What Is a Black Hole?
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A black hole is a region where matter has been packed so densely that gravity becomes strong enough to stop anything escaping, including light. That sounds exotic, but black holes follow the same gravity that keeps the Moon in orbit, taken to an extreme.
What it is made of
A black hole is not made of a special substance. It is ordinary matter compressed into a small enough space that its gravity overwhelms every force that would normally hold it apart.
Squeeze anything far enough and it becomes a black hole. The Earth would need to be crushed to about the size of a marble. The reason this does not happen is that ordinary matter resists compression, and only the collapse of a massive star provides enough force to overcome that.
The event horizon
The event horizon is the boundary of no return. Outside it, an object moving fast enough can escape. Inside it, every possible path leads inwards, and nothing, not even light, can get back out.
The horizon is not a solid surface. Someone falling through it would notice nothing special at the moment of crossing, though from outside they would appear to slow down and fade as light from them stretches to longer and longer wavelengths.
Where black holes come from
There are two well established populations:
- •Stellar mass black holes, formed when a massive star collapses at the end of its life
- •Supermassive black holes, millions to billions of times the Sun's mass, sitting at the centres of galaxies
- •Intermediate mass black holes, a rarer in between class now being detected
How we detect something invisible
Black holes cannot be seen directly, so astronomers watch their effects. Stars near the centre of our galaxy orbit an invisible object at enormous speed, and tracking those orbits over decades revealed a mass of about four million Suns in a tiny volume.
Gas falling towards a black hole also heats up and emits X-rays before it crosses the horizon. Since 2015, detectors have recorded gravitational waves from black holes merging, giving a completely independent line of evidence.
The first pictures
In 2019 the Event Horizon Telescope released an image of the black hole at the centre of the galaxy M87, produced by linking radio telescopes across the planet into one effective instrument the size of Earth. A second image, of our own galaxy's black hole, followed in 2022.
What the images show is not the black hole itself but the bright ring of hot material around it, with a dark shadow in the middle where the horizon blocks the light. The size and shape of that shadow matched predictions closely.
Do black holes last forever
Stephen Hawking showed in the 1970s that black holes should not be entirely black. Quantum effects near the event horizon should cause them to emit a faint thermal radiation, now called Hawking radiation.
If correct, black holes slowly lose mass and will eventually evaporate. The timescales are absurd: a stellar mass black hole would take vastly longer than the current age of the universe. The prediction has never been observed, since the radiation is far too faint against the cosmic background, but it is theoretically important because it links gravity, quantum mechanics and thermodynamics in a single result.
The takeaway
A black hole is matter compressed until its gravity traps light, bounded by an event horizon, detected through its effect on nearby stars and gas, and now directly imaged as a dark shadow ringed by glowing material.