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.
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.
- 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.
- Light pollution
Artificial lighting that brightens the night sky and drowns faint stars, nebulae and the Milky Way. A dome of light spreads for tens of kilometres around towns. It also harms wildlife and health; some towns now switch off street lights in the middle of the night.
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