(See description below)
This stunning emission-line image of the Rosette nebula (NGC2237) in the constellation Monoceros (the Unicorn) was taken at the National Science Foundation's 0.9-m telescope on Kitt Peak with the Mosaic camera, and is presented here in false color (hydrogen alpha, OIII oxygen, and SII sulfur respectively red, green and blue, using five ten-minute exposures each). The Rosette is a prominent star formation region, glowing due to ultraviolet light from the young, hot, blue stars whose winds also cleared the central hole. It is enormously large on the sky, covering more than six times the area of the full moon. An approximately true color version from the Oschin Schmidt Telescope on Palomar Mountain is also available.
In the heart of the Rosette Nebula (NGC 2244) lies a bright open cluster of stars that lights up the nebula. The stars of NGC 2244 formed from the surrounding gas only four million years ago and emit light and wind that define the nebula's appearance today. High energy light from the bright young stars of NGC 2244 ionizes the surrounding hydrogen gas clouds to create the red emission nebula appearance. The hot wind of particles that streams away from the cluster stars contributes to an already complex menagerie of gas and dust filaments while slowly evacuating the cluster center. NGC 2244 measures about 50 light-years across, lies about 5200 light-years away, and is visible with binoculars towards the constellation of Monoceros.
NGC 2244 contains at least 17 OB stars; it is an OB-association. Some of them are visible as bright objects to the lower left in this CFH12K image. O and B stars are massive, about 20 times more so than our sun, or even more. They drive powerful stellar winds and shine brightly in the ultraviolet. Because of that these stars, although fairly rare, have a strong impact on their environment.
For example, the combined stellar wind of the cluster's OB stars is presently blowing the dust and gas away from the center of NGC 2244 at a velocity of about 20 km/sec. The empty cavity near the OB stars is already somewhat visible on the image. The wave of expanding material is expected to trigger events of extensive star formation as the gas and dust is compressed. Their strong ultraviolet flux is also responsible for the propagation of an ionisation front that will also help trigger star formation. This large UV Flux is responsible for the illumination of the bright extended reddish features seen in the image