Black holes are regions of space where gravity is so strong that nothing, not even light, can escape once it crosses a certain boundary. They feature in countless films, yet they are real objects that astronomers study with telescopes and gravitational wave detectors.
Key takeaways
- A black hole is a region where gravity is strong enough to trap light.
- The boundary of no return is called the event horizon.
- Many black holes form when very massive stars collapse at the end of their lives.
- Supermassive black holes sit at the centres of most large galaxies, including ours.
- Black holes do not "suck in" everything nearby; at a distance they pull like any object of the same mass.
How a black hole forms
Stars shine by fusing atoms in their cores, and the outward pressure from that fusion balances the inward pull of gravity. When a very massive star runs out of fuel, the core can no longer resist gravity and collapses. If the remaining core is massive enough, it collapses into a black hole while the outer layers are blown off in a supernova explosion. Black holes formed this way are called stellar-mass black holes.
The event horizon
The event horizon is the boundary around a black hole. Inside it, the escape speed would be greater than the speed of light, so nothing can get out. It is not a solid surface but a limit on what can ever be observed from outside. The size of the horizon depends on the black hole's mass: more massive black holes have larger horizons.
Supermassive black holes
Much larger black holes, millions to billions of times the mass of the Sun, lie at the centres of most large galaxies. Astronomers have measured the orbits of stars around the centre of the Milky Way and found they circle an invisible, extremely compact object called Sagittarius A*. How such giants grow so large is an area of active research.
How we know they exist
We cannot see a black hole directly, but we can detect its effects. Gas falling toward a black hole heats up and glows brightly in X-rays. Stars orbit around invisible companions in ways that reveal their mass. Detectors have recorded gravitational waves, ripples in space-time produced when two black holes merge. In 2019, the Event Horizon Telescope collaboration released the first image of the shadow of a black hole, in the galaxy Messier 87, and later an image of the one at the centre of our own galaxy.
Myths to retire
Black holes are not cosmic vacuum cleaners. If the Sun were replaced by a black hole of the same mass, Earth would continue to orbit it in the same way, although it would be very cold and dark. Black holes are also not necessarily "holes": they are extremely dense objects whose gravity affects nearby space and time.
What happens to time near one
Einstein's general theory of relativity predicts that time runs more slowly close to a strong gravitational field. This effect has been tested in weaker fields, such as with precise clocks at different altitudes, and it is part of how GPS satellites are corrected.
Black holes come in sizes
Astronomers sort black holes by mass. Stellar-mass black holes form from collapsing stars and typically weigh a few to a few tens of times the mass of the Sun. Supermassive black holes lie at the centres of galaxies and weigh millions to billions of solar masses. Between them are intermediate-mass black holes, which have been hard to find, though candidates exist. Some theories propose primordial black holes formed in the early universe, but there is no confirmed evidence for them. The size of a black hole's event horizon grows in proportion to its mass.
Listening to collisions
In September 2015, the LIGO detectors in the United States recorded gravitational waves from two black holes, about thirty times the mass of the Sun each, spiralling together and merging roughly 1.3 billion years ago. The detection was announced in February 2016, confirmed a prediction made by Albert Einstein a century earlier and earned the 2017 Nobel Prize in Physics for its leaders. Since then, detectors around the world have recorded many more mergers of black holes and neutron stars, opening a new way of observing the universe.
Photographing a shadow
In 2019, the Event Horizon Telescope collaboration, which links radio telescopes around the world to act like one giant dish, released the first image of a black hole's shadow, in the galaxy Messier 87. In 2022 it released a similar image for Sagittarius A*, the black hole at the centre of the Milky Way. The pictures show a bright ring of hot gas around a dark centre, and their shapes match predictions from Einstein's general relativity.
Hawking radiation and open questions
In 1974, Stephen Hawking showed that quantum effects should cause black holes to emit a faint glow, now called Hawking radiation, and slowly lose mass. For black holes of astronomical size, the effect is far too small to detect. It also raised the information paradox, a puzzle about whether information that falls into a black hole is destroyed, which would conflict with quantum theory. Physicists have proposed many ideas, and the problem remains a focus of research on how to unify gravity with quantum mechanics.
What would happen if you fell in?
For a person falling into a stellar-mass black hole, tidal forces would stretch the body, a process nicknamed spaghettification. Falling into a very large black hole, the tides at the horizon are weaker, so you might cross it without noticing anything unusual locally. To a distant observer, however, you would appear to slow down and fade as time dilation grows. What happens at the centre is unknown, because our theories break down there.
Frequently asked questions
Could a black hole destroy Earth?
There is no known black hole near enough to pose a threat to the Solar System.
What is inside a black hole?
Nobody knows for sure. Our current physics breaks down at the centre, and scientists are working on theories that combine gravity and quantum physics.
Do black holes last forever?
Theory predicts that black holes can very slowly lose energy through Hawking radiation, but this process is far too slow to matter for known black holes.
What is the closest known black hole to Earth?
As of recent discoveries, the closest known is Gaia BH1, found in 2022 using data from the Gaia spacecraft, about 1,560 light-years away. It orbits a Sun-like star. Others may exist closer but be undiscovered, because isolated black holes are hard to detect.
Can we see a black hole directly?
Not in the usual sense, because no light escapes. Astronomers see the glowing gas around them, measure the motion of nearby stars and detect gravitational waves from mergers. The Event Horizon Telescope images show the shadow cast by the black hole against the glowing gas.
Are black holes infinitely dense?
General relativity predicts a singularity at the centre, where density becomes infinite, but most physicists believe this signals that the theory is incomplete there. A full theory combining gravity and quantum physics is needed to say what really happens.
Sources and further reading
Written by
FactNest Editorial Team
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