An R association is a group of young stars, typically less than a million years old, of intermediate mass, embedded in a reflection nebula. The nebula is illuminated by the stars, which unlike OB stars are not luminous enough to drive the material of the nebula away.

 

R associations are one of three recognized types of stellar association. The R classification was first suggested by the the Canadian astronomer Sidney van den Bergh (1929–).

 

Being relatively common, R associations are useful markers of the structure of our galaxy's spiral arms. An example is Vela R2, a very young group at a distance of about 2,800 light-years (850 pc).