猎户座内的马头星云是星空中最具辨识度的星云之一， 它是巨大黑暗分子云的一部分。马头星云亦称为巴纳德33, 19世纪末，其独特的形状被首次发现于一张照片底板上。星云赤红的辉光来自其后方被亮星参宿一电离的氢气。除了左下部是“脖子”的投影，“马头”的暗部主要由密不透光的尘埃构成。另外，贯穿星云的强大磁场，正迫使大量的气体飞离星云。尚处于孕育期的年轻恒星用星光点缀了马头星云。这张照片由夏威夷的3.5米口径的法加夏望远镜（CFHT）拍摄，实际呈现的是马头星云1500年前的样子。
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. The darkness of the Horsehead is caused mostly by thick dust, although the lower part of the Horsehead‘s neck casts a shadow to the left. Streams of gas leaving the nebula are funneled by a strong magnetic field. Bright spots in the Horsehead Nebula‘s base are young stars just in the process of forming. Light takes about 1,500 years to reach us from the Horsehead Nebula. The featured image was taken with the large 3.6-m Canada-France-Hawaii Telescope in Hawaii, USA.