Summary of Key Ideas

• If a stellar corpse is more massive than about 3 M_⊙, gravitational compression overcomes neutron degeneracy and forces it to collapse further and become a black hole.
• A black hole is an object so dense that the escape velocity from it exceeds the speed of light.

The Relativity Theories

• Special relativity reveals that space and time are intimately connected and change with an observer’s relative motion.
• As seen by observers moving more slowly, the faster an object moves, the slower time passes for it (time dilation) and the shorter it becomes (length contraction).
• According to general relativity, mass causes space to curve and time to slow down. These effects are significant only near large masses or compact objects.

Inside a Black Hole

• The event horizon of a black hole is a spherical boundary where the escape velocity equals the speed of light. No matter or electromagnetic radiation can escape from inside the event horizon. The distance from the center of the black hole to the event horizon is the Schwarzschild radius.
• The matter inside a black hole collapses to a singularity. The singularity for nonrotating matter is a point at the center of the black hole. For rotating matter, the singularity is a ring inside the event horizon.
• Matter inside a black hole has only three physical properties: mass, angular momentum, and electric charge.
• Nonrotating black holes are called Schwarzschild black holes. Rotating black holes are called Kerr black holes. The event horizon of a Kerr black hole is surrounded by an ergoregion in which all matter must constantly move to avoid being pulled into the black hole.
• Matter that approaches a black hole’s event horizon is stretched and torn by the extreme tidal forces generated by the black hole, light from the matter is redshifted, and time slows down.
• Black holes can evaporate by the Hawking process, in which virtual particles near the black hole become real. These transformations of virtual particles into real ones decrease the mass of a black hole until, eventually, it disappears.

Evidence of Black Holes

• Observations indicate that some binary star systems harbor black holes. In such systems, gases captured by the black hole from the companion star heat up and emit detectable X-rays and jets of gas.
• Supermassive black holes exist in the centers of most galaxies. Intermediate-mass black holes appear to exist in globular clusters of stars. Very low mass (primordial) black holes may have formed at the beginning of the universe.

Gamma-Ray Bursts

• Gamma-ray bursts are events believed to be caused by some supernovae and by the collisions of dense objects, such as neutron stars or black holes. Some occur in the Milky Way and nearby galaxies, whereas many others occur billions of light-years away from Earth.
• Typical gamma-ray bursts occur for a few tens of seconds and emit more energy than the Sun will radiate over its entire 10-billion-year lifetime.