Black Holes :: space black hole

Introduction Black holes are one of the more interesting topics in astrophysics. Even though they are commonly accepted as being real, their actual existence has yet to have been proven2. Black holes are objects that have such a concentration of mass that nothing can escape their gravitational pull once it crosses the black hole's event horizon or Schwarzschild radius. The Schwarzschild radius is the radius where, for anything inside, the escape velocity is greater than the speed of light. Since the speed of light is the maximum possible velocity, nothing escapes from a black hole. They are given their name from the fact that they do not emit any light, since light is also pulled into them. History The term "black hole" was first introduced in 1971 by John Archibald Wheeler1. The actual phenomena the term describes was theorized to exist back in 1916 by Karl Schwarzschild. Then, it was just a curiosity as a possible solution to Einstein's general theory of relativity which described gravity as a curvature of space-time2. It wasn't until 1939 that Oppenheimer and Snyder seriously considered the possibility that massive stars could collapse and become what is now called black holes at the end of their lives. Today, black holes are a commonly accepted theory that even the average person has some idea of what they are. There was even a recent Associated Press story about two black holes in the NGC6240 galaxy that scientists predict will someday merge. Properties Black holes have many interesting properties. The most obvious from their name is the fact that they can't be seen since no light is emitted from them. One of the ways they can be detected is by the x-rays given off by the matter being pulled into them before it crosses the Schwarzschild radius. As the matter is pulled in, it gains kinetic energy, heats up, ionizes, and when it reaches a few million Kelvin, emits x-rays3. Black holes can also be detected by the way nearby objects are affected by their immense gravity. As you get nearer to a black hole, light begins to be bent toward it. Black holes have a radius around them that forms what is called the photonsphere. At this point, photons of light can actually orbit the black hole4.