Explanation:
The Horsehead
Nebula is one of the most famous nebulae on the sky.
It is visible as the dark indentation to the red
emission nebula seen just below
and left of center in the
this photograph.
The brightest star on the left is located in the belt of the familiar
constellation Orion.
The horse-head
feature is dark because it is really an opaque
dust cloud that lies in
front of the bright red
emission nebula.
Like clouds in
Earth's atmosphere, this cosmic cloud has
assumed a
recognizable
shape by chance.
After many thousands of years, the internal motions of the cloud
will alter its appearance.
The emission nebula's red color is caused by
electrons recombining
with
protons
to form
hydrogen atoms.
Also visible in the picture are blue
reflection nebulae that
preferentially reflect the blue light from
nearby stars.
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