They are often associated with more diffuse nebulosity, as well. A typical open cluster is less than 50 light-years across. Emission Nebulae Emission nebulae are clouds of high temperature gas. Emission nebulae are usually the sites of recent and ongoing star formation. Reflection Nebulae Reflection nebulae are clouds of dust which are simply reflecting the light of a nearby star or stars.
Reflection nebulae are also usually sites of star formation.
They are usually blue because the scattering is more efficient for blue light. Reflection nebulae and emission nebulae are often seen together and are sometimes both referred to as diffuse nebulae. Dark nebulae are clouds of dust which are simply blocking the light from whatever is behind. Dark nebulae are also often seen in conjunction with reflection and emission nebulae. A typical diffuse nebula is a few hundred light-years across.