  Because of their large number of stars spread over the entire stellar mass
spectrum, starburst clusters are highly suitable to benchmark and calibrate
star formation models and theories. Among the handful of Galactic starburst
clusters, Westerlund 1 with its estimated 150 O-stars, W-R stars, supergiants
and hypergiants is the most massive young cluster identified to date in the
Milky Way. While previous studies of Westerlund 1 focused largely on optical
and X-ray observations of its evolved massive stellar population, we have
analyzed near-infrared data, resulting in the first in depth study of the
``lower-mass'' main sequence and pre-main sequence cluster population, i.e., of
stars in the mass range 0.4 to 30 solar masses. The derived properties of the
cluster population allow us to test theoretical evolutionary tracks. By
comparison of Westerlund 1's half-mass radius with younger starburst clusters
like NGC 3603 YC and Arches, and somewhat older massive clusters like RSGC1 and
RSGC2, we find evidence for a rapid dissolution of Galactic starburst clusters,
which has interesting implications for the long-term survival of starburst
clusters, and the question to which extent Galactic starburst clusters might
mimic proto-globular clusters.
