October 15, 2013
The Rosette Nebula, an emission nebula in Monoceros
Image Credit & Copyright: J-P Metsävainio (http://astroanarchy.zenfolio.com)
The Rosette Nebula (Caldwell 49 or Sh2-275) is a circular emission nebula of some 130 light-years across that lies near one end of the giant the Rosette Molecular Cloud in the Perseus Arm of the Galaxy, about 5,200 light-years away from Earth in the constellation of Monoceros (the Unicorn).
Its parts have been assigned different NGC numbers: 2237, 2238, 2239, and 2246. The mass of the Rosette Nebula is estimated to be around 10,000 solar masses, what makes it one of the more massive diffuse nebulae.
At the core of this large H II region lies the open cluster NGC 2244 (Caldwell 50), with hot, young stars which recently formed from the nebula’s matter. The age of this cluster has been estimated to be less than 5 million years. The cluster contains hundreds of lower-mass stars but also several massive O and B type stars, superhot stars that generate large amounts of radiation and strong stellar winds.
The two brightest stars of NGC 2244 are HD 46223, 400,000 times brighter than the Sun, and approximately 50 times more massive, and HD 46150, with a luminosity 450,000 time larger than that of our star, and is up to 60 times more massive, but it may actually be a double star.
The radiation from the hot stars excite the atoms in the nebula, causing them to emit radiation what makes the nebula shine, while the stellar winds have cleared the central hole. These massive stars have also heated the surrounding gas, with a volume of about 3,000 cubic light-years at the center of the Rosette Nebula, to a temperature in the order of 6 million kelvins causing them to emit copious amounts of X-rays.
It is believed that stellar winds from a group of O and B stars are exerting pressure on interstellar clouds to cause compression, followed by star formation in the nebula. This star formation is currently still ongoing.
One young (proto)star in the Rosette Nebula is ejecting a complex jet of material stretching for more than 8,000 astronomical units (1 AU = 150 million kilometers). Known as Rosette HH1, this jet contains a prominent knot and hints of others, which can be interpreted as “bullets” of material being ejected from the rapidly rotating young star at hypersonic velocities on the order of 2,500 kilometers per second. Bow shocks on the other side of the star suggest the existence of a degenerated counterjet extending in the opposite direction.
Stripped of its normally opaque surroundings by the intense ultraviolet radiation produced by nearby massive stars, this young stellar object is likely one of the last of its generation in this region of space.
Because most young stars are embedded in very dense molecular clouds, the view of the early stages of star formation is normally impossible with optical telescopes. This is one of only a few cases where a protostar is visible, making it a valuable young star that will be studied in detail.