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What you will learn in Nebulae

1. Four kinds of nebula
Gas glowing by itself, dust scattering starlight, dust blocking light, and a dying star's shed envelope — plus the wreck of an exploded one. What a nebula looks like is decided by how it makes its light.
2. Why they glow
A star hot enough to ionise hydrogen lights the gas around it. At 37,100 K, 17% of θ¹ Orionis C's photons can; at 14,500 K, only 0.11% of Merope's can, which is why the Pleiades reflect and Orion glows.
3. What colour is a nebula?
Through an eyepiece, grey. In a Hubble-palette image, gold and teal, because someone assigned hydrogen's red light to green. Every colour picture of a nebula is a choice, and you can make your own.
4. What you would actually see
Andromeda is magnitude 3.4 and yet invisible from most places, because its light is spread over six full Moons of sky. Integrated magnitude predicts visibility only for points.

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1. Four kinds of nebula
Gas glowing by itself, dust scattering starlight, dust blocking light, and a dying star's shed envelope — plus the wreck of an exploded one. What a nebula looks like is decided by how it makes its light.
2. Why they glow
A star hot enough to ionise hydrogen lights the gas around it. At 37,100 K, 17% of θ¹ Orionis C's photons can; at 14,500 K, only 0.11% of Merope's can, which is why the Pleiades reflect and Orion glows.
3. What colour is a nebula?
Through an eyepiece, grey. In a Hubble-palette image, gold and teal, because someone assigned hydrogen's red light to green. Every colour picture of a nebula is a choice, and you can make your own.
4. What you would actually see
Andromeda is magnitude 3.4 and yet invisible from most places, because its light is spread over six full Moons of sky. Integrated magnitude predicts visibility only for points.