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Black Holes

What Are Black Holes and How Do They Form?


Computer artwork of black hole
Science Photo Library - MARK GARLICK/ Brand X Pictures/ Getty Images

Often the subject of science fiction novels, black holes are mysterious objects that, while very real, have a certain mythology that surrounds them. Some of these myths actually arise out of scientific truth, while others are the result of wild imagination. So what is fact and what is fiction? And where do black holes come from anyway?

What Is a Black Hole?

Simply put, a black hole is a region of space that is so incredibly dense that not even light can escape from the surface. However, it is this fact that often leads to miss-understanding. Black holes, strictly speaking, don't have any greater gravitational reach than any other star of the same mass. If our Sun suddenly became a black hole of the same mass the rest of the objects, including Earth, would be unaffected gravitationally. The Earth would remain in its current orbit, as would the rest of the planets. (Of course other things would be affected, such as the amount of light and heat that Earth received. So we would still be in trouble, but we wouldn't get sucked into the black hole.)

There is a region of space surrounding the black hole from where light can not escape, hence the name. The boundary of this region is known as the event horizon, and it is defined as the point where the escape velocity from the gravitational field is equal to the speed of light. The calculation of the radial distance to this boundary can become quite complicated when the black hole is rotating and/or is charged.

For the simplest case (a non-rotating, charge neutral black hole), the entire mass of the black hole would be contained within the event horizon (a necessary requirement for all black holes). The event horizon radius (Rs) would then be defined as Rs = 2GM/c2.

How Do Black Holes Form?

This is actually somewhat of a complex question, namely because there are different types of black holes. The most common type of black holes are known as stellar mass black holes as they are roughly up to a few times the mass of our Sun. These types of black holes are formed when large main sequence stars (10 - 15 times the mass of our Sun) run out of nuclear fuel in their cores. The result is a massive supernova explosion, leaving a black hole core behind where the star once existed.

The two other types of black holes are supermassive black holes -- black holes with masses millions or billions times the mass of the Sun -- and micro black holes -- black holes with extremely small masses, perhaps as small as 20 micrograms. In both cases the mechanisms for their creation is not entirely clear. Micro black holes exist in theory, but have not been directly detected. While supermassive black holes are found to exist in the cores of most galaxies.

While it is possible that supermassive black holes result from the merger of smaller, stellar mass black holes and other matter, it is possible that they form from the collapse of a single, extremely high mass star. However, no such star has ever been observed.

Meanwhile, micro black holes would be created during the collision of two very high energy particles. It is thought that this happens continuously in the upper atmosphere of Earth, and is likely to happen in particle physics experiments such as CERN. But no need to worry, we are not in danger.

How Do We Know Black Holes Exist If We Can't "See" Them?

Since light can not escape from the region around a black hole bound by the event horizon, it is not possible to directly "see" a black hole. However, it is possible to observe these objects by their effect on their surroundings.

Black holes that are near other objects will have a gravitational effect on them. Going back to the earlier example, suppose that our Sun became a black hole of one solar mass. An alien observer somewhere else in the galaxy studying our solar system would see the planets, comets and asteroids orbiting a central point. They would deduce that the planets and other objects were bound in their orbits by a one solar mass object. Since they would see no such star, the object would correctly be identified as a black hole.

Another way that we observe black holes is by utilizing another property of black holes, specifically that they, like all massive objects, will cause light to bend -- due to the intense gravity -- as it passes by. As stars behind the black hole move relative to it, the light emitted by them will appear distorted, or the stars will appear to move in an unusual way. From this information the position and mass of the black hole can be determined.

There is another type of black hole system, known as a microquasar. These dynamic objects consist of a stellar mass black hole in a binary system with another star, usually a large main sequence star. Due to the immense gravity of the black hole, matter from the companion star is funneled off onto a disk surrounding the black hole. This material then heats up as it begins to fall into the black hole through a process called accretion. The result is the creation of X-rays that we can detect using telescopes orbiting the Earth.

Hawking Radiation

The final way that we could possibly detect a black hole is through a mechanism known as Hawking radiation. Named for the famed theoretical physicist and cosmologist Stephen Hawking, Hawking radiation is a consequence of thermodynamics that requires that energy escape from a black hole.

The basic (perhaps oversimplified) idea is that, due to natural interactions and fluctuations in the vacuum (the very fabric of space time if you will), matter will be created in the form of an electron and anti-electron (called a positron). When this occurs near the event horizon, one particle will be ejected away from the black hole, while the other will fall into the gravitational well.

To an observer, all that is "seen" is a particle being emitted from the black hole. The particle would be seen as having positive energy. Meaning, by symmetry, that the particle that fell into the black hole would have negative energy. The result is that as a black hole ages it looses energy, and therefore loses mass (by Einstein's famous equation, E=Mc2).

Ultimately, it is found that black holes will eventually completely decay unless more mass is accreted. And it is this same phenomenon that is responsible for the short lifetimes expected by micro black holes.

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