A planetary nebula is expelled stellar gas illuminated and ionized by a hot central remnant after an evolved star loses its outer envelope. It belongs to late stellar evolution, despite its historical name suggesting a planet. Ultraviolet photons from the exposed hot core cause photoionization; subsequent atomic processes produce prominent emission lines.[ref]
Planetary Nebula by European Space Agency. Definition and explanation of ionization of the expelled outer layers by the central star
The planetary-nebula spectral energy distribution can combine those lines with continua and thermal emission from dust. Spectroscopy therefore measures more than a colour: line ratios diagnose gas conditions, while Doppler shifts trace expansion. The Zanstra method connects nebular light to the central star's ionizing output. The central remnant subsequently follows white-dwarf cooling as the surrounding gas disperses.
A planetary nebula is an astronomical phenomenon that occurs during the late stages of a star's evolution, particularly for stars of intermediate mass (generally 1 to 8 times the mass of the Sun). It is the result of a star exhausting the nuclear fuel in its core, leading to changes in its structure and composition.
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