Where Black Holes Come From

A star lives in balance: energy from reactions in its core pushes outward, while its own gravity pulls inward. As long as there is fuel, the balance holds. When the reactions stop, the outward pressure disappears.

What remains in the end depends on the mass. In lighter stars, it is a dense, cooling sphere. In heavier ones, it is an object made of matter of unimaginable density. Only in the most massive ones does nothing stop the collapse.

The boundary between the second and third outcomes is narrow and known only approximately. That is why objects near it are so interesting: they help scientists pin down exactly where matter stops resisting its own gravity.

The shed shell appears as a flash as bright as an entire galaxy. These flashes are how we most often learn that a collapse has happened somewhere - the object left behind emits no light.

The gap is enormous, and simple accumulation cannot easily explain it. Matter can only be absorbed up to a point: radiation from the infalling gas pushes away the gas that follows, and the growth rate hits a ceiling.

Observations of the early universe made the problem even sharper. They reveal giant objects that already existed within the first hundreds of millions of years. There clearly was not enough time for gradual growth from stellar size.

This has led to several hypotheses about their seeds: the direct collapse of enormous gas clouds without a star stage, the merger of many objects in dense clusters, and unusually massive first stars. None has yet been accepted as definitive.

Observations of mergers help. Bursts detected by detectors of spacetime ripples revealed intermediate-mass objects - exactly the sizes that had rarely been found before.

There is also a third path, confirmed recently: mergers. Two black holes orbiting each other eventually merge into one, and the event is detected through ripples in spacetime, not light.