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.
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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.
- 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.
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