OB associations and their origins
  OB associations are unbound groups of young stars made prominent by their
bright OB members, and have long been thought to be the expanded remnants of
dense star clusters. They have been important in astrophysics for over a
century thanks to their luminous massive stars, though their low-mass members
have not been well studied until the last couple of decades. This has changed
thanks to data from X-ray observations, spectroscopic surveys and astrometry
from Gaia that allows their full stellar content to be identified and their
dynamics to be studied, which in turn is leading to changes in our
understanding of these systems and their origins, with the old picture of
Blaauw (1964) now being superseded. It is clear now that OB associations have
considerably more substructure than once envisioned, both spatially,
kinematically and temporally. These changes have implications for the star
formation process, the formation and evolution of planetary systems, and the
build-up of stellar populations across galaxies.
