A black hole is a region of spacetime where gravity is so intense that nothing — not even light — can escape once it crosses the boundary called the event horizon. They are not vacuum cleaners that suck things in; objects fall into them the same way they fall towards any massive body, just with no way back past the event horizon.
The surface of no return is the event horizon, whose radius (for a non-rotating black hole) is the Schwarzschild radius: r = 2GM/c². For an object as massive as the Sun, this radius is about 3 km. For Earth, it would be less than 9 mm. Any object compressed smaller than its Schwarzschild radius becomes a black hole.
At the very centre lies the singularity — the point (in classical general relativity) where density becomes infinite and our current physics breaks down. Most physicists believe quantum mechanics must modify our understanding of what actually happens at the singularity.
Stellar-mass black holes (roughly 5–100 solar masses) form from the collapse of massive stars after supernovae, as described in Chapter 2. They are sprinkled throughout galaxies.
Supermassive black holes, ranging from millions to billions of solar masses, lurk at the centres of most large galaxies, including our own. How they formed is still a major open question in astrophysics — they may have grown from smaller seeds over cosmic time, or formed through a different early-universe mechanism.
Intermediate-mass black holes (hundreds to thousands of solar masses) have been detected more recently and bridge the gap between stellar and supermassive. There may also exist primordial black holes formed in the very early universe, though none have been confirmed.
In 1974, Stephen Hawking applied quantum field theory to the region just outside a black hole's event horizon and found something remarkable: black holes should slowly emit thermal radiation. This Hawking radiation arises from quantum effects near the horizon — tightly paired virtual particle–antiparticle pairs can be split by the intense gravity, with one particle escaping and one falling in.
Over an immense timescale (far longer than the current age of the universe for any stellar-mass black hole), this radiation would cause a black hole to slowly lose mass and eventually evaporate. This creates the Black Hole Information Paradox: does the information about what fell into the black hole get destroyed when it evaporates? It's one of the deepest unsolved problems in theoretical physics.
Because black holes emit no light themselves, we detect them indirectly. One method is observing the effects of their gravity on nearby stars or gas. Astronomers tracked stars orbiting the centre of the Milky Way for decades, revealing the mass of Sgr A* precisely.
When a black hole accretes (pulls in) surrounding material, that infalling matter forms an accretion disc — a spinning disc of superheated gas that radiates brilliantly in X-rays. These X-ray binaries are among the brightest sources in the X-ray sky.
In 2019, the Event Horizon Telescope produced the first direct image of a black hole's shadow — the supermassive black hole M87*, 6.5 billion solar masses, at the centre of the galaxy M87. In 2022, they followed with an image of Sgr A* in our own galaxy. These ring-shaped images show the bright photon sphere around the dark shadow of the event horizon.
Test what you've just learned.
1.What is the event horizon of a black hole?
2.What is Hawking radiation?
3.How do astronomers observe black holes if they emit no light?
4.In what year did the Event Horizon Telescope first image a black hole's shadow?