cassandra santiago's List: black holes

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• All matter in a black hole is squeezed into a region of infinitely small volume, called the central singularity. The event horizon is an imaginary sphere that  measures how close to the singularity you can safely get. Once you have passed the event horizon, it becomes impossible to escape: you will be drawn in by the  black hole's gravitational pull and squashed into the singularity.

   

  
  The size of the event horizon (called the Schwarzschild radius, after the German physicist who discovered it while fighting in the first World War) is  proportional to the mass of the black hole. Astronomers have found black holes with event horizons ranging from 6 miles to the size of our solar system. But in  principle, black holes can exist with even smaller or larger horizons.
• Some black holes spin around an axis, and their situation is more complicated. The surrounding space is then dragged around, creating a cosmic whirlpool.  The singularity is an infinitely thin ring instead of a point. The event horizon is composed of two, instead of one, imaginary spheres. And there is a region  called the ergosphere, bounded by the static limit, where you are forced to rotate in the same sense as the black hole although you can still escape.
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• Black holes obey all laws of physics, including the laws of gravity
• He first showed, based on the fact that light moves at a fixed  speed (671 million miles per hour), that space and time must be connected. Then in 1915, he showed that massive objects distort the four-dimensional space-time  continuum, and that it is this distortion that we perceive as gravity. Einstein's predictions have now been tested and verified through many different  experiments. For relatively weak gravitational fields, such as those here on Earth, the predictions of Einstein's and Newton's theories are nearly identical. But  for very strong gravitational fields, such as those encountered near black holes, Einstein's theory predicts many fascinating new phenomena.

• A black hole is an object that is so compact (in other words, has enough mass in a small enough volume) that its gravitational force is strong enough to prevent  light or anything else from escaping.
• The existence of black holes was first proposed in the 18th century, based on the known laws of gravity. The more massive an object, or the smaller its  size, the larger the gravitational force felt on its surface. John Michell and Pierre-Simon Laplace both independently argued that if an object were either  extremely massive or extremely small, it might not be possible at all to escape its gravity. Even light could be forever captured.
• The name "black hole" was introduced by John Archibald Wheeler in 1967.
• They form whenever massive but otherwise normal stars die. We  cannot see black holes, but we can detect material falling into black holes and being attracted by black holes.

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• Black holes are the evolutionary endpoints of stars at least 10 to 15 times  as massive as the Sun. If a star that massive or larger undergoes a  supernova  explosion, it may leave behind a fairly massive burned out stellar remnant.  With no outward forces to oppose gravitational forces, the remnant will  collapse in on itself. The star eventually collapses to the point of zero  volume and infinite  density, creating  what is known as a " singularity ". As the density increases, the path of light rays emitted from the  star are bent and eventually wrapped irrevocably around the star. Any emitted  photons are trapped into an orbit by the intense gravitational field; they  will never leave it. Because no light escapes after the star reaches this  infinite density, it is called a black hole.
• To be "sucked" into a black hole, one has to cross inside the   Schwarzschild radius. At this radius, the escape speed is equal to the  speed of  light, and once light passes through, even it cannot escape.
• However, if a black hole passes through a cloud of interstellar matter, or  is close to another "normal" star, the black hole can accrete  matter into itself. As the matter falls or is pulled towards the black hole, it  gains kinetic energy, heats up and is squeezed by tidal forces. The heating  ionizes  the atoms, and when the atoms reach a few million degrees  Kelvin, they emit  X-rays. The X-rays  are sent off into space before the matter crosses the Schwarzschild radius and  crashes into the singularity. Thus we can see this X-ray emission.

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