# Astronomy: Black Holes

Topics: Black hole, General relativity, Schwarzschild radius Pages: 17 (6484 words) Published: April 7, 2013
Could a Black Hole Destroy Earth?
Black holes do not wander around the universe, randomly swallowing worlds. They follow the laws of gravity just like other objects in space. The orbit of a black hole would have to be very close to the solar system to affect Earth, which is not likely.

If a black hole with the same mass as the sun were to replace the sun, Earth would not fall in. The black hole with the same mass as the sun would keep the same gravity as the sun. The planets would still orbit the black hole as they orbit the sun now.

Will the Sun Ever Turn Into a Black Hole?
The sun does not have enough mass to collapse into a black hole. In billions of years, when the sun is at the end of its life, it will become a red giant star. Then, when it has used the last of its fuel, it will throw off its outer layers and turn into a glowing ring of gas called a planetary nebula. Finally, all that will be left of the sun is a cooling white dwarf star.

Could Earth ever get sucked into a black hole?
No, because the Sun is much too small to ever become a black hole. Even if it did, we are too far from the Sun for the gravitational pull to drag us in. Kerr black holes rotate, unlike static Schwarzschild black holes. Roy Kerr, a mathematician from New Zealand who studied rotating stars in the 1960s, hypothesized that since stars rotate, black holes probably rotate also. Kerr black holes can have ring-shaped singularities. Spinning black holes also have what's called a static limit. The static limit has a radius larger than the event horizon, and is sometimes called the edge, or outer boundary, of the black hole. Once you cross the static limit of a rotating black hole, you are subject to its effects, which in this case means that it is impossible to stay still—everything inevitably gets dragged along in the direction of rotation of the black hole. Since the static limit is outside the event horizon, however, crossing this boundary is not final. It is possible to enter and leave the zone between the static limit and the event horizon at will, and an object or light ray only gets trapped forever once it crosses the event horizon. Reissner-Nordstrøm nonrotating black holes are similar to static ones, but they are charged and have two event horizons. This type of black hole is thought to have an electric charge (called Q in physics equations), but it would likely attract the opposite charge as well, balancing it out. The outer event horizon is called the static limit, similar to the outer edge of a rotating black hole. Extra Features

Two other features can characterize a black hole - the accretion disk and jets. An accretion disk is matter that is drawn to the black hole. In rotating black holes and/or ones with a magnetic field, the matter forms a disk due to the mechanical forces present. In a Schwarzschild black hole, the matter would be drawn in equally from all directions, and thus would form an omni-directional accretion cloud rather than disk. The matter in accretion disks is gradually pulled into the black hole. As it gets closer, its speed increases, and it also gains energy. Accretion disks can be heated due to internal friction to temperatures as high as 3 billion K, and emit energetic radiation such as gamma rays. This radiation can be used to "weigh" the black hole. By using the doppler effect, astronomers can determine how fast the material is revolving around the black hole, and thus can infer its mass. Jets form in Kerr black holes that have an accretion disk. The matter is funneled into a disk-shaped torus by the hole's spin and magnetic fields, but in the very narrow regions over the black hole's poles, matter can be energized to extremely high temperatures and speeds, escaping the black hole in the form of high-speed jets.

They were actually the brainchild of John Mitchell.

AP
I always attributed the discovery of black holes to Einstein. While Einstein did revive the theory in 1916, John Mitchell...