The Rosette Nebula is a vast emission nebula located about 5,200 light years away. The star forming region lies near a large molecular cloud in the constellation Monoceros, the Unicorn. It is closely associated with the young open star cluster NGC 2244.
Also known as the Satellite Cluster, NGC 2244 appears in the centre of the Rosette. The hot young stars of the cluster were formed from the nebula’s material in the last 5 million years.
The stars in the Satellite Cluster are responsible for the nebula’s glow. Their radiation ionizes the surrounding clouds of nebulosity, causing them to emit their own light. The nebula glows in the red part of the spectrum because the powerful ultraviolet radiation from the stars strips electrons from the nebula’s hydrogen atoms.
The Rosette Nebula is a very active stellar nursery. It is home to numerous Herbig-Haro objects and Herbig Ae/Be stars, Bok globules, T Tauri stars and clusters of newly formed stars. The dark filaments of dust extending toward the centre of the nebula, sometimes called “elephant trunks,” are shaped by the stellar winds and radiation from hot young stars and electromagnetic forces.
The Rosette (or Rosetta) Nebula spans 130 light years and has an angular size of 1.3 degrees. It is much larger than the better-known Orion Nebula, which is about 24 light years across. However, the Orion Nebula is much closer to us at 1,344 light years and appears brighter but smaller than the Rosette.
The Rosette Nebula has an estimated mass of about 10,000 solar masses. It is home to about 2,500 stars. The most massive O- and B-type stars power the nebula and cause it to expand. The nebula will disperse in the next few million years, leaving behind only the central cluster, NGC 2244.
The Rosette Nebula was named after the stylized flower design used in sculptural objects since antiquity. The nebula’s appearance in optical light resembles the rosette. It is sometimes called the Skull Nebula because it also closely resembles the human skull.
The region of the Rosette Nebula contains several stellar groups in a range of 98 light-years (30 parsecs). The open cluster NGC 2244 occupies the central cavity and contains several tens of young, blue O- and B-type stars. It is also home to many low-mass young stellar objects: protostars and pre-main sequence stars (Herbig Ae/Be and T Tauri stars). Smaller stellar groups are found at the rim of the nebula.
NGC 2244: Satellite Cluster
The Satellite Cluster is a bright open cluster within the Rosette Nebula. It has an apparent size of 24 arcminutes, corresponding to a linear radius of about 18 light years.
Most of the stars in NGC 2244 are less than 5 million years old. The cluster has an estimated age of about 2 million years. A 2021 study found a mass of about 879 solar masses and a half-mass radius of 5.9 arcminutes (2.4 parsecs or 7.83 light years) for the cluster. The same study found that the cluster had a pattern of radial expansion and rotation.
NGC 2244 contains several exceptionally hot O-type stars that are mostly responsible for ionizing the surrounding nebula. The most massive members, catalogued as HD 46223 and HD 46150, are hot blue stars with stellar classifications O4V and O5V. HD 46223 has a mass about 50 times that of the Sun and shines with 400,000 solar luminosities, while HD 46150 is up to 60 times more massive than the Sun and 450,000 times more luminous. The latter may be a double star. Both stars are less than 2 million years old. They are mainly responsible for blowing a bubble in the nebula’s centre.
Several stars that lie north and south of NGC 2244 are moving away from the cluster. Most of these stars were members of NGC 2244 about 2 million years ago.
The K-type giant 12 Monocerotis lies in the same line of sight as the cluster and appears as its brightest star. However, the star is much closer to us and is not a member of NGC 2244.
The open cluster NGC 2237 lies near the heads of the gas pillars in the nebula. Most stars in the cluster are moving toward the south, in contrast to the H II bubble, which is expanding west.
The stars of Rosette Nebula are members of the Monoceros OB2 association, a young association of stars that are not gravitationally bound. The Satellite Cluster forms the core of the association. It is the smallest of the three subassociations, spanning 36 – 40 light years. The Rosette Nebula is the most active star forming region in the Monoceros OB2 association.
The massive stars in the nebula play a part in regulating star formation in their vicinity. Stellar winds and far-ultraviolet radiation disperse the surrounding gas, reducing the efficiency of star formation and ultimately ending star formation altogether. However, the massive H II bubbles driven by massive stars can compress the nebula’s material, triggering star formation away from the region excavated by the massive stars.
A 2003 study that used X-ray images obtained with the Chandra X-ray Observatory Advanced CCD Imaging Spectrometer (ACIS) instrument reported soft diffuse X-ray emission between the stars in the bubble. The extended emission is believed to be associated with hot plasma with temperatures in the range from 1 million to 10 million K. This is considerably hotter than plasmas with temperatures of about 10,000 K generally observed in H II regions. It is believed to come from shock-heated winds from the massive young stars. The fast stellar winds of the nebula’s O-type stars are heated either by collisions with winds of other stars or by a termination shock against the surrounding media.
A 2021 kinematic study of the young stars in the Rosette Nebula found that new stars in nebula may form through the expansion of stellar clusters, hierarchical star formation in turbulent clouds, and feed-back-driven star formation. The study identified a total of 338 young stellar objects within a region of 1 by 1 degrees.
A 2015 study found that 22% of the stars – about 560 stars – in the Rosette Nebula did not belong to a cluster. They may have formed in molecular filaments outside the main cluster. The filaments may later merge with the cluster.
Various parts of the Rosette Nebula have different designations in the New General Catalogue, and some of these are now obsolete (NGC 2238, NGC 2246). The designation NGC 2237, which denotes an open cluster west of NGC 2244, is often used for the whole nebula. More commonly, the Rosette Nebula is referred to by its Caldwell designation, Caldwell 49, or by the designation Sharpless 275 (Sh2-275) from the Sharpless catalogue of H II regions.
Both the Rosette Nebula and the Satellite Cluster are included in the Caldwell catalogue of bright deep sky objects that can be observed with amateur telescopes. The two objects have separate Caldwell designations. The nebula is listed as Caldwell 49 and the cluster as Caldwell 50.
Different parts of the Rosette Nebula were discovered by different astronomers from the 17th to the 19th centuries. This is why the nebula has several designations in the New General Catalogue: NGC 2237, NGC 2238, NGC 2239, NGC 2244, and NGC 2246. The open cluster NGC 2244 was discovered by the English astronomer John Flamsteed in 1690. German-British astronomer William Herschel discovered the cluster independently about a century later, but like Flamsteed, he was unable to see the surrounding nebulosity. His son John Herschel is credited for the discovery of NGC 2239 in 1830. The Simbad database lists NGC 2239 as another designation for NGC 2244.
German astronomer Albert Marth was the first to see the Rosette Nebula. He discovered NGC 2238, a small bright patch in the nebula, in 1864 using a 48-inch reflector. American astronomer Lewis Swift was the first to report NGC 2246 (a bright patch on the eastern side of the Rosette) in 1886. The different parts of the nebula are collectively catalogued as NGC 2237. The full extent of the nebula became known only after American astronomer Edward Emerson Barnard started his photographic work at the Lick Observatory at the University of California.
The Rosette Nebula lies in the Perseus Arm, one of our galaxy’s two major spiral arms. The Perseus Arm is also home to the Crab Nebula (M1) in Taurus, the Double Cluster (NGC 869 and NGC 884) in Perseus, and the open clusters Messier 36, Messier 37, and Messier 38 in Auriga.
The Rosette Nebula is one of the most popular targets for wide-field astrophotography. The nebula’s appearance, size, and location make it an attractive target for amateur telescopes. However, it is not an easy target for visual astronomy. While the Satellite Cluster can be spotted even in binoculars and is best seen at low magnification, the surrounding nebula is best observed in larger telescopes at low power, with OIII or UHC filters. The Rosette may not fit in a single field of view, but its dark lanes will be more visible. The nebula requires a clear, dark sky to be seen. Its red colour cannot be seen visually. It can only be captured photographically.
On April 16, 2019, the Rosette Nebula was adopted as the official astronomical object of the State of Oklahoma. The bill was signed into law by Governor Kevin Stitt on April 22, 2019.
How to find the Rosette Nebula?
The Rosette Nebula is easy to find because it lies between two first-magnitude stars. It appears just south of the imaginary line that connects Betelgeuse in Orion and Procyon in Canis Minor. Betelgeuse is identified as the right shoulder of Orion (left from our point of view). It forms a prominent triangle with Procyon and Sirius, known as the Winter Triangle. Sirius in Canis Major and Alhena in Gemini can be used for orientation. The nebula lies just right of the imaginary line connecting the two stars.
On a clear night, the Rosette Nebula can be found within a faint triangle of stars that outline the head of the celestial Unicorn. The three stars – the A-type subgiant Epsilon Monocerotis (mag. 4.39), K-type giant 18 Monocerotis (mag. 4.47), and white supergiant 13 Monocerotis (mag. 4.5) – are all evolved, luminous stars, but they lie too far away to be visible from light-polluted areas.
The Rosette Nebula lies in the northern part of the constellation Monoceros, the Unicorn. Monoceros is one of the fainter constellations. Its brightest stars, the giants Alpha and Gamma Monocerotis, shine at magnitudes 3.94 and 3.96. They are visible to the unaided eye, but do not stand out in this region of the sky, which contains more first-magnitude stars than any other area. However, the constellation can be identified on a clear, dark night because a good portion of it lies within the Winter Triangle, a prominent asterism formed by Sirius, Betelgeuse, and Procyon.
The best time of year to observe the stars and deep sky objects in Monoceros is during the northern hemisphere winter, especially in the month of February, when the Winter Triangle dominates the evening sky.
Rosette Nebula – Caldwell 49
|Right ascension||06h 32m 26.76s|
|Declination||+04° 47′ 37.1″|
|Distance||5,200 light years (1,600 parsecs)|
|Radius||65 light years|
|Names and designations||Rosette Nebula, Caldwell 49, Sh2-275, CTB 21, ASB 27, LBN 949, LBN 206.39-01.87, LBN 948, 3C 163|
Satellite Cluster – NGC 2244
|Right ascension||06h 32m 10.8s|
|Declination||+04° 54′ 50′′|
|Distance||5,200 light years|
|Radius||18 light years|
|Age||< 5 million years|
|Names and designations||Satellite Cluster, NGC 2244, NGC 2239, Caldwell 50, Collinder 99|