A black hole is a region of space where gravity is so strong that nothing, not even light, can escape from within its boundary.
The event horizon
The event horizon is not a physical surface but a boundary of no return. Its size depends only on mass: the more massive the black hole, the larger the horizon. From outside, an object falling in appears to slow and fade as it approaches, never quite seen to cross.
How they form
The most common route is the collapse of a massive star. When a star exhausts its nuclear fuel, its core can no longer resist gravity. If the remaining mass is large enough, collapse continues past the neutron star stage and a black hole forms.
Supermassive black holes, millions to billions of times the mass of the Sun, sit at the centres of most large galaxies. How they grew so large so early remains an open research question.
Observing the invisible
Black holes are detected by their effects:
- the motion of stars orbiting an unseen mass;
- radiation from hot gas in an accretion disc spiralling inward;
- gravitational waves produced when two black holes merge;
- direct imaging of the shadow cast against surrounding emission.
Common misconceptions
A black hole does not "suck in" everything nearby. At a distance, its gravity behaves like that of any other object of the same mass — replacing the Sun with a black hole of equal mass would leave Earth's orbit unchanged, though the sky would go dark.
Open questions
What happens to information falling into a black hole, and how gravity behaves at the centre, are not settled. These questions sit precisely where general relativity and quantum mechanics fail to agree, which is why they receive so much attention.