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Rich in HII regions, giant molecular clouds are natural laboratories to studymassive stars and sequential star formation. The Galactic star forming complexW33 is located at l=~12.8deg and at a distance of 2.4 kpc, has a size of ~10 pcand a total mass of (~0.8 - ~8.0) X 10^5 Msun. The integrated radio and IRluminosity of W33 - when combined with the direct detection of methanol masers,the protostellar object W33A, and protocluster embedded within the radio sourceW33 main - mark the region out as a site of vigorous ongoing star formation. Inorder to assess the long term star formation history, we performed an infraredspectroscopic search for massive stars, detecting for the first time fourteenearly-type stars, including one WN6 star and four O4-7 stars. The distributionof spectral types suggests that this population formed during the last ~2-4Myr, while the absence of red supergiants precludes extensive star formation atages 6-30 Myr. This activity appears distributed throughout the region and doesnot appear to have yielded the dense stellar clusters that characterize otherstar forming complexes such as Carina and G305. Instead, we anticipate that W33will eventually evolve into a loose stellar aggregate, with Cyg OB2 serving asa useful, albeit richer and more massive, comparator. Given recent distanceestimates, and despite a remarkably similar stellar population, the richcluster Cl 1813-178 located on the north-west edge of W33 does not appear to bephysically associated with W33.

Massive Stars in the W33 Giant Molecular Complex