SUMMARY OF KEY IDEAS

• Stars with higher masses fuse more elements into existence than do stars with lower masses.

• Stars lose mass via stellar winds throughout their lives.

• An intermediate-mass (between 0.4 and 8 M_⊙) main-sequence star becomes a giant when hydrogen shell fusion begins. It becomes a horizontal-branch star when core helium fusion begins. It enters the asymptotic giant branch and becomes a supergiant when helium shell fusion starts.

• Stellar winds during the thermal pulse phase eject mass from the star’s outer layers.

• The burned-out core of an intermediate-mass star becomes a dense carbon-oxygen body, called a white dwarf, with about the same diameter as that of Earth. The maximum mass of a white dwarf (the Chandrasekhar limit) is 1.4 M_⊙.

• Explosive hydrogen fusion may occur in the surface layer of a white dwarf in some close binary systems, producing sudden increases in luminosity that we call novae.

• An accreting white dwarf in a close binary system can also become a Type Ia supernova when carbon fusion ignites explosively throughout such a degenerate star.

• After exhausting its central supply of hydrogen and helium, the core of a high-mass (above 8 M_⊙) star undergoes a sequence of other thermonuclear reactions. These reactions include carbon fusion, neon fusion, oxygen fusion, and silicon fusion. This last fusion eventually produces an iron core.

• A high-mass star dies in a supernova explosion that ejects most of the star’s matter into space at very high speeds. This Type II supernova is triggered by the gravitational collapse and subsequent bounce of the doomed star’s core.

• Neutrinos were detected from Supernova 1987A, which was visible to the naked eye. Its development supported theories of Type II supernovae.

• The core of a high-mass main-sequence star containing between 8 M_⊙ and 25 M_⊙ becomes a neutron star. A neutron star is a very dense stellar corpse consisting of closely packed neutrons in a sphere roughly 20 km in diameter.

• A pulsar is a rapidly rotating neutron star with a powerful magnetic field tilted relative to the star’s rotation axis. The spinning field makes the neutron star a source of periodic radio and other electromagnetic pulses. Energy pours out of the polar regions of the neutron star in intense beams that sweep across the sky.

• Some X-ray sources exhibit regular pulses. These objects are believed to be neutron stars in close binary systems with ordinary stars.

• Explosive helium fusion may occur in the surface layer of a companion neutron star, producing a sudden increase in X-ray radiation, called an X-ray burster.

• 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.

• 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.

• 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.

• 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.

  386

• 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 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.