Objects · Milky Way scale
Black Hole
A region where gravity is so strong that nothing — not even light — can escape from inside its boundary.
In a sentence
The supermassive black hole at the centre of the Milky Way, Sagittarius A*, has a mass of ~4 million solar masses.
Source: ESA — Black Holes
More on Black Holes
A black hole forms when a sufficiently massive object collapses under its own gravity to an extremely small volume. The boundary beyond which nothing can escape is the event horizon. Inside the event horizon, all paths through spacetime lead toward the singularity — a point where density becomes infinite and general relativity breaks down.
Most stellar-mass black holes form via a supernova: the violent collapse of a massive star's core. If the remaining core mass exceeds roughly 3 solar masses, neutron degeneracy pressure cannot halt the collapse, and a black hole forms. Read the full story in our guide to black holes.
Frequently asked questions
- The Schwarzschild radius is the radius at which a given mass would need to be compressed to form a black hole. For the Sun, it is about 3 km. For Earth, it is about 9 mm. The formula is rs = 2GM/c², where G is the gravitational constant, M is mass, and c is the speed of light. NASA Science explains the physics.
- Yes. Stellar-mass black holes (3–100 M☉) form from collapsing massive stars. Intermediate-mass black holes (100–100,000 M☉) are rarer and still being studied. Supermassive black holes (millions to billions of M☉) sit at the centres of most large galaxies, including the Milky Way (Sgr A*). Primordial black holes — hypothetical objects formed in the early universe — have been proposed but not yet confirmed. See ESA's black hole overview.
- Proposed by Stephen Hawking in 1974, Hawking radiation is a quantum mechanical process by which black holes slowly emit thermal radiation and lose mass over very long timescales. It arises from quantum effects near the event horizon. For stellar-mass black holes, the rate is negligible — they absorb far more from the cosmic microwave background than they radiate. It has not yet been directly observed. ESA provides context.
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