Horsehead Nebula, optical image. North is at left. The distinctive horsehead shape is formed by the dark nebula B33 being silhouetted against the bright emission nebula IC 434. The blue-white area at lower left is the reflection nebula NGC 2023. An emission nebula is a cloud of gas and dust that glows as the hydrogen gas it contains is ionised by radiation from hot stars within it. A reflection nebula does not emit its own light, but reflects the light of stars in or around it. This complex of gas and dust lies around 1500 light years from Earth in the constellation Orion.