Nebulae
Nebulae are insterstellar cloud of dust or gases.
They are distingued into 2 main families: diffuse nebulae and dark nebulae.
Diffuse ones may be emissives, or reflectives (light from a near star is reflected on the nebula).
When nebulae emit their own light (emissives), they can be categorized into different types: HII region, supernova remnant, Wolf-Rayet nebula or planetary nebula.
