To the east of the constellation Orion sits the little known and hard-to-see constellation called Monoceros. Its name is Greek for unicorn, and was probably penned by a Dutch cartographer named Petrus Plancius in the 17th century. In the unicorn’s head, which is immediately to the left of Orion’s chest, sits the Rosette Nebula. It appears on Earth to be 1 degree across, which is about 5 times the size of the full Moon and has an actual radius of 65 light years, although estimates vary. The nebula's apparent magnitude is a relatively dim 9, which is impossible to see with the unaided human eye. The nebula's dimness made it rather difficult to discover.
This large cloud of gas and dust sits about 5,000 light years from the Earth, although other estimates of the Rosetta Nebula's distance can be as low as 1,500 light years. It is composed mostly of hydrogen, but contains dust and other elements as well. The nebula is classified as an H II region, meaning that the gas has been ionized by by the intense light being emitted by stars that have formed in it. The huge amounts of gas in the Rosette Nebula are driving the formation of very large and bright O and B type stars. The extremely energetic solar wind from these stars has created an area roughly 3,000 cubic light years in volume at the nebula's center with a temperature of 6 million kelvins. This hot gas itself emits X-rays, and its discovery shed new light on how solar wind from massive stars interact to create violent shocks.
The Rosette Nebula itself is a part of the much larger Rosette Molecular Cloud, and is differentiated by its emission of light. It is further broken down into several smaller parts which have been assigned NGC numbers; 2237, 2238, 2239, and 2246. In the center of the nebula sits the open star cluster NGC 2244, fueling the extreme temperatures. This open cluster was discovered in 1690 by John Flamsteed, but the Rosette Nebula took much more effort to find. The various parts of the nebula were discovered by three different astronomers: John Herschel found 2239, Albert Marth found 2238, and Lewis Swift found 2237 and 2246.