Astronomy Picture of the Day

Explanation: 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 supernovae 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 colour 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.