Key Ideas

Late Evolution of Low-Mass Stars: A star of moderately low mass (about 0.4 M to about 4 M) becomes a red giant when shell hydrogen fusion begins, a horizontal-branch star when core helium fusion begins, and an asymptotic giant branch (AGB) star when the helium in the core is exhausted and shell helium fusion begins.

Planetary Nebulae and White Dwarfs: Helium shell flashes in an old, moderately low-mass star produce thermal pulses during which more than half the star’s mass may be ejected into space. This exposes the hot carbon-oxygen core of the star.

Late Evolution of High-Mass Stars: Unlike a moderately low-mass star, a high-mass star (initial mass more than about 4 M) undergoes an extended sequence of nuclear reactions in its core and shells. These include carbon fusion, neon fusion, oxygen fusion, and silicon fusion.

Core-Collapse Supernovae: A star with an initial mass greater than 8 M dies in a violent cataclysm in which its core collapses and most of its matter is ejected into space at high speeds. The luminosity of the star increases suddenly by a factor of around 108 during this explosion, producing a supernova.

White Dwarf Supernovae: An accreting white dwarf in a close binary system can also become a supernova when carbon fusion ignites explosively throughout such a degenerate star. This is called a thermonuclear supernova.

Neutron Stars: A neutron star is a dense stellar corpse consisting primarily of closely packed degenerate neutrons.

Pulsars: A pulsar is a source of periodic pulses of radio emission. These pulses are produced as beams of radio waves from a neutron star’s magnetic poles sweep past Earth.

Novae and Bursters: Material from an ordinary star in a close binary can fall onto the surface of the companion white dwarf or neutron star to produce a thin surface layer in which nuclear reactions can explosively ignite.