  We investigated the nature of the massive star [OMN2000] LS1 and used these
results to constrain the history of star formation within the host complex W51.
A combination of near-IR spectroscopy and non-LTE model atmosphere analysis was
used to derive the physical properties of [OMN2000] LS1 and a combination of
theoretical evolutionary calculations and Monte Carlo simulations to apply
limits on the star formation history of W51. The spectrum of [OMN2000] LS1 is
consistent with that of a P Cygni supergiant, but with a temperature of
13.2-13.7kK and log(L/L_sun)<5.75, it is significantly cooler, less luminous,
and massive than proposed by previous authors. The presence of such a star
within W51 shows that star formation has been underway for at least 3Myr, while
the formation of massive O stars is still on going. We find no evidence of
internally triggered, sequential star formation within W51, and favour the
suggestion that star formation has proceeded at multiple indepedent sites
within the GMC. [ABRIDGED]
