The Horsehead Nebula in Orion 1,500 light years from us, one of the most identifiable nebulae in the sky, is part of a large, dark, molecular cloud.
The above image is a digital combination of images taken in blue, green, red, and hydrogen-alpha light from the Argentina, and an image taken in infrared light by the orbiting Hubble Space Telescope. image credit: Optical: Aldo Mottino & Carlos Colazo, OAC, Córdoba; Infrared: Hubble Legacy Archive
Also known as Barnard 33, the unusual shape was first discovered on a photographic plate in the late 1800s. The red glow originates from hydrogen gas predominantly behind the nebula, ionized by the nearby bright star Sigma Orionis.
The darkness of the Horsehead is caused mostly by thick dust, although the lower part of the neck casts a shadow to the left. Streams of gas leaving the nebula are funneled by a strong magnetic field. Bright spots in the base are young stars just in the process of forming.