The shell of gas thrown off by a Sun-like star at the end of its life, lit by the hot core left behind. Nothing to do with planets: the name comes from their small disc, which reminded early observers of a planet. The Ring Nebula (M57) and the Dumbbell (M27) are classics.
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Related terms
- Nebula
A cloud of gas and dust in space. Depending on how it shines, or does not, it is called an emission, reflection, dark or planetary nebula. The word once also covered galaxies, whose nature was unknown.
- Nebula filter
A filter that passes only the few colours emitted by nebulae, notably ionised oxygen (OIII, around 500 nm) and hydrogen. It darkens the sky background and brings out emission and planetary nebulae, such as the Veil Nebula. It does not help with galaxies or star clusters.
- Supernova remnant
The expanding debris of a massive star that exploded. The Crab Nebula (M1) is what remains of the supernova seen in 1054; the Veil Nebula in Cygnus is far older and very large. An OIII filter helps a lot with the latter.
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