All objects have mass, which means they exert a force on other objects near them, which we call gravity. Gravity is the reason you are currently standing on the surface of the Earth, and not being thrown into space by the earth's rotation. To overcome gravity and get into Earth orbit, such as astronauts do to visit the International Space Station, you need to be travelling at a speed of over 17,000 mph! That is almost 500 times the speed limit in Guernsey! NASA Artist concept of a Black Hole To actually escape the Earth's gravity completely you need to be travelling around 25,000 mph. A dense object, which means an object with a lot of mass in a small volume, will have a strong gravitational pull on nearby objects and will require a much higher speed for objects to be able to escape its gravity. Scientists have shown that in a vacuum, light travels at a fixed speed of 299, 792, 458 metres per second.
Putting these facts together, clever scientists proposed a theoretical object with an escape velocity, higher than the speed of light. This means that not even light could escape! They called these objects black holes, because they don't emit any light and are very dark.
Event Horizon Telescope picture of SMBH in M87
We can't observe a black hole directly, but a lot of black holes have a disc of orbiting gas and dust which is very bright, due to friction in the disk. We can observe this accretion disk, which can have a temperature of over 1 milion °C and can shine really brightly with very high energy light, such as x-rays and gamma rays! Supermassive black holes are also thought to often have jets of gas coming out of the poles of the black hole at a very fast speed, as can be seen in the photo on the right.
We think that at the centre of most galaxies there are supermassive black holes. A black hole is a really massive object, which is so dense that even light can’t escape. The black hole at the centre of our galaxy is over 4 million times heavier than the Sun! Last year astronomers from all over the world worked together to take a picture of the black hole at the centre of a galaxy called M87. This was an incredible achievement that people thought was impossible only a few years before! Obviously you can’t see the black hole itself because it doesn’t emit any light, but we can see the hot gas and dust that is orbiting the black hole.
At the centre of our galaxy we think there is a supermassive black hole called Saggitarius A*, which is around 5 million times the mass of our Sun! You can see a timlapse video here of stars close to this black hole being affected by its strong gravity!
