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The dense molecular cores of GMCs are the sites of massive star formation in the Milky Way. Millimeter and submillimeter wavelength spectroscopy are used to probe the molecular gas, while far-infrared data obtained by IRAS and MMX and near-infrared images obtained from ground-based telescopes and from 2MASS probe the star formation activity. Combining these data, one can probe the relationship between the dense molecular gas and the newly formed stars.

The first figure shown below is a CO image of the W3/4/5 molecular cloud complex, at a distance of about 2 Kpc, overlaid on a 21 cm continuum image from the DRAO Galactic Plane Survey. The CO image was obtained at FCRAO as part of the FCRAO CO Survey of Second Quadrant of the Galaxy. This complex has a mass of about 1 million solar masses. In the second figure, images of CO, ¹³CO, near-infrared emission and contours of stellar density are shown in the four panels for three rich stellar clusters in W3/4/5. These figures are from Carpenter, Heyer, and Snell (2000).

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