The Horsehead Nebula, also known as Barnard 33, is a small dark nebula silhouetted against the glow of the emission nebula IC 434. Only by chance does the dark nebula resemble the head of a horse - but its coincidental appearance has led to its becoming one of the most photographed objects in the sky.
This narrow patch of nebulosity extends a degree south of the bright star Zeta Orionis (Alnitak), the leftmost star in Orion's Belt. Amateur astronomers often use the Horsehead as a test of their observing skills; it is one of the more difficult objects to see visually in an amateur-sized telescope, requiring dark skies with excellent transparency. The Horsehead is best seen in long exposure photographs as a dark 4' x 6' notch against the 60' strip of faint nebulosity that is IC 434.
Although William Henry Pickering was official credited with its discovery in 1889, the Horsehead Nebula was first recorded on a photographic plate taken by Williamina Paton Fleming at the Harvard College Observatory in 1888. The first published description of the Horsehead Nebula was given by E. E. Barnard in 1913, and it was first cataloged by him in 1919.
Barnard 33 is the most interesting feature of a huge region of gas and dust situated 1,600 light years away in the constellation Orion. It is a dark globule of dust and non-luminous gas, obscuring the light coming from the moderately bright nebula IC 434 behind it. The red glow of IC 434 originates from ionized hydrogen gas. The bright, bluish reflection nebula near the Horsehead is NGC 2023.
The underside of the "neck" of the Horsehead is especially dark, and actually casts a shadow on the field below the "muzzle". The entire region is illuminated by the bright OB star Sigma Orionis, which is also responsible for ionizing the emission nebula IC 434. The much brighter Zeta Orionis is a foreground star, not related to the nebulosity.
The marked change in star density on either side of IC 434 indicates that this strip of glowing hydrogen marks the edge of a substantial dark cloud. As a cloud core emerging from its parent, and as an active site of low-mass star formation, the Horsehead is a fascinating, active, and complex neighborhood. The 'streamers' visible in the brighter region appear to outflowing matter, funneled by a strong magnetic field. Small red spots in the Horsehead Nebula's base are protostars in the process of forming, and red streaks near the yellowish nebula surrounding V615 Orionis are Herbig-Haro objects, which are jets of material ejected from protostars.