Black Holes and Extreme Gravity

A black hole is a place in space where gravity is so strong that nothing, not even light, can escape from it. Black holes are the most extreme objects in the universe. They form when very massive stars collapse, or when matter clumps together over billions of years in the centres of galaxies. Although a black hole is invisible (no light comes out), astronomers can detect them by their powerful gravitational effects on stars and gas nearby. The first direct image of a black hole was finally taken in 2019, more than 100 years after Einstein predicted they could exist.

  • What it isRegion where light cannot escapeExtreme gravity well
  • EdgeEvent horizonPoint of no return
  • Stellar black hole3-100x Sun massFormed from giant star collapse
  • SupermassiveMillions to billions of SunsAt the centre of galaxies
  • Milky Way centreSagittarius A*4.3 million Sun masses
  • First image taken2019M87 galaxy black hole

How black holes form

Most black holes form when a very massive star runs out of fuel and collapses. Throughout its life, a star is held up by the outward push of nuclear fusion at its core. When the fuel runs out, gravity wins. The cores outer layers crash inward. If the original star was about 20 times the mass of the Sun or more, the collapse cannot be stopped, and the core collapses to a point of essentially zero size and infinite density called a singularity.

The other main kind, supermassive black holes, sit at the centres of nearly all galaxies. They contain millions or billions of times the mass of the Sun. Scientists are not yet certain exactly how they got so massive: perhaps they started small and grew by swallowing matter, or they may have formed very early in the universe directly from huge clouds of collapsing gas.

The event horizon

The event horizon is the invisible boundary around a black hole beyond which nothing can ever return. Anything that crosses it is gone forever from the outside universe. The event horizon is not a physical surface but a one-way line in space: outside it, you can still escape if you move fast enough; inside it, you cannot. The size of the event horizon depends on the black holes mass.

  • A black hole the mass of the Sun would have an event horizon only about 6 km across.
  • The supermassive black hole at the centre of our galaxy has an event horizon about 24 million km across (still tiny compared to the galaxy itself).
  • The biggest known supermassive black holes have event horizons larger than our entire solar system.
Fact If you turned the Sun into a black hole (which would never actually happen because it is not massive enough), it would only be about 6 km across. The planets, including Earth, would keep orbiting exactly as they do now. We would freeze without sunlight, but we would not be sucked in. Black holes are dangerous only if you get very close to them. From far away, they behave like any other heavy object.

What happens near a black hole?

Black holes do strange things to space and time around them. As you approach one:

  • Time slows down: clocks near a black hole tick more slowly than clocks far away. If you fell in, far-away observers would see your clock run slower and slower, and you would appear to slow down and fade just outside the event horizon.
  • Light bends: photons passing near a black hole follow curved paths, like marbles rolling on a stretched sheet. Sometimes a black hole acts as a giant magnifying lens, brightening galaxies behind it (called gravitational lensing).
  • Tidal forces grow: gravity is much stronger at your feet than at your head, stretching you. Near a small stellar black hole, you would be ripped into a long stream of atoms (a process scientists call spaghettification). Near a giant supermassive black hole, the tidal forces are much gentler and you could probably cross the event horizon without immediately being shredded.

How we know they exist

Black holes are invisible, but we have many ways of finding them:

  • Stars orbiting nothing visible: telescopes can watch stars whipping around an invisible centre at huge speeds, betraying a hidden massive object. This is how astronomers found the supermassive black hole at the centre of the Milky Way.
  • Glowing accretion discs: matter falling toward a black hole spirals around it in a glowing hot disc before crossing the event horizon. The glow can be detected.
  • Gravitational waves: when two black holes collide, they send out ripples in space-time detected by ground-based detectors like LIGO.
  • Direct imaging: the Event Horizon Telescope, a network of telescopes across the world acting as one, has now produced direct images of black holes including the one in galaxy M87 (released in 2019) and our own galaxys Sagittarius A* (released in 2022).
Did you know? Black holes can actually shrink over time. In 1974, the physicist Stephen Hawking showed that black holes give off a tiny amount of radiation (now called Hawking radiation) due to quantum effects near the event horizon. As they give off this radiation, they lose mass. Tiny black holes shrink quickly; big ones essentially never shrink at all. A black hole the mass of the Sun would take about 10^67 years to evaporate, far longer than the current age of the universe.

Three kinds of black hole

  • Stellar black holes: 3 to 100 times the mass of the Sun. Formed from collapsing massive stars. Tens of millions are thought to exist in our galaxy alone.
  • Intermediate-mass black holes: 100 to 100,000 times the mass of the Sun. Surprisingly rare. Scientists are still studying how they form.
  • Supermassive black holes: millions to billions of solar masses. Sit at the centres of nearly all big galaxies, including our Milky Way. They probably form alongside their galaxies.

The black hole at the heart of the Milky Way

At the centre of our own galaxy is a supermassive black hole called Sagittarius A* (pronounced "Sagittarius A star"). It contains about 4.3 million times the mass of the Sun, packed into a region about as wide as Mercurys orbit around the Sun.

Astronomers found it by watching stars near the galaxy centre. Some of those stars orbit so fast (one star takes just 16 years to make a complete orbit) that only a huge invisible mass at the centre can explain their motion. The astronomers Reinhard Genzel and Andrea Ghez won the 2020 Nobel Prize in Physics for this discovery.

Try this Watch a video animation showing star orbits around Sagittarius A* (search online for "stars orbiting black hole Milky Way centre"). Many telescopes have watched these stars over years; the sped-up footage looks like a frantic ballet around an invisible point. You are watching the gravity of a supermassive black hole at work, even though the hole itself is not visible. The fastest of these stars reaches about 24,000 km per second at its closest pass, almost a tenth of the speed of light.
Deeper dive: what happens if you fall into a black hole?

Lets imagine you bravely (or foolishly) decided to fall into a supermassive black hole. What would you see and feel?

From your point of view:

  • As you fall, the universe behind you appears to bunch up into a brighter, smaller circle as light gets bent.
  • You cross the event horizon without feeling anything special (for a supermassive black hole, where tidal forces at the horizon are gentle).
  • Inside the event horizon, all paths lead to the central singularity. You cannot turn around or escape.
  • You reach the singularity in finite time, where the tidal forces eventually become infinite and you are ripped into a stream of particles. That is the end.

From an outside observers point of view:

  • They never actually see you cross the event horizon.
  • As you approach the event horizon, your image appears to slow down and grow more and more red-shifted (your colours shift towards red, then infrared, then beyond visibility).
  • You appear to freeze at the edge of the event horizon, fading slowly out of view as your light loses energy.
  • From the outside, you take an infinite amount of time to disappear; from your point of view, the fall is over in a finite time. Both stories are simultaneously true. Black holes are like that.

What happens at the very centre, in the singularity, is unknown. Our current theories of physics break down there. Solving this mystery is one of the great unfinished tasks of modern physics, and it may require a future theory that combines gravity with quantum mechanics.

For more, see what is gravity and black holes.