With a dynamical mass M _ { dyn } \sim 1.3 \times 10 ^ { 5 } \ > M _ { \odot } and a lower limit M _ { cl } > 5 \times 10 ^ { 4 } \ > M _ { \odot } from star counts , Westerlund 1 is the most massive young open cluster known in the Galaxy and thus the perfect laboratory to study massive star evolution .
We have developed a comprehensive spectral classification scheme for supergiants based on features in the 6000 – 9000Å range , which allows us to identify > 30 very luminous supergiants in Westerlund 1 and \sim 100 other less evolved massive stars , which join the large population of Wolf-Rayet stars already known .
Though detailed studies of these stars are still pending , preliminary rough estimates suggest that the stars we see are evolving to the red part of the HR diagram at approximately constant luminosity .