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The Rosette Nebula - an HII Region

The Rosette Nebula is a stunning HII region, shown at right.  Here, the colors denote three ionized elements - hydrogen (H-alpha) is shown in red, oxygen (OIII) in green, and sulfur (SII) in blue.  (For further information about observing HII regions in these elements, see Stephen Portillo’s nebula predictor).  We can see a hydrogen rich gaseous nebula, with luminous ionizing stars at the center.  Overlaid fine dark, filamentary structures are visible, due to dust.  Another interesting feature of the Rosette is the central cavity that has been cleared out around the central cluster of stars.

NGC 2244 - ionizing and blowing out material

At the heart of the Rosette Nebula lies NGC 2244 - an open cluster containing at least 17 O and B stars (CFHT reference).  Of these, the hottest are believed to be HD 46223, an O4V star, and HD 46150, an O5V star.  These stars have large ionizing luminosities, responsible for the HII region itself.

NGC 2244 is also responsible for stellar winds that blow gas outward from the cluster center, at roughly 20 km/s.  These winds are so strong that the central gas can be cleared out, leaving a noticeably visible cavity.  Stellar winds play a significant role in many HII regions, causing the regions to deviate from ideal uniform Strömgren spheres.

T. A. Rector et al, WIYN & NOAO/AURA/NSF