  The open cluster Stephenson 2 contains the largest collection of red
supergiants known in the Galaxy, and at present is the second most massive
young cluster known in the Milky Way. We have obtained multi-epoch,
intermediate-resolution spectra around the CaII triplet for more than 30 red
supergiants in Stephenson~2 and its surroundings. We find a clear separation
between a majority of RSGs having spectral types M0-M2 and the brightest
members in the NIR, which have very late spectral types and show strong
evidence for heavy mass loss. The distribution of spectral types is similar to
that of RSGs in other clusters, such as NGC 7419, or associations, like Per
OB1. The cluster data strongly support the idea that heavy mass loss and maser
emission is preferentially associated with late-M spectral types, suggesting
that they represent an evolutionary phase.
