Rich in HII regions , giant molecular clouds are natural laboratories to study massive stars and sequential star formation .
The Galactic star forming complex W33 is located at l = \sim 12 \fdg 8 and at a distance of 2.4 kpc , has a size of \approx 10 pc and a total mass of \approx ( 0.8 - − 8.0 ) \times 10 ^ { 5 } M _ { \odot } .
The integrated radio and IR luminosity of W33 - when combined with the direct detection of methanol masers , the protostellar object W33A , and protocluster embedded within the radio source W33 main - mark the region out as a site of vigorous ongoing star formation .
In order to assess the long term star formation history , we performed an infrared spectroscopic search for massive stars , detecting for the first time fourteen early-type stars , including one WN6 star and four O4-7 stars .
The distribution of spectral types suggests that this population formed during the last \sim 2 - 4 Myr , while the absence of red supergiants precludes extensive star formation at ages 6 - 30 Myr .
This activity appears distributed throughout the region and does not appear to have yielded the dense stellar clusters that characterize other star forming complexes such as Carina and G305 .
Instead , we anticipate that W33 will eventually evolve into a loose stellar aggregate , with Cyg OB2 serving as a useful , albeit richer and more massive , comparator .
Given recent distance estimates , and despite a remarkably similar stellar population , the rich cluster Cl 1813 - 178 located on the north-west edge of W33 does not appear to be physically associated with W33 .