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
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.
