National Aeronautics and Space Administration

Black holes can be said to "come in all sizes", meaning that they have a wide range of masses. There are at least two different types of black holes. The types differ by their masses. We are perhaps most familiar with "stellar-size black holes"; these are the black holes which form from the death of a very massive single star. They tend to have masses in the range of a few to a few tens of solar masses. Next, there are what are called the "supermassive black holes"; these objects have the mass of a few billion to hundreds of billions of solar masses. They exist in the centers of galaxies.

The most common types of black holes have a mass of between about four and a few tens of solar masses. They are the remains of supernovae - the explosions of massive stars. To understand how such black holes can form, let us briefly review the life cycle of a massive star.

For stars some 10 or more times as massive as our Sun, fate has something very special in store when they begin to run out of hydrogen to fuse into helium. After the outer layers of the star have swollen into a red supergiant (i.e., a very big red giant), the core begins to yield to gravity and starts to shrink. As it shrinks, it grows hotter and denser, and a new series of nuclear reactions begin to occur, temporarily halting the collapse of the core. However, when the core becomes essentially just iron, it has nothing left to fuse (because the nuclear structure of iron does not permit its atoms to fuse into heavier elements) and fusion ceases. In less than a second, the star begins the final phase of its gravitational collapse. The core temperature rises to over 100 billion degrees as the iron atoms are crushed together. The repulsive force between the nuclei overcomes the force of gravity, and the core recoils out from the heart of the star in an explosive shock wave.

So what, if anything, remains of the core of the original star? Unlike in smaller stars, where the core becomes essentially all carbon and stable, the intense pressure inside the supergiant causes the electrons to be combined with the protons, forming neutrons. In fact, the whole core of the star becomes nothing but a dense ball of neutrons. It is possible that this core will remain intact after the supernova, and be called a neutron star. However, if the original star was very massive (say 10 or more times the mass of our Sun), even the neutrons will not be able to withstand the core collapse and a black hole will form.

Such black holes may exist all by themselves in the vast reaches of space, or may be part of a binary system of stars. There are certain conditions under which a star can start out in a binary system (the usual condition for a star), undergo a supernova explosion, and yet still remain locked into the binary system. It is possible in such systems that there will be a flow of gas from the outer layers of the normal star into the gravitational field of the black hole companion. This gas cannot simply fall onto the black hole, the orbital motion of the pair of stars will make it go into rotation and form a disk around the black hole. Friction between the disk components will heat the gas to 10,000,000 Kelvin long before it reaches the event horizon, and a gas of such temperature emits X-rays.