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.

Low-Mass Stars and Planetary Nebulae

• A low-mass (below 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 a low-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.

High-Mass Stars and Supernovae

• 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.

Neutron Stars and Pulsars

• 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.