What are black holes?

Black holes vary in size. Some are only as big as a single atom, but their mass is huge. The mass is just the amount of matter or "stuff" in the object. 

There is another type of black hole that's called "stellar" these have up to twenty times more than the mass of the sun. There might be many stellar black holes in the Earth's galaxy (the Milky Way) 

The largest black holes are called "supermassive." This type of black holes have masses that total to more than 1 million suns together. Scientists have found proof that every large galaxy contains a supermassive black hole at its centre. The supermassive black hole at the centre of the Milky Way galaxy is called Sagittarius A. It has a mass equal to about 4 million suns and would fit inside a very large ball that could hold a few million Earths. 

NASA is using satellites and telescopes that are traveling in space to learn more about black holes.

How Do Black Holes Form?

Scientists believe that the black holes were formed when the universe was first formed. Stellar black holes are made when the center of a very big star falls in upon itself, or collapses. When this happens, it causes a supernova. A supernova is an exploding star that blasts part of the star into space. 

Scientists think supermassive black holes were made at the same time as the galaxy they are in, so for example made at the same time the Milky Way was formed. 

This image of the center of the Milky Way galaxy was taken by the Chandra X-ray Observatory. Image Credit: NASA/CXC/MIT/F.K. Baganoff et al.

If Black Holes are "black" why do scientists know they exist? 

A black hole can not be seen because strong gravity pulls all of the light into the middle of the black hole. However scientists can see how the strong gravity affects the stars and gas that surround the black hole. Scientists can study stars to find out if they are flying around, or orbiting, the black hole.

When a black hole and a star are close together, high-energy light is made. This kind of light can not be seen with human eyes. Scientists use satellites and telescopes in space to see the high-energy light. 

Could a Black Hole Destroy Earth?

Black holes do not go around in space eating stars, moons and planets. Earth will not fall into a black hole because no black hole is close enough to the solar system for Earth to do that. 

Even if a black hole the same mass as the sun were to take the place of the sun, Earth still would not fall in. The black hole would have the same gravity as the sun. Earth and the other planets would orbit the black hole as they orbit the sun now. 

The sun will never turn into a black hole. The sun is not a big enough star to make a black hole.