Reference Work Entry

Encyclopedia of Astrobiology

pp 1159-1159

OB Association

  • Steven StahlerAffiliated withDepartment of Astronomy, University of California Email author 


An OB association is a loose grouping of several thousand stars, a small fraction of which are of spectral type O and B. These latter are the most massive members of the group and also the most luminous, allowing identification of OB associations out to great distances. In external spiral galaxies, they trace the spiral arms. OB associations range in size, with diameters from tens of parsecs to about 100 parsecs. It is thought that all the stars formed together in a tighter configuration, and observations of stellar velocities reveal that the groups are in a state of expansion. The oldest recognizable OB associations have ages of order 10 million years.

See also

Elephant Trunks

Pre-Main-Sequence Star

Stellar Cluster

T Association

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