Black holes
Key facts
| Escape speed at the event horizon | The speed of light, about 186,000 miles per second |
|---|---|
| Typical stellar black hole | About 5 to several dozen times the Sun's mass |
| Sagittarius A* mass | About 4 million Suns |
| Distance to Sagittarius A* | About 27,000 light-years |
| M87* mass | About 6.5 billion Suns |
| Distance to M87* | About 55 million light-years |
| First image of a black hole | M87*, released April 2019 |
| First gravitational waves detected | September 14, 2015 (LIGO) |
In short
- A black hole is a region where matter is packed so tightly that nothing, not even light, can escape its gravity.
- Its boundary is the event horizon, a point of no return rather than a solid surface.
- Stellar black holes form when massive stars collapse; supermassive ones sit at the centers of galaxies.
- The Event Horizon Telescope imaged M87* in 2019 and our galaxy's Sagittarius A* in 2022.
- Black holes do not suck everything in; from a safe distance they pull like any other object of the same mass.
What is a black hole?
A black hole is a place where a large amount of mass has been squeezed into a tiny space. The result is gravity so strong that, past a certain boundary, nothing can climb back out, not even light.
Because no light escapes, a black hole itself is invisible. We find black holes by watching what happens around them: stars orbiting something unseen, or gas heating up and glowing as it spirals inward.
The idea grew out of Einstein's general theory of relativity, which describes gravity as the bending of space and time by mass. Near a black hole, that bending becomes extreme.
The event horizon
The event horizon is the edge of a black hole. Outside it, an object moving fast enough can still get away. Inside it, the speed needed to escape would be greater than the speed of light, which nothing can reach.
It is not a solid surface. A traveler would not feel anything special when crossing it, but from that moment there would be no way back.
The size of the horizon depends on the mass. For a black hole with the mass of the Sun it would be only about 4 miles across. For Sagittarius A* it is about 15 million miles across, smaller than Mercury's orbit.
Stellar and supermassive black holes
Stellar black holes form when a very massive star runs out of fuel and its core collapses, usually in a supernova explosion. They typically weigh about 5 to several dozen times as much as the Sun. Our galaxy may contain around 100 million of them, though only a small number have been found.
Supermassive black holes are millions to billions of times the Sun's mass and live at the centers of most large galaxies. How they grew so big so early in cosmic history is still an active research question.
Astronomers also look for intermediate-mass black holes, in between these two groups, and a few good candidates have been reported.
Sagittarius A*, our galaxy's center
At the heart of the Milky Way, about 27,000 light-years away in the direction of the constellation Sagittarius, lies a supermassive black hole called Sagittarius A* (said "A-star"). It has about 4 million times the mass of the Sun.
Its existence was proven by tracking stars that whip around it in tight orbits over many years. Reinhard Genzel and Andrea Ghez, who led those studies, shared the 2020 Nobel Prize in Physics for this work.
Sagittarius A* is fairly quiet right now. It is not swallowing much material, so it shines only faintly compared with the bright cores of some other galaxies.
The first pictures
In April 2019, the Event Horizon Telescope collaboration released the first image of a black hole: M87*, at the center of the galaxy Messier 87, about 55 million light-years away and about 6.5 billion times the Sun's mass.
The Event Horizon Telescope is not one dish but a network of radio observatories around the world working together, which makes it act like a telescope the size of Earth.
The image shows a glowing ring of hot gas around a dark center, the black hole's shadow. In May 2022, the same team released an image of Sagittarius A*, which looks strikingly similar despite being about 1,500 times less massive. That match is what general relativity predicts.
Gravitational waves
Einstein predicted that moving masses create ripples in space and time called gravitational waves. They are incredibly weak by the time they reach Earth.
On September 14, 2015, the two LIGO detectors in the United States picked up such a signal for the first time. It came from two black holes, about 29 and 36 times the Sun's mass, spiraling together and merging about 1.3 billion light-years away. The discovery won the 2017 Nobel Prize in Physics.
Since then, LIGO, Virgo in Italy and KAGRA in Japan have recorded hundreds of mergers, turning gravitational waves into a new way to study black holes.
Common myths
Myth: black holes suck everything in. In fact, from a safe distance a black hole pulls just like any object of the same mass. If the Sun were replaced by a black hole of equal mass, Earth would keep the same orbit, although it would get very cold and dark.
Myth: black holes are cosmic vacuum cleaners roaming the galaxy. The nearest known black hole is more than 1,500 light-years away and poses no danger to us.
Myth: black holes are holes. They are actually extremely dense concentrations of matter. Many also spin, and some fire powerful jets of particles that stretch far beyond their host galaxies.
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