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We present near-infrared multi-object spectroscopy and JHKs imaging of themassive stellar content of the Galactic star-forming region W3 Main, obtainedwith LUCI at the Large Binocular Telescope. We confirm 15 OB stars in W3 Mainand derive spectral types between O5V and B4V from their absorption linespectra. Three massive Young Stellar Objects are identified by their emissionline spectra and near-infrared excess. The color-color diagram of the detectedsources allows a detailed investigation of the slope of the near-infraredextinction law towards W3 Main. Analysis of the Hertzsprung Russell diagramsuggests that the Nishiyama extinction law fits the stellar population of W3Main best (E(J-H)/E(H-Ks) = 1.76 and R_(Ks) = 1.44). From ourspectrophotometric analysis of the massive stars and the nature of theirsurrounding HII regions we derive the evolutionary sequence of W3 Main and wefind evidence of an age spread of at least 2-3 Myr. While the most massive star(IRS2) is already evolved, indications for high-mass pre-main-sequenceevolution is found for another star (IRS N1), deeply embedded in an ultracompact HII region, in line with the different evolutionary phases observed inthe corresponding HII regions. We derive a stellar mass of W3 Main of (4 +- 1)10^3 Msun, by extrapolating from the number of OB stars using a Kroupa IMF andcorrecting for our spectroscopic incompleteness. We have detected thephotospheres of OB stars from the more evolved diffuse HII region to the muchyounger UCHII regions, suggesting that these stars have finished theirformation and cleared away their circumstellar disks very fast. Only in thehyper-compact HII region (IRS5), the early type stars seem to be stillsurrounded by circumstellar material.

Age spread in W3 Main: LBT/LUCI near-infrared spectroscopy of the massive stellar content