We have compiled a significantly updated and comprehensive census of massive stars in the nearby Cygnus OB2 association by gathering and homogenising data from across the literature .
The census contains 169 primary OB stars , including 52 O-type stars and 3 Wolf-Rayet stars .
Spectral types and photometry are used to place the stars in a Hertzprung-Russell diagram , which is compared to both non-rotating and rotating stellar evolution models , from which stellar masses and ages are calculated .
The star formation history and mass function of the association are assessed , and both are found to be heavily influenced by the evolution of the most massive stars to their end states .
We find that the mass function of the most massive stars is consistent with a ‘ universal ’ power-law slope of \Gamma = 1.3 .
The age distribution inferred from stellar evolutionary models with rotation and the mass function suggest the majority of star formation occurred more or less continuously between 1 and 7 Myr ago , in agreement with studies of low- and intermediate mass stars in the association .
We identify a nearby young pulsar and runaway O-type star that may have originated in Cyg OB2 and suggest that the association has already seen its first supernova .
Finally we use the census and mass function to calculate the total mass of the association of 16500 ^ { +3800 } _ { -2800 } M _ { \odot } , at the low end , but consistent with , previous estimates of the total mass of Cyg OB2 .
Despite this Cyg OB2 is still one of the most massive groups of young stars known in our Galaxy making it a prime target for studies of star formation on the largest scales .