Alnilam nebula (NGC 1990), optical image. North is at left. The Alnilam nebula (blue cloud) is a faint reflection nebula in the middle of Orion's belt, in the constellation Orion. Reflection nebulae are clouds of gas and dust which do not emit light of their own, but rather reflect the light of bright stars around them. They appear blue because blue light is scattered more than red. The light of Alnilam (Epsilon Orionis), the central star of Orion's belt, is just visible at top right. This image is based on data from the Oschin Telescope at the Mount Palomar Observatory.