An Etymological Dictionary of Astronomy and Astrophysics

An → OB association consisting of several dozen → hot stars of → spectral types O and B (→ O star, → B star). The Orion OB1 association consists of several subgroups, first divided by Blaauw (1964) into four subgroups. The subgroups differ in age and gas and dust content:

Orin OB1a, which contains the stars to the northwest of → Orion’s Belt stars. Within this group lies another subgroup, known as the → 25 Orionis group;

Orion OB1b, containing the group of stars located in and around the Belt;

Orion OB1c, including the stars around → Orion’s Sword; and

Orion OB1d, which contains the stars in and close to the → Orion Nebula (including the → Trapezium cluster).

See also: → Orion; → OB association.