Massive star formation
The birth of massive stars remains a major puzzle for theorists and observers alike. What we do know is that they are formed within Giant Molecular Clouds (such as Orion).
Once the hot star starts to 'shine', it illuminates its dense surrounding region – called an UltraCompact HII region – the surrounding material prevents direct observation of the star until the natal gas has been blown away, but one can study the birth environment of massive stars at radio and mid-infrared wavelengths. This is shown in the image for two Galactic UCHII regions, based on observations with the Midcourse Space Experiment.
Near-IR spectroscopy from VLT/ISAAC has provided robust classification of the ionising star of the G23.96+0.15 UCHII region, only the second such case due to high interstellar and circumstellar dut extinction.
New observations are also being made with the Spitzer mid-IR telescope, primarily through the GLIMPSE survey, shown on the bottom-right for RCW49, plus our own IRAC and IRS study of the W31 star forming region.
Galactic UCHII regions
Star formation in RCW49
More information
Near and mid infrared observations of ultracompact HII regions, Crowther 2005.
On the central ionising star of G23.96+0.15 and near-IR spectral classification of O stars, Crowther and Furness 2008.
Mid-infrared diagnostics of metal-rich HII regions from VLT and Spitzer spectroscopy of young massive stars in W31, Furness et al. 2010.