Clusters rich in red supergiants
  In the past few years, several clusters containing large numbers of red
supergiants have been discovered. These clusters are amongst the most massive
young clusters known in the Milky Way, with stellar masses reaching a few
$10^4\:$M$_\odot$. They have provided us, for the first time, with large
homogeneous samples of red supergiants of a given age. These large populations
make them, despite heavy extinction along their sightlines, powerful
laboratories to understand the evolutionary status of red supergiants. While
some of the clusters, such as the eponymous RSGC1, are so obscured that their
members are only observable in the near-IR, some of them are easily accessible,
allowing for an excellent characterisation of cluster and stellar properties.
The information gleaned so far from these clusters gives strong support to the
idea that late-M type supergiants represent a separate class, characterised by
very heavy mass loss. It also shows that the spectral-type distribution of red
supergiants in the Milky Way is very strongly peaked towards M1, while
suggesting a correlation between spectral type and evolutionary stage.
