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Emission nebulae are usually red, because hydrogen, the most common gas in the universe, most commonly emits red light. Reflection nebulae are clouds of dust that simply reflect the light of a nearby star or stars. Reflection nebulae are usually blue, because blue light scatters more easily.Emission and reflection nebulae are often seen together and are sometimes both referred to as diffuse nebulae.In some nebulae, the star formation regions are so dense and thick that light cannot get through.eflection nebulae are usually blue, because blue light scatters more easily. Emission and reflection nebulae are often seen together and are sometimes both referred to as diffuse nebulae. In some nebulae, the star formation regions are so dense and thick that light cannot get through.
The Horsehead Nebula, one of the most famous dark nebulae, is located in the constellation Orion. 1  Emission nebulae are huge bright gas clouds with irregular shapes and dimensions that span many light years across. Emission nebulae are where stars are born. When stars begin to shine, they heat up the remaining gas around them and that gas starts to shine as well. One of the most famous emission nebula is M42, which is located in the constellation Orion.