OB association

A gravitationally unbound grouping of O class and B class stars. These young hot stars form within giant molecular clouds in the disks of spiral galaxies. OB associations often contain 10-100 massive stars grouped into one or more star clusters, with multiple O stars in their cores. In addition, these associations also contain hundreds or thousands of low- and intermediate-mass stars. These associations are typically the most visually obvious features of spiral galaxies and often mark spiral structures such as arms, arcs or rings.

Association members are believed to form within the same small volume inside a giant molecular cloud. Once the surrounding dust and gas is blown away, the remaining stars become unbound and begin to drift apart. It is believed that the majority of all stars in the Milky Way were formed in OB associations. O class stars are short-lived, and will expire as supernovae after roughly a million years. As a result, OB associations are generally only a few million years in age or less. The O-B stars in the association will have burned all their fuel within 10 million years.

An example is the Scorpius-Centaurus Association. OB associations have also been found in the Large Magellanic Cloud and the Andromeda Galaxy. These associations can be quite sparse, spanning 1,500 light years in diameter.