A large, faint emission nebula about 1500 l.y. away in Orion, discovered by E. E. Barnard. It is a semicircular arc, 14° across, centred approximately on the Orion Nebula. Powerful young stars from the Orion OB1 association ionize the loop, which is thought to have been formed by a supernova explosion 3 million years ago. It is expanding at 10–20 km/s.
Subjects: Astronomy and Astrophysics.