Planetary Nebulae & White Dwarfs: Structures & Vocabulary

Planetary nebulae are glowing shells of gas expelled by dying low- and intermediate-mass stars, while white dwarfs are the compact stellar cores left behind. The two are linked evolutionary stages despite the misleading historical term 'planetary nebula.'

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.

Planetary nebulae show shells, bipolar lobes, knots, and jets surrounding very hot central stars. White dwarfs compress roughly stellar mass into Earth-like radii and may have hydrogen- or helium-dominated atmospheres over carbon-oxygen, helium, or oxygen-neon interiors.