Reflection nebulae, such as M78 shown here, are visible because they reflect the light from a nearby star. The size of the particles in the nebula preferentially reflect blue light so we tend to see reflection nebulae as blue.

@evilscientistca Colors notwithstanding, how are astronomers able to discern reflection from emission nebulae? I'd imagine it'd have something to do with their respective spectra, but it's not clear to me how the spectra contributes to that knowledge.

For example, I'd imagine reflection nebulae to have absorption lines exactly where other types to have emission lines. Is that the case?

@vertigo Yes. Reflection nebula would have more continuum in their spectra and absorption lines. Emission nebula emit almost exclusively emission lines and little to no continuum.

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