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Formation
When a star dies, one of several different things can happen based on the size of the star.

In medium-sized stars, like the sun, once the core stops burning hydrogen, the core starts to shrink. This begins the burning of shell hydrogen since the surrounding hydrogen is being heated. The outside of the star expands and cools because of the enormous amount of energy being produced (Shipman, 30). These stars are called "red giants." Stars smaller then four solar masses are not hot enough to begin thermonuclear reactions that use carbon or oxygen as fuel. These stars instead end up with just a hot carbon-oxygen core because the outer layers are eliminated. The core cools and the star becomes a "white dwarf," which is about the same size as earth - relatively small compared to other stars in the universe.

However, is a star is over four times larger than the sun, its death is much more dramatic. At first it goes through a similar process as the medium-sized stars, burning and expanding out to a "red supergiant." Once it runs out of fuel, it begins to collapse, but because of its huge mass the reactions get out of control and result in a huge explosion - a supernova. When a supernova occurs, most of the star's matter is blown out into space. All that is left is an extremely small, extremely dense core, made of electrons and protons that have combined to make neutrons. These are called neutron stars (Moore, 71). As they collapse, neutron stars begin to spin more rapidly because they become more compact. Some of these rapidly spinning neutron stars develop very strong magnetic fields near its surface. The stars emit radiation in two beams, which can be picked up by our telescopes. When one of the beams is pointed towards earth, we see a pulse of radiation, one for every rotation of the neutron star (Freedman, 528). This type of neutron star is called a pulsar.

If a star is more than ten times the size of the sun, there is no supernova. Once it runs out of fuel, the star begins to collapse. The gravity of the star is so strong since the burning of the star's fuel is not exerting an outward force that it begins to collapse and cannot stop. The matter collapses inside its own Schwarzschild radius, leaving so much matter packed into such a small space that not even light can escape its gravity. This is a black hole.

This image is an artist's rendition of a supermassive black hole, with matter being pulled into it "like water down the drain of a bathtub."

Black Holes in the Center of Galaxies
No galaxies have currently been proven to have a black hole at its center, but many astronomers believe that many galaxies may. Astronomers, using instruments lie the Hubble Space Telescope, have discovered that many, maybe even all, large galaxies have a black hole in the center. This is because in these galaxies studied, the rotation of stars around the center of the galaxy increases as you move closer to the center of the galaxy. This means that in order to keep these stars in orbit, the object in the middle must be millions of times more massive than the Sun. When the available volume in the middle of the galaxy is considered, combined with how huge the mass must be, it becomes very likely that it is a black hole in the center of these galaxies.

Joseph Weber, an astronomer at the University of Maryland, found "gravitational waves" radiating from our galaxy's center (Moore, 107). If his findings are true, and these waves are being radiated in all directions from the center of the galaxy, it presents and interesting possibility for the center of our own galaxy. This could mean that, because the signal is so strong, that a black hole must be at the center of our own Milky Way galaxy. If this is true, it means that thousands of stars are being pulled into the black hole every year. However, there has not been much further evidence to back up this claim, so it is not considered a definite fact yet.
Did you know?

Did you know that time does not exist inside a black hole?
   
 
Astronomy 9: Concepts of the Cosmos (Spring 2002)