BARNARD'S LOOP AROUND THE HORSEHEAD NEBULA - 2009 FEBRUARY 24

Why is the Horsehead Nebula surrounded by a bubble?

Although hard to make out, the famous Horsehead Nebula is the slight dark indentation in the streak just to the left of the image center.

Glowing like an emission nebula, the origin of the bubble, known as Barnard's Loop, is currently unknown. Progenitor hypotheses include winds from bright Orion stars and the supernovas of stars long gone.

Barnard's Loop is too faint to be identifies with the unaided ey. 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 a small dark indentation, is the photogenic Flame Nebula.

See: http://apod.nasa.gov/apod/ap090224.html