A large association of very young stars of spectral types O and B, centred on the Orion Nebula, nearly 1500 l.y. away. It is about 400 l.y. across and contains all the main stars of Orion, except Betelgeuse, which is closer to us. The association is split into three main regions: Orion OB1a to the north-west of Orion’s Belt is the oldest, at about 8–12 million years; Orion OB1b, which includes the belt stars, is the next oldest; and Orion OB1c south of the belt is the youngest. The association is located adjacent to the Orion molecular cloud, which in turn is split into cloud A to the south and cloud B to the north. Cloud B is associated with NGC 2023/4 near the Horsehead Nebula, while cloud A is associated with the Orion Nebula and the Trapezium cluster of stars (part of Orion 1c). The area is still partly shrouded in gas from which stars continue to form.