A group of dozens of young, massive, and hot stars of spectral types O and B found in the spiral arms of galaxies. OB associations can be tens or hundreds of light years across. The stars are moving apart at several kilometres per second, so associations disperse among the background stars within about 30 million years. Famous examples are the Orion Association and the Sco–Cen Association.
Subjects: Astronomy and Astrophysics.