Why are O-type and B-type stars often found in stellar associations?
Most of the brightest stars are older yellow stars, but a few young blue stars are also visible. This picture is a composite of three images taken by the Hubble Space Telescope.
O-type and B-type stars are frequently found in stellar associations—specifically OB associations—because they share a common origin and are relatively young.12
These massive stars form from the same large molecular cloud cores. Because they are born at essentially the same time in the same region of space, they have not yet had sufficient time to disperse far from their birthplace.31
The formation of an association rather than a bound star cluster depends on the efficiency of the star-formation process. If only a small fraction of the cloud's mass is converted into stars, the remaining gas is typically blown away by stellar winds or expanding H II regions. Without enough mass to remain gravitationally bound, the stars disperse, resulting in a loose, unbound association. Because O and B stars are very massive and have extremely short lifespans, their presence in these groups is a clear indicator of recent star formation.31