Emission Nebula
A cloud of ionized gas emitting light of various colors, often found in regions of active star formation and contributing to the reddish hues observed in galaxies.
Source: esa.int
APODs including "Emission Nebula"
Barnard's Loop around the Horsehead Nebula
17/04/2006
Why is the Horsehead Nebula surrounded by a bubble? Although glowing like an emission nebula, the origin of the bubble, known as Barnard's Loop, is currently unknown. Progenitor hypotheses include the winds from bright Orion stars and the supernovas of stars long gone. Barnard's Loop is too faint to be identified with the unaided eye. The nebula was discovered only in 1895 by E. E. Barnard on long duration film exposures. The above image was taken in a single specific color emitted by hydrogen to bring out detail. To the left of the Horsehead Nebula, visible as the small dark indentation near the image top, is the photogenic Flame Nebula.