We present new spectroscopic and photometric observations of the young Galactic open cluster Westerlund 1 ( Wd 1 ) that reveal a unique population of massive evolved stars .
We identify \sim 200 cluster members and present spectroscopic classifications for \sim 25 % of these .
We find that all stars so classified are unambiguously post-Main Sequence objects , consistent with an apparent lack of an identifiable Main Sequence in our photometric data to V \sim 20 .
We are able to identify rich populations of Wolf Rayet stars , OB supergiants and short lived transitional objects .
Of these , the latter group consists of both hot ( Luminous Blue Variable and extreme B supergiants ) and cool ( Yellow Hypergiant and Red Supergiant ) objects - we find that half the known Galactic population of YHGs resides within Wd1 .
We obtain a mean V - M _ { V } \sim 25Â mag from the cluster Yellow Hypergiants , implying a Main Sequence turnoff at or below M _ { V } = -5 ( O7 V or later ) .
Based solely on the masses inferred for the 53 spectroscopically classified stars , we determine an absolute minimum mass of \sim 1.5 \times 10 ^ { 3 } M _ { \odot } for Wd 1 .
However , considering the complete photometrically and spectroscopically selected cluster population and adopting a Kroupa IMF we infer a likely mass for Wd 1 of \sim 10 ^ { 5 } M _ { \odot } , noting that inevitable source confusion and incompleteness are likely to render this an underestimate .
As such , Wd 1 is the most massive compact young cluster yet identified in the Local Group , with a mass exceeding that of Galactic Centre clusters such as the Arches and Quintuplet .
Indeed , the luminosity , inferred mass and compact nature of Wd 1 are comparable with those of Super Star Clusters - previously identified only in external galaxies - and is consistent with expectations for a Globular Cluster progenitor .