The morphology and stellar content of the Cygnus OB2 association has been determined using 2MASS infrared observations in the J , H , and K bands .
The analysis reveals a spherically symmetric association of \sim 2 ^ { \circ } in diameter with a half light radius of 13 ^ { \prime } , corresponding to R _ { h } = 6.4 pc at an assumed distance of 1.7 kpc .
The interstellar extinction for member stars ranges from A _ { V } \approx 5 ^ { m } to 20 ^ { m } , which led to a considerable underestimation of the association size and richness in former optical studies .
From the infrared colour-magnitude diagram , the number of OB member stars is estimated to 2600 \pm 400 , while the number of O stars amounts to 120 \pm 20 .
This is the largest number of O stars ever found in a galactic massive star association .
The slope of the initial mass function has been determined from the colour-magnitude diagram to \Gamma = -1.6 \pm 0.1 .
The total mass of Cyg OB2 is estimated to ( 4 - 10 ) \times 10 ^ { 4 } M _ { \sun } , where the primary uncertainty comes from the unknown lower mass cut-off .
Using the radial density profile of the association , the central mass density is determined to \rho _ { 0 } = 40 - 150 M _ { \sun } pc ^ { -3 } .

Considering the mass , density , and size of Cyg OB2 it seems untenable to classify this object still as OB association .
Cygnus OB2 more closely resembles a young globular cluster like those observed in the Large Magellanic Cloud or in extragalactic star forming regions .
It is therefore suggested to re-classify Cygnus OB2 as young globular cluster – an idea which goes back to [ Reddish et al .
( 1966 ) ] .
Cygnus OB2 would then be the first object of this class in the Milky Way .