= Planetary nebula
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= Planetary nebulae
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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>.<Planetary Nebula by European Space Agency>[Definition and explanation of ionization of the expelled outer layers by the central star]{ref}
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.
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