What Is a Black Hole?

A black hole is a region of space where gravity is so strong that nothing that enters it can escape, not even light. Because no light comes back out, we can’t see a black hole directly. That is where the name comes from.

How a black hole forms

Many black holes are the end point in the life of a very massive star. When such a star runs out of the fuel that keeps it shining, its core can collapse under its own weight, squeezing a huge amount of matter into a very small space. Much larger black holes, called supermassive black holes, are found at the centre of many galaxies, including our own Milky Way. Exactly how these giants first grew is still an active area of research.

The point of no return

The boundary around a black hole is called the event horizon. It is not a solid surface. It is the line beyond which escaping would require travelling faster than light, which nothing can do. Once something crosses it, it cannot get back out.

Not a cosmic vacuum cleaner

Black holes are often described as things that suck everything in. That is a misleading picture. From far away, a black hole’s gravity is just like the gravity of any other object with the same mass. If the Sun were somehow replaced by a black hole of the same mass, the planets would carry on orbiting as before. It would simply be dark and cold. You would have to come very close to a black hole before its gravity became dramatic.

How we know they exist

We can’t see a black hole itself, but we can see what it does to its surroundings: stars orbiting an invisible object, and hot, glowing gas swirling just before it falls in. Scientists have also detected tiny ripples in space, called gravitational waves, produced when two black holes merge. In 2019, astronomers released the first picture of the shadow of a black hole, a dark centre ringed by glowing gas.

Key points

• A black hole is a place where gravity is so strong that light cannot escape.
• Many form when very massive stars collapse, and giant ones sit at the centres of galaxies.
• The event horizon is the point of no return, not a solid surface.
• From a distance, a black hole’s gravity is no stronger than that of any object with the same mass.