The Galactic open cluster Westerlund 1 was found only a few years ago to be much more massive than previously thought , with evidence suggesting its mass to be in excess of \sim 10 ^ { 5 } M _ { \odot } , in the range spanned by young extragalactic star clusters .
Unlike those clusters its proximity makes spatially resolved studies of its stellar population feasible .
It is therefore the ultimate template for a young , massive star cluster , permitting direct comparison of its properties with measurements of velocity dispersion and dynamical mass for spatially unresolved extragalactic clusters .

To this end , we used the long slit near-infrared spectrograph VLT/ISAAC to observe the CO bandhead region near 2.29 \mu m , scanning the slit across the cluster centre during the integration .
Spatially collapsing the spectra along the slit results in a single co-added spectrum of the cluster , comparable to what one would obtain in the extragalactic cluster context .
This spectrum was analysed the same way as the spectra of almost point-like extragalactic clusters , using red superiant cluster members as velocity templates .
We detected four red supergiants which are included in the integrated spectrum , and our measured velocity dispersion is 5.8 km/s .
Together with the cluster size of 0.86 pc , derived from archival near-infrared SOFI-NTT images , this yields a dynamical mass of 6.3 \times 10 ^ { 4 } M _ { \odot } .
While this value is not to be considered the final word , there is at least so far no sign for rapid expansion or collapse .