Section 2 of 6
Nebulae
Gas lit from inside or from nearby. Gas made visible two different ways, and the one accessory that genuinely changes what reaches the eye.
A dying star lit from inside
A planetary nebula is a shell of shed atmosphere fluorescing under the naked core. The name is a mistake nobody corrected.
Emission and reflection
Gas that glows, and dust that only borrows light.
Dark nebulae
The objects defined by absence — cold dust blocking the stars behind.
The filter question
Why cutting most of the spectrum can make a nebula easier to see.