Planetary Nebulae & White Dwarfs: Physical Mechanisms

A planetary nebula contains an expanding, often highly structured ionized shell surrounding a very hot central star. The central remnant contracts to approximately Earth size while retaining a substantial fraction of a solar mass and is supported against further collapse by electron degeneracy pressure.

Strong late-stage winds and pulsations remove the envelope of an asymptotic-giant-branch star. The exposed core becomes hot enough to ionize the expanding ejecta, causing the nebula to glow. With no sustained fusion, the white dwarf gradually radiates stored thermal energy and cools.

Imaging reveals expanding shells with characteristic emission lines, while spectra measure expansion speeds and ionization states. White-dwarf masses, radii, and temperatures match degenerate-matter models, including the counterintuitive trend that more massive white dwarfs are smaller.

Late stellar winds remove the envelope. As the hot core is exposed, ultraviolet radiation ionizes the expanding gas. The remnant is supported mainly by electron degeneracy pressure and cools because it has no long-lived internal fusion source.