Monoceros (Greek: Μονόκερως) is a faint constellation on the celestial equator. Its name is Greek for unicorn. Its definition is attributed to the 17th-century Dutch cartographer Petrus Plancius. It is bordered by Orion to the west, Gemini to the north, Canis Major to the south and Hydra to the east. Other bordering constellations include Canis Minor, Lepus and Puppis.
The constellation Monoceros, from Urania’s Mirror, a set of star charts from 1825. Includes Canis Minor and the obsolete constellation Atelier Typographique.
In Western astronomy, Monoceros is a relatively modern constellation. Its first certain appearance was on a globe created by the Dutch cartographer Petrus Plancius in 1612 or 1613 and it was later charted by German astronomer Jakob Bartsch as Unicornu on his star chart of 1624.
German astronomers Heinrich Wilhelm Olbers and Ludwig Ideler indicate that the constellation may be older, quoting an astrological work from 1564 that mentioned ‘the second horse between the Twins and the Crab has many stars, but not very bright;’ these references may ultimately be due to the 13th century Scotsman Michael Scot, but refer to a horse and not a unicorn, and its position does not quite match. Joseph Scaliger is reported to have found Monoceros on an ancient Persian sphere. French astronomer Camille Flammarion believed that a former constellation, Neper (the ‘Auger’), occupied the area of the sky now home to Monoceros and Microscopium, but this is disputed.
Chinese asterisms Sze Fūh, the Four Great Canals; Kwan Kew; and Wae Choo, the Outer Kitchen, all lay within the boundaries of Monoceros.
Constellations of Monoceros and Canis Minor
[http://www.davidmalin.com/fujii/source/Mon.html]
A map showing the constellation Monoceros, east of Orion. The massive object called Plaskett’s Star is highlighted by a circular marker at center-left.
[http://oneminuteastronomer.com/2425/constellation-monoceros/]
Monoceros is not easily seen with the naked eye, containing only a few fourth magnitude stars. However, Monoceros does have some interesting features to observe with the aid of a small telescope.
Alpha Monocerotis is the brightest star in Monoceros. It is an orange giant with the stellar classification of K0 III. It has an apparent visual magnitude of 3.94 and is approximately 144 light years distant from the Sun. The star has a mass 2.02 times that of the Sun and a radius 10.1 times solar.
Gamma Monocerotis is the second brightest star in the constellation. It is another orange giant, belonging to the stellar class K1.5III. The star has an apparent magnitude of 3.98 and is approximately 645 light years distant from the solar system. It is the primary star in a multiple star system.
Delta Monocerotis is a white main sequence star with the stellar classification of A2V. It has an apparent visual magnitude of 4.15 and is about 375 light years distant from the Sun. It is the third brightest star in Monoceros.
Zeta Monocerotis is a massive, luminous, yellow supergiant star in Monoceros. It belongs to the stellar class G2Ib. It has a visual magnitude of 4.37 and is approximately 1,852 light years distant from Earth. The star lies about three quarters of a degree from the border with Hydra constellation. It is 2,535 times more luminous than the Sun and has a radius 62 times solar.
Epsilon Monocerotis is a double star approximately 128 light years from Earth. It has an apparent magnitude of 4.31. The primary component in the system is a white, class A5 subgiant star with an apparent magnitude of 4.44, and the companion is a yellow-white main sequence dwarf of the spectral type F5, with a visual magnitude of 6.72. The two components are separated by 12.1 arcseconds. They have luminosities 20 and 2.5 times that of the Sun, radii 2.2 and 1.2 times solar, and masses 1.9 and 1.25 times that of the Sun.
The primary component is a very rapid rotator, with a projected rotational velocity of 137 km/s. The two stars have an orbital period of at least 6,000 years and are at least 500 astronomical units apart. The brighter star has a dim, line-of-sight companion.
Epsilon Monocerotis lies just to the west of the famous Rosette Nebula, one of the best known diffuse nebulae in the sky.
[http://www.constellation-guide.com/constellation-list/monoceros-constellation/]
V838 Monocerotis, a variable red supergiant star, had an outburst starting on January 6, 2002; in February of that year, its brightness increased by a factor of 10,000 in one day. After the outburst was over, the Hubble Space Telescope was able to observe a light echo, which illuminated the dust surrounding the star:
Light Echoes from V838 Mon
What caused this outburst of V838 Mon? For reasons unknown, star V838 Mon’s outer surface suddenly greatly expanded with the result that it became the brightest star in the entire Milky Way Galaxy in January 2002. Then, just as suddenly, it faded. A stellar flash like this had never been seen before- supernovas and novas expel matter out into space. Although the V838 Mon flash appears to expel material into space, what is seen in the above image from the Hubble Space Telescope is actually an outwardly moving light echo of the bright flash.
In a light echo, light from the flash is reflected by successively more distant rings in the complex array of ambient interstellar dust that already surrounded the star. V838 Mon lies about 20,000 light years away toward the constellation of the unicorn (Monoceros), while the light echo above spans about six light years in diameter.
[https://www.nasa.gov/multimedia/imagegallery/image_feature_2472.html]
Plaskett’s Star or HD 47129 is a narrow double of two blue supergiants beyond their main-sequence stadium. Both have strong stellar winds which collide in the middle and produce a cloud around the stars. Interestingly the smaller one (A) is optically brighter, probably because much light of the bigger one is absorbed by the cloud.
[https://jumk.de/astronomie/big-stars/plaskett.shtml]
Plaskett’s Star, also known as HR 2422 and V640 Monocerotis, is a spectroscopic binary at a distance of around 6600 light-years. It is one of the most massive binary stars known, with a total mass of around one hundred times that of the Sun, although it is a binary system.
The pair have a combined visual magnitude of 6.05. The secondary is a rapid rotator with a projected rotational velocity of 300 km·s−1, giving it a pronounced equatorial bulge.
The luminosities of each component are much lower than expected for their spectral types. The masses derived from the binary orbit are also somewhat higher than expected from the spectral types, but with considerable uncertainty due to assumptions about the inclination.
This system is named after John Stanley Plaskett, the Canadian astronomer who discovered its binary nature in 1922.
[https://en.wikipedia.org/wiki/Plaskett%27s_star]
Ross 614 (V577 Monocerotis) is a red dwarf UV Ceti flare star and it is the primary member of a nearby binary star system in the constellation of Monoceros. This star has a magnitude of about 11, making it invisible to the unaided eye even though it is one of the stars nearest to the Sun. The binary star system consists of two closely spaced low-mass red dwarfs. The secondary star is a dim magnitude 14 lost in the glare of the nearby primary star. This system is among the closest to the Sun at an estimated distance of about 13.3 light years. Because it is so close to the Earth it is often the subject of study, hence the large number of designations by which it is known.
[https://en.wikipedia.org/wiki/Ross_614]
COROT-7 is a G-type main sequence star, slightly smaller and cooler than the Sun. It has an apparent magnitude 11.67, fainter than Proxima Centauri (mag. 11.05), the nearest star to the Sun. This star is located in the Monoceros constellation, in the LRa01 field of view of the COROT spacecraft. It is about 500 light years from earth.
According to the project website, this field is in the Monoceros constellation. Published data lists the stellar properties as being a G9V yellow dwarf with a temperature of 5250 K, a radius of about 82% of the Sun and a mass of about 91% of the Sun. But other sources have been known to list it is a (K0V) orange dwarf. The metallicity, i.e. the quantity [M/H] of the star, which is the base-10 logarithm of the ratio of the star’s metal abundance (given by the ratio of metals to hydrogen) to that of the Sun is 0.12 ± 0.06. The star has an estimated age range 1.2 – 2.3 billion years, and it is younger than our own star which has an age of 4.6 billion years.
The star is reported to be orbited by the super-Earth extrasolar planets COROT-7b and COROT-7c, both discovered in 2009. The existence of a possible third planet COROT-7d remains unconfirmed. The discovery of the inner planet was made using the astronomical transit method by the COROT program. CoRoT-7b is notable for its small size.
[https://en.wikipedia.org/wiki/COROT-7]
Monoceros contains many clusters and nebulae, most notable among them:
Sirius and M41 (lower right), M50 (upper left), and NGC 2360 (lower left)
Messier 50 (also known as M 50 or NGC 2323) is an open cluster in the constellation Monoceros. It was perhaps discovered by G. D. Cassini before 1711 and independently discovered by Charles Messier in 1772. M50 is at a distance of about 3,000 light-years away from Earth. It is described as a ‘heart-shaped’ figure.
[https://en.wikipedia.org/wiki/Messier_50]
The Rosette Nebula (NGC 2237, 2238, 2239, and 2246) is a diffuse nebula in Monoceros. It has an overall magnitude of 6.0 and is 4900 light-years from Earth. The Rosette Nebula, over 100 light-years in diameter, has an associated star cluster and possesses many Bok globules in its dark areas. It was independently discovered in the 1880s by Lewis Swift (early 1880s) and Edward Emerson Barnard (1883) as they hunted for comets:
The Rosette Nebula
The Rosette Nebula is not the only cosmic cloud of gas and dust to evoke the imagery of flowers- but it is the most famous. At the edge of a large molecular cloud in Monoceros, some 5,000 light years away, the petals of this rose are actually a stellar nursery whose lovely, symmetric shape is sculpted by the winds and radiation from its central cluster of hot young stars. The stars in the energetic cluster, cataloged as NGC 2244, are only a few million years old, while the central cavity in the Rosette Nebula, cataloged as NGC 2237, is about 50 light-years in diameter. The nebula can be seen firsthand with a small telescope toward the constellation of the Unicorn (Monoceros).
[http://apod.nasa.gov/apod/ap120214.html]
The Christmas Tree Cluster (NGC 2264) is another open cluster in Monoceros. Named for its resemblance to a Christmas tree, it is fairly bright at an overall magnitude of 3.9; it is 2400 light-years from Earth. The variable star S Monocerotis represents the tree’s trunk, while the variable star V429 Monocerotis represents its top:
A Fox Fur, a Unicorn, and a Christmas Tree
What do the following things have in common: a cone, the fur of a fox, and a Christmas tree? Answer: they all occur in the constellation of the unicorn (Monoceros). Pictured as a star forming region cataloged as NGC 2264, the complex jumble of cosmic gas and dust is about 2,700 light-years distant and mixes reddish emission nebulae excited by energetic light from newborn stars with dark interstellar dust clouds. Where the otherwise obscuring dust clouds lie close to the hot, young stars they also reflect starlight, forming blue reflection nebulae. The image spans about 3/4 degree or nearly 1.5 full moons, covering 40 light-years at the distance of NGC 2264. Its cast of cosmic characters includes the Fox Fur Nebula, whose convoluted pelt lies below center, bright variable star S Mon immersed in the blue-tinted haze, and the Cone Nebula near the tree’s top. Of course, the stars of NGC 2264 are also known as the Christmas Tree star cluster. The triangular tree shape traced by the stars appears here with its apex at the Cone Nebula and its broader base centered near S Mon.
[http://apod.nasa.gov/apod/ap120410.html]
Hubble’s Variable Nebula (NGC 2261) is a nebula with an approximate magnitude of 10, 2500 light-years from Earth. Though it is named for Edwin Hubble, it was discovered in 1783 by William Herschel. Hubble’s Variable Nebula is illuminated by R Monocerotis, a young variable star embedded in the nebula; the star’s unique interaction with the material in the nebula makes it both an emission nebula and a reflection nebula. One hypothesis regarding their interaction is that the nebula and its illuminating star are a very early stage planetary system:
Hubble’s Variable Nebula (NGC 2261)
Hubble’s variable nebula is named (like the Hubble telescope itself) after the American astronomer Edwin P. Hubble, who carried out some ofthe early studies of this object. It is a fan-shaped cloud of gas and dust which is illuminated by R Monocerotis (R Mon), the bright star at the bottom end of the nebula. Dense condensations of dust near the star cast shadows out into the nebula, and as they move the illumination changes, giving rise to the variations first noted by Hubble. The star itself, lying about 2,500 light-years from Earth, cannot be seen directly, but only through light scattered off of dust particles in the surrounding nebula. R Mon is believed to have a mass of about 10 times that of the Sun, and to have an age of only 300,000 years. There is probably a symmetrical counterpart of the fan-shaped nebula on the southern side of the star, but it is heavily obscured from view by dust lying between this lobe and our line of sight.
[https://www.spacetelescope.org/images/opo9935c/]
The Red Rectangle Nebula, so called because of its red color and unique rectangular shape, is a protoplanetary nebula in the Monoceros constellation. Also known as HD 44179, the nebula was discovered in 1973 during a rocket flight associated with the AFCRL Infrared Sky Survey called Hi Star. The binary system at the center of the nebula was first discovered by Robert Grant Aitken in 1915:
Hubble Frames a Unique Red Rectangle
The star HD 44179 is surrounded by an extraordinary structure known as the Red Rectangle. It acquired its moniker because of its shape and its apparent color when seen in early images from Earth. This strikingly detailed Hubble image reveals how, when seen from space, the nebula, rather than being rectangular, is shaped like an X with additional complex structures of spaced lines of glowing gas, a little like the rungs of a ladder.
The star at the center is similar to the sun, but at the end of its lifetime, pumping out gas and other material to make the nebula, and giving it the distinctive shape. It also appears that the star is a close binary that is surrounded by a dense area of dust- both of which may help to explain the very curious shape.
The Red Rectangle is an unusual example of what is known as a proto-planetary nebula. These are old stars, on their way to becoming planetary nebulae. Once the expulsion of mass is complete a very hot white dwarf star will remain and its brilliant ultraviolet radiation will cause the surrounding gas to glow. The Red Rectangle is found about 2,300 light-years away in the constellation Monoceros (the Unicorn).
The High Resolution Channel of the NASA/ESA Hubble Space Telescope’s Advanced Camera for Surveys captured this view of HD 44179 and the surrounding Red Rectangle nebula- the sharpest view so far. Red light from glowing Hydrogen was captured through the F658N filter and colored red. Orange-red light over a wider range of wavelengths through a F625W filter was colored blue. The field of view is about 25 by 20 arcseconds.
[https://www.nasa.gov/image-feature/goddard/2016/hubble-frames-a-unique-red-rectangle]
IC 2177 is a region of nebulosity that lies along the border between the constellations Monoceros and Canis Major. It is a roughly circular H II region centered on the Be star HD 53367. This nebula was discovered by Welsh amateur astronomer Isaac Roberts and was described by him as “pretty bright, extremely large, irregularly round, very diffuse.”
The name Seagull Nebula is sometimes applied by amateur astronomers to this emission region, although it more properly includes the neighboring regions of star clusters, dust clouds and reflection nebulae. This latter region includes the open clusters NGC 2335 and NGC 2343:
[https://en.wikipedia.org/wiki/IC_2177]
The Wings of the Seagull Nebula
This new image from ESO shows a section of a cloud of dust and glowing gas called the Seagull Nebula. These wispy red clouds form part of the “wings” of the celestial bird and this picture reveals an intriguing mix of dark and glowing red clouds, weaving between bright stars. This new view was captured by the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile.
Running along the border between the constellations of Canis Major (The Great Dog) and Monoceros (The Unicorn) in the southern sky, the Seagull Nebula is a huge cloud mostly made of hydrogen gas. It’s an example of what astronomers refer to as an HII region. Hot new stars form within these clouds and their intense ultraviolet radiation causes the surrounding gas to glow brightly.
The reddish hue in this image is a telltale sign of the presence of ionised hydrogen. The Seagull Nebula, known more formally as IC 2177, is a complex object with a bird-like shape that is made up of three large clouds of gas- Sharpless 2-292 forms the ‘head,’ this new image shows part of Sharpless 2-296, which comprises the large ‘wings,’ and Sharpless 2-297 is a small, knotty addition to the tip of the gull’s right ‘wing.’
These objects are all entries in the Sharpless nebula catalogue, a list of over 300 glowing clouds of gas compiled by American astronomer Stewart Sharpless in the 1950s. Before he published this catalogue Sharpless was a graduate student at the Yerkes Observatory near Chicago, USA, where he and his colleagues published observational work that helped to show that the Milky Way is a spiral galaxy with vast, curved arms.
Spiral galaxies can contain thousands of HII regions, almost all of which are concentrated along their spiral arms. The Seagull Nebula lies in one of the spiral arms of the Milky Way. But this is not the case for all galaxies; while irregular galaxies do contain HII regions, these are jumbled up throughout the galaxy, and elliptical galaxies are different yet again- appearing to lack these regions altogether. The presence of HII regions indicates that active star formation is still in progress in a galaxy.
This image of Sharpless 2-296 was captured by the Wide Field Imager (WFI), a large camera mounted on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile. It shows only a small section of the nebula, a large cloud that is furiously forming hot stars in its interior. The frame shows Sharpless 2-296 lit up by several particularly bright young stars- there are many other stars scattered across the region, including one so bright that stands out as the gull’s ‘eye’ in pictures of the entire complex.
[https://www.eso.org/public/news/eso1306/]
[https://en.wikipedia.org/wiki/Monoceros]
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