the big bang, some 13.7 billion years ago, and then quickly evaporated. Astronomers also suspect that a class of objects called intermediate-mass black holes exist in the universe, although evidence for them is so far debatable.

No matter their starting size, black holes can grow throughout their lives, slurping gas and dust from any objects that creep too close. Anything that passes the event horizon, the point at which escape becomes impossible, is in theory destined for spaghettification thanks to a sharp increase in the strength of gravity as you fall into the black hole.