For centuries, scientists have speculated about the existence of objects so dense that even light could not escape their gravitational fields. In the twentieth century, scientists became convinced that such objects, called black holes, could actually form. A black hole would form when a very massive star exploded at the end of its life, leaving a core three or more times as heavy as the Sun. No known force can prevent the core from collapsing into a black hole. In the Milky Way, there are typically one or two supernova explosions every century, so black holes should be common in our galaxy. It has been estimated that the Milky Way contains about 10 million black holes.
A black hole can be detected if it is in a binary system with a companion star circling close to it. The black hole’s gravity will pull material off the companion to form a swirling disk around the black hole. As this material spirals inward, it becomes so hot it can emit X-rays. In the 1970’s, with the advent of orbiting X-ray telescopes, astronomers found the first possible black hole binary, Cygnus-1, about 8,000 light-years away. Not only does Cygnus-1 emit X-rays, it’s bright star has a wobble that reveals a dark companion at least seven times as massive as the Sun; such a dark, heavy object could only be a black hole. Since then, hundreds of other possible black holes have been discovered in our galaxy. Observations from NASA’s Chandra X-ray telescope suggest that there are tens of thousands of dense compact objects near the center of the galaxy, many of them black holes. These black holes probably formed further out and will gradually sink into the center of the galaxy over billions of years.
A supermassive black hole lies at the heart of the Milky Way. Three million times as massive as the Sun, it is so large that its gravity prevents anything, even light, escaping from inside a radius of about 7.7 million kilometers, about twice the distance from Earth to the Moon. Astronomers have found that most, if not all, large galaxies have a supermassive black hole. Nobody knows how these black holes originated. They may have formed before the creation of galaxies, when giant clouds collapsed in the starless Universe, or they may have formed later when many smaller black holes merged within galaxies that already exited. Compared to the black holes in “active galaxies,” which give off enormous amounts of energy, the Milky Way’s black hole is quite dim. Other galaxies have black holes 100 million times brighter than the Milky Way’s. However, in 2003, a satellite named Integral revealed extremely energetic X-rays coming from a cloud of hydrogen called Sagittarius B2, about 350 light-years to one side of the black hole. This observation has led astronomers to deduce that only 350 years ago, the Milky Way was a million times brighter, viewed from Earth, than it is now. The fact that the Milky Way was an active galaxy so recently suggests that it might become one again in the future.
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