Black holes are the strangest objects in the universe. They are formed when a celestial body collects so much mass that the force of gravity overcomes the forces that hold atoms together. When that happens, the entire mass of the body collapses into a tiny point in space. This collapse often results in a supernova explosion. A massive amount of gravity then becomes condensed into a single point, and some very strange things happen. The laws that govern the normal three dimensional space break down. Gravity becomes so strong that the fabric of space folds in on itself. A spherical boundary forms between this region of folded space and the normal flat space around it. This spherical boundary is called an event horizon. The force of gravity inside this boundary is so strong that not even light can escape, so the boundary looks black. Hence the name “Black Hole.” Anything that falls inside of this region of space is lost to our universe forever. Nothing can escape a black hole. Every time something falls into the black hole, the event horizon gets bigger. The more mass in the black hole, the bigger the hole. Some black holes have been found that have billions of times the mass of our Sun. These are known as supermassive black holes.
While black holes can form anywhere in a galaxy, it’s now believed that nearly all galaxies in the universe have a supermassive black hole at the very center of the galaxy. These black holes act as an anchor for all of the gas and stars in the galaxy.
Black holes attract everything in their local environment. Clouds of gas and dust, and even stars and planets can be drawn into the gravitational field of a black hole. All of this matter orbits around the black hole in a hot disk known as the accretion disk. As matter falls closer to the black hole it speeds up and heats up until it glows white hot. While the black hole itself doesn’t emit any light, the accretion disk around the black hole glows brightly.
For the first time in history, a group of radio telescopes across planet Earth have succeeded in collecting pictures of black holes. The first was the supermassive black hole at the heart of the giant elliptical galaxy Messier 87, but the same group has now succeeded in taking a picture of the black hole at the heart of our Milky Way galaxy. Time-lapsed pictures also have now revealed the motion of the accretion disk around the black holes.
Sometimes two black holes become mutually trapped by their massive gravity fields. The two black holes orbit each other, falling closer together until they merge into a single larger black hole. These black hole mergers happen in seconds, and are the most violent events in the universe. The collision of the black holes creates ripples in the fabric of space, similar to the waves that are made by dropping a rock into water. These gravitational waves ripple all the way across the universe. Scientists on Earth have created instruments that can detect gravitational waves. The study of gravitational waves is teaching scientists about the formation and evolution of the universe.
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