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We present near-infrared JHKs imaging as well as K-band multi-objectspectroscopy of the massive stellar content of W3 Main using LUCI at the LBT.We confirm 13 OB stars by their absorption line spectra in W3 Main and spectraltypes between O5V and B4V have been found. Three massive Young Stellar Objectsare identified by their emission line spectra and near-infrared excess. Fromour spectrophotometric 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 have detected the photospheres of OB starsfrom the more evolved diffuse HII region to the much younger UCHII regions,suggesting that the OB stars have finished their formation and cleared awaytheir possible circumstellar disks very fast. Only in the hyper-compact HIIregion (IRS5), the early type stars are still surrounded by circumstellarmaterial.

Dissecting high-mass star-forming regions; tracing back their complex formation history