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.
ToolsInteractive atlas
Related terms
- Emission nebula
A gas cloud lit up by hot young stars that make it glow, like a neon sign. Its hydrogen emits a red light that photos reveal; the eye mostly sees grey or pale green. The Orion Nebula (M42), about 1,350 light-years away, is visible to the naked eye.
- Reflection nebula
A dust cloud that does not glow by itself but scatters the light of nearby stars. Just as our sky scatters sunlight, it looks blue in photos. The haze around the Pleiades is an example; nebula filters do not help with them.
- Dark nebula
A dust cloud dense enough to hide the stars and glowing gas behind it. It shows up in silhouette, like the Horsehead in Orion or the Coalsack near the Southern Cross. The dark rifts in the summer Milky Way are made of them.
- Planetary nebula
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.
- 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.
← The whole glossary