The Horsehead Nebula, found in the
constellation Orion,
is probably one of the most identifiable
nebulae in the sky.
It is part of a dark
molecular cloud, and is shaped like - you guessed
it - a horsehead. It is visible as it is in front of a larger red diffuse nebula (IC 434) which consists mostly of
hydrogen.
It is located near Alnitak (or Zeta Orionis), which is the easternmost star in Orion's Belt, and whose radiation causes the background nebula
to glow.
It is approximately 1,500 light years away.