A black hole is a region of space where gravity is so strong that nothing, not even light, can escape once it crosses a boundary called the event horizon. Black holes are not really "holes" at all. They are super-dense leftover cores of dead stars, or much larger objects sitting at the centre of almost every galaxy in the universe. Our own Milky Way has one called Sagittarius A* (said "Sagittarius A-star") with the mass of about 4 million Suns.
- Types of black hole3Stellar, intermediate, supermassive
- Smallest knownapprox. 3 solar massesThree times the mass of our Sun
- Largest knownTON 618Approx. 40 billion solar masses
- Our galaxy's centreSagittarius A*approx. 4 million solar masses
- Escape speed at the event horizonSpeed of light299,792 km/s, too fast for anything to reach
- Closest known to Earthapprox. 1,560 light yearsGaia BH1 in the constellation Ophiuchus
How does a black hole form?
Most stellar black holes form when a very massive star runs out of fuel and explodes as a supernova. The outer layers of the star are blasted out into space, while the heavy core collapses in on itself. If the leftover core is more than about 3 times the mass of our Sun, gravity wins. The core crushes itself smaller and smaller until it becomes a black hole.
The supermassive black holes at the centres of galaxies are far bigger, with millions or even billions of times the mass of the Sun. Scientists are still working out exactly how they grow that big.
Can anything escape?
Once something crosses the event horizon, it can never come back. To escape Earth's gravity, a rocket has to travel at about 11 km/s. To escape a black hole's gravity from inside the event horizon, you would have to travel faster than the speed of light, which nothing can do. Even light itself gets pulled back in. That is why a black hole looks completely black: no light can leave to show us what is inside.
Why do scientists study black holes?
Black holes are nature's most extreme laboratories. Their gravity is so strong, and the conditions around them so unusual, that they let scientists test our best theories of physics, especially Albert Einstein's general theory of relativity, in places no laboratory on Earth could ever copy. So far, every time scientists have checked, Einstein's theory has matched what they see almost perfectly.
Black holes also help shape the galaxies they live in. The supermassive black holes at the centres of galaxies appear to control how fast stars form around them. When two black holes spiral into each other and merge, they create ripples in space itself called gravitational waves, which scientists on Earth can now detect with giant L-shaped detectors called LIGO and Virgo. The first one was spotted in 2015.
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