(Courtesy to Microsoft Encarta for these photographs.)

Throughout the life of a star, it burns up its fuel of hydrogen. As it hits rock bottom of hydrogen, sometimes, if it is large enough, it can fuse together other elements into heavier elements until finally, it cannot fuse together the final heaviest element anymore and the star dies. Although a star eventually will die, its fuel reserves are huge. For example, the Sun converts 4 million tons of its own matter into energy per second.

There are different phases in a star's life other than a protostar and its normal self. If the star is about the size of the Sun, it will eventually grow into a what is called a red giant. This phase of the star is about a hundred times larger than its original self. This happens because when the star runs out of hydrogen, it starts to fuse together helium and it requires more energy to fuse helium. Gravity pushes down more on the star's core when it fuses helium and the outsides are pushed out and cool down. That is why it is a red color. The same thing happens to a very large star, except it becomes a red supergiant.

There are different ways stars die, and all of them usually depend on the size and the mass of the star. Our Sun will probably become a red giant someday and eventually expell its outer layers off. Those outer layers are a planetary nebula and what's left of the Sun is a very dense surface of its core. Gravity has collapsed the Sun's core until the inner particles cannot be packed anymore. A white dwarf is extremely dense and the smaller it is, the heavier it weighs. A matchbox of an average white dwarf's material would weigh as much as an elephant! Because nuclear fusion does not occur anymore, the white dwarf slowly cools down and becomes what is called a brown dwarf. So far there have been no brown dwarf sightings because they are hard to see in the night sky. All red giants die this way, but red supergiants die in a different way. Red supergiants' do not last very long in this expanded state, only about a hundred thousand years. What happens next is amazing. These huge stars die out with a bang called a supernova. This happens when the star's core tries to fuse iron, but instead of releasing energy, the iron core takes in energy, therefore the energy bounces off of the core and blasts out the star's outer layers. This explosion was powered by very vast amounts of neutrinos. After the supernova explosion, a planetary nebula is also created.

When the red supergiant explodes in a supernova, the star's core still remains. This core, if its mass is less than three *Solar masses, it becomes a spinning neutron star. To get an idea of how dense a neutron star is, one pinhead of neutron star material would weigh the same amount as a million tons or twice as much as the world's heaviest supertanker. Also, if the core collapsed in only a few seconds, it might even become a pulsar. These are neutron stars that beam out radiation as they spin. Pulsars usually spin about once per second, but the highest recording of pulsar rotation is 642 times per second. If the star's core after the supernova is larger than three *Solar masses, gravity pushes the core until it becomes a black hole.

Black holes were first discovered in 1989 in the Cygnus constellation by a Japanese satellite. Later on, at the William Herschel Telescope in the Canary Islands, to look for the black hole. Astronomer's found a star in a duel star system that was in orbit around an object that was smaller than the star. The difference was that the smaller object had many more Solar masses than the star. This black hole came to be known as the V404 Cygni black hole. Other black holes were also discovered afterward. Some of them are the LMC X-3 and the Cygnus X-1 black holes.

Like a human, black holes have different parts to them. All of them have an accretion disk, an event horizon, and a singularity. The singularity is what's left of the star's core. The accretion disk is the orbitting gas and dust that eventually gets swallowed up by the gravity of the black hole. As gases and other particles hit the accretion disk, it sends out x-rays. This is how scientists can find out about black holes, by detecting the x-rays using radio wave telescopes or radio telescopes. Anything passing the event horizon will have no chance getting out of the black hole. If anything hits the singularity, it is immediately destroyed. Singularities are what is left of the star core. Black holes also have an immense gravitational force, and even light cannot escape the attraction force of a black hole.

There are different kinds of black holes. Black holes can also have a spin and/or a charge. Three of the main kinds of black holes are the Schwarzschild, Reissner-Nordstrom, and Kerr black holes. Of all of these kinds of black holes, the simplest in composition is the Schwarzchild black hole. It only has a singularity and an event horizon. Reissner-Nordstrom black holes have a charge but have no spin to them, but they have two event horizons. In the space between the outer event horizon and the inner event horizon, matter is pushed inward toward the inner event horizon. What's different is that once anything is inside the inner event horizon, it is not sucked inward anymore. A Kerr black hole has spin and it also has two event horizons. Outside of the two event horizons, there is something called an ergosphere. In the ergosphere, objects are spun around and sucked inward at the same time. In a Kerr black hole, the singularity is turned into a oval shaped ring.

*= Solar mass or one mass of the Sun.
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