Astronomy Picture of the Day . ... Credit Copyright: Nigel Sharp ( NOAO ), KPNO , AURA , NSF Explanation: One of the most identifiable nebulae in the sky, the Horsehead Nebula in Orion , is part of a large, dark, molecular cloud . 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 . ... Horsehead and Orion Nebulas . ...