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.

• As a moderately low-mass star ages, convection occurs over a larger portion of its volume. This takes heavy elements formed in the star’s interior and distributes them throughout the star.

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.

• Ultraviolet radiation from the exposed core ionizes and excites the ejected gases, producing a planetary nebula.
• No further nuclear reactions take place within the exposed core. Instead, it becomes a degenerate, dense sphere about the size of Earth and is called a white dwarf. It glows from thermal radiation; as a white dwarf cools, it becomes dimmer.

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.

• In the last stages of its life, a high-mass star has an iron-rich core surrounded by concentric shells hosting the various nuclear reactions. The sequence of nuclear reactions stops here, because the formation of elements heavier than iron requires an input of energy rather than causing energy to be released.

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.

• More than 99% of the energy from a core-collapse supernova is emitted in the form of neutrinos from the collapsing core.
• The matter ejected from the supernova, moving at supersonic speeds through interstellar gases and dust, glows as a nebula called a supernova remnant.
• A Type II supernova is the result of the collapse of the core of a massive star, as are supernovae of Type Ib and Type Ic.

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.

• One scenario for Type Ia supernovae is an accreting white dwarf in a close binary with a star; another scenario involves the merger of two white dwarfs.

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

• Neutron stars form in a Type II core-collapse supernova. They are held up from further collapse by degenerate neutron pressure.
• A neutron star typically has a diameter of about 20 km, a mass less than 3 M_⊙, a magnetic field 10¹² times stronger than that of the Sun, and a rotation period of roughly 1 second.

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.

• Explosive hydrogen fusion may occur in the surface layer of a companion white dwarf, producing the sudden increase in luminosity that we call a nova.
• Explosive helium fusion may occur in the surface layer of a companion neutron star. This produces a sudden increase in X-ray radiation, which we call a burster.