Image Description

This image is centered on the top of Orion's "sword" as viewed looking eastward on an early winter evening.  The easternmost of three "belt" stars, Zeta Orionis or or Alnitak, is the bright blue star left of center.  Brilliant nebulosity sweeps south down the sword.  Embedded in this nebulosity is the Horsehead dark nebula. 
View a Horsehead closeup.

The Horsehead is a fine examble of a Barnard catalog object or dark nebulae.  It is number 33 in this catalog.  NGC2024, below Zeta Orionis, is commonly referred to as the Flame or Christmas Tree.  Reflection nebulae surround several of the brighter stars. 
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Reflection nebulae are cold interstellar clouds of dust and gas which reflect light of nearby stars.  Red light passes through or is absorbed by these cold clouds while blue light is scattered and reflected, consequently, they usually appear blue in astrophotos.  Emission nebulae are hot ionized clouds of gasses, mostly hydrogen, which fluoresce with a red light when illuminated by ultraviolet radiation from nearby stars.