Black holes are arguably the strangest and most mysterious objects in the universe. Their bizarre properties can challenge the laws of physics and even the very nature of reality itself. To understand black holes, we must learn to think "outside the box" and use a little imagination. Black holes are formed from the cores of super massive stars and can best be described as regions of space where so much mass is concentrated that nothing, not even light, can escape the gravitational pull. It is an area where the escape velocity is greater than the speed of light. The more massive an object is, the faster you have to travel to escape its gravity. This is known as the escape velocity. Black holes are so massive that their escape velocity is faster than the speed of light. Since nothing can travel faster than light, nothing can escape the gravity of a black hole. It was Einstein's general theory of relativity that provided the first clue to understanding black holes. His theory also states that gravity affects time. The more massive an object is, the more it can slow down time. The gravity of a black hole is so great that time nearly stands still. If you were on the outside of a black hole watching a spaceship fall in, you would see the spaceship appear to slow down until it finally disappears. A common myth about black holes is that they will suck in all matter around them. This is not true. They will suck in matter within a certain distance from them, but beyond that, they act no differently than a massive star. If our Sun could become a black hole, the planets would continue to orbit around it the same as they do today.
Since light cannot escape from the massive beasts, they can't be seen. To search for them, we must rely on indirect evidence for their existence. One way of searching for a black hole is to look for areas of space that exhibit a large amount of mass in a small, dark space. By searching for these types of objects, astronomers have found them in two main areas: in the centers of galaxies and in binary star systems within our own galaxy. In fact, most astronomers now believe that a super massive black hole may exist in the center of our own Milky Way galaxy. Does this mean that it will eventually swallow all of the matter in the galaxy? Actually, no. The black hole has the same mass as the original star that it was formed from. As long as nothing gets too close to the event horizon, it is safe. It is likely that the billions of stars in our galaxy will continue to orbit around this giant black hole for billions of years to come. The evidence for this and other black holes can be confirmed by searching for x-rays. Astronomers believe that black holes emit strong amounts of x-rays. Many of the stars in our galaxy exist as binary star systems. Sometimes one of the two stars in a binary system becomes a black hole. When this happens, the black hole may begin to suck matter from the other star. This matter swirls around the black hole in a formation known as an acceleration disk, moving faster and faster as it nears the center. It is believed that this matter emits radiation in the form of x-rays as it enters the black hole and is crushed out of existence. Binary star systems that emit strong amounts of x-rays are good black hole candidates. Once such a system has been identified, astronomers next try to determine the mass of the star's companion. By measuring the orbital speed of the visible star, they can figure out the mass of its invisible companion. If the mass of the companion object is large enough, then it may very well be a black hole. One of the likeliest candidates today for a black hole is Cygnus X-1. It is an intense x-ray radio source located in the constellation Cygnus.