The Rosette Nebula is arguably one of the finest HII regions in the northern sky. Located in the constellation Monoceros, the nebula's center has been excavated centrally by the radiation pressure and powerful stellar winds of the massive OB stars belonging to its central cluster NGC 2244. The Rosette Nebula represents an ionization front along the edge of an enormous molecular cloud complex extending across 300 light years of winter sky called the Rosette molecular cloud (RMC). The RMC contains enough gas and dust to create over 100,000 suns. The hot stars of NGC 2244 are about 3 million years old (range of 0.3 to 6.4 Myr) and form the core cluster of the larger Monoceros OB2 stellar association. Monoceros OB2 contains at least three subassociations, the smallest being NGC 2244 at about 40 light years in diameter. Star formation is occurring along the edge of an expanding gas shell coincident with the interface of the Rosette Nebula with the surrounding RMC suggesting that sequentially triggered star formation is at work and progressing outward in the Rosette. The process by which massive molecular clouds transform themselves into clusters of stars is controversial. A likely explanation is that imbedded clusters form by compression of the molecular cloud by external shocks or triggering events. Triggering events can be supernova shock fronts, stellar winds from massive stars or expanding shells of neutral gas which all serve to compress and fragment the cloud into clumps which become the precursors of new stars.
The O and B type giants of the young open cluster NGC 2244 provide the excitation for the wreath-like emission cloud known as the Rosette Nebula. The cluster contains 30 OB type stars, including at least four O type giants (including a massive O4 type) and at least nine other massive stars earlier than B2 class, which all power the nebula. The winds from these powerful stars impart momentum to the ambient gas of the Rosette, expanding the nebula at about 4 kilometers per second. The Rosette HII region covers approximately 130 light years and contains some 10,000 solar masses of gas and dust.
The Rosette is undoubtedly a very active region of star formation. Herbig-Haro objects, Herbig Ae/Be stars, T Tauri stars, and Bok globules exist within the Rosette all pointing to the presence of infant stars imbedded within the nebulosity. Infant star clusters have been observed at infrared wavelengths (invisible optically) along the outer perimeter of the Rosette at its contact point with the RMC. Centrally in the Rosette the energy output of NGC 2244 creates a violent and tenuous environment which may serve to abort the formation of low mass stars and their planet progeny in close proximity to the massive hot stars. When the cold gases of a molecular cloud are heated too quickly by nearby hot stars the gases will evaporate off leaving insufficient material for star birth. Recent observations by the Chandra observatory have detected abundant x-ray emission deep within NGC 2244. The source of the x-rays is gas superheated to temperatures of almost 6 million degrees. The tremendous temperatures are believed to be generated when colliding shock fronts produced by the massive OB stars of NGC 2244 produce superheated gases which then emit high energy X-rays.
Dark filaments of dust appear to radiate towards the center of the Rosette and are sometimes referred to as "elephant trunks". The twisted helical pattern of these filaments is thought to be molded by the interplay of stellar winds, radiation, and electromagnetic forces. The resulting forces produce a double helix lining up along a magnetic field pointing towards the central cluster of hot young stars. Like other HII regions the Rosette will have a finite life of a few million years and will eventually disperse and disappear from view leaving behind the aging cluster NGC 2244.