Stars are formed and held together by gravity. At each stage of stellar development that we have considered, stars are prevented from collapsing further by the outward forces generated by fusion, electromagnetism, and electron or neutron degeneracy pressure. In this chapter we explore the fate of stellar remnants with more than 3 M_⊙, in which gravitation “wins” the battle, but eventually loses the war between the inward and outward forces.

When the gravitational force of an object is so great that it overcomes all opposing repulsive forces or pressures (like neutron degeneracy pressure), the object collapses in on itself. Its gravitational attraction then becomes so strong that nothing—not even light—can escape from it. When this happens, the matter and the space around it become a black hole.

Black holes inspire awe, fear, and uncertainty. Many people harbor the mistaken belief that black holes are giant vacuum cleaners destined to “suck up” all matter in the universe. Happily, the equations that describe black holes reveal that they are more benign than that, but they are still truly strange.

The idea that gravity could prevent matter from escaping was first put forward in the eighteenth century, but it was not until the twentieth century, when Albert Einstein presented his theories of relativity (explaining how motion, mass, and energy affect the behavior of everything in the universe), that the modern concept of a black hole developed. We must first explore his relativity theories in order to understand these exotic objects.