2020 Nobel Prize in physics has been awarded to Roger Penrose, Reinhard Genzel, and Andrea Ghez for furthering the understanding of black holes. What Is a Black Hole? A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because the matter has been squeezed into a tiny space. This can happen when a star is dying. Because no light can get out, people can't see black holes. They are invisible. Space telescopes with special tools can help find black holes. How Big Are Black Holes? Black holes can be big or small. Scientists think the smallest black holes are as small as just one atom. These black holes are very tiny but have the mass of a large mountain. Mass is the amount of matter, or "stuff," in an object. Another kind of black hole is called "stellar." Its mass can be up to 20 times more than the mass of the sun. There may be many, many stellar-mass black holes in Earth's galaxy. Earth's galaxy is called the Milky Way . How Do Black Holes Form? Scientists think the smallest black holes formed when the universe began. Stellar black holes are made when the centre 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. If Black Holes Are "Black," How Do Scientists Know They Are There? A black hole can not be seen because strong gravity pulls all of the light into the middle of the black hole. But scientists can see how strong gravity affects the stars and gas around the black hole. Scientists can study stars to find out if they are flying around, or orbiting, a black hole.