The Horsehead Nebula in Orion, is part of a large, dark, molecular cloud. The unusual shape was first discovered on a photographic plate in the late 1800s. The glow originates from hydrogen gas predominantly behind the nebula, ionized by the nearby bright star Sigma Orionis. The darkness of the Horsehead is caused by thick dust. Streams of gas leaving the nebula are funneled by a strong magnetic field. Light takes about 1500 years to reach us from the Horsehead Nebula.