Introduction

Our Galaxy and others in the Universe harbor many young clusters and associations that each contain hundreds to thousands of massive OB stars, and as many as a million low-mass stars. These massive star forming regions are the major star factories in the Universe and their astrophysical scope is vast, influencing galactic evolution at one extreme, and terms in the Drake Equation that determines the probability of extra-terrestrial intelligence, at the other. They are a mini-universe of starburst astrophysics in which super- and hypernovae, O-star and Wolf-Rayet winds inject nuclear-processed material into galactic disks and halos, whilst shaping and eroding parent molecular clouds, and triggering new waves of star formation via compression waves and ionization fronts. Such regions likely dominate cosmic ray acceleration in galaxies. Understanding how these regions work is crucial to our knowledge of the first stars and starburst galaxies, galactic nucleosynthesis, and element mixing in the ISM, as well as star and planet formation processes and feedback in our own Galaxy.

Studies of our Galaxy's super-cluster candidates is generally hampered by their great distances (Arches at 10 kpc, W49 at 12 kpc, NGC 3603 at 6 kpc. Westerlund 1 at 5.5 kpc). There is just one exception. In the last few years realization has grown that Cygnus OB2 (D = 1.5 kpc), the pre-eminent cluster and central engine of the massive molecular-gas dominated Cygnus X region approaches supercluster dimensions. With an age of 2-5 Myr, it has been estimated to contain about 2000 OB stars, of which as many as 100 may be O-type stars. Cygnus OB2 belongs to the elite club of massive clusters in hosting two O3 stars, of which only ~10 Galactic examples are known. Cygnus OB2 is therefore among the most extreme OB associations in our Galaxy. It is potentially dense and massive enough to stand comparison with globular clusters and the superclusters that characterize starbursts.

To read more about Cygnus OB2:
- Massey & Thompson (1991), AJ, 101, 4.
- Knodlseder (2000), A&A, 360, 539.
- Wright et al. (2010), ApJ, 713, 871.

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