Sh2-276, Barnard's Loop, is the largest emission nebula in the image above and one of the largest in the night sky. It is seen as a semi-circle around the belt and sword of Orion. At its estimated distance of 1600 light-years, the loop is 300 light-years across in space. This entire area is part of the Orion Molecular Cloud, a gigantic star forming region that covers most of the constellation of Orion including the Orion Nebula, Horsehead Nebula and Witch Head Nebula. Hot young stars in the Orion Nebula are believed to be powering the ionization that illuminates the nebulosity in the region. Barnard's Loop is thought to be part of the inner surface of a large hydrogen bubble that contains the Orion OB1 association, a group of hot young stars of spectral type O and B that have formed recently and that energize the nebula. The bubble may have been formed 3 million years ago by a series of supernova explosions. While Barnard's Loop is named after E. E. Barnard, who imaged the nebula and published a description in 1894, it was probably observed by William Herschel in, 1786. Barnard originally called it the "Orion Loop".