Star Constellation Facts: Monoceros

Star Constellation Facts: Monoceros

Monoceros (“The Unicorn”) is a faint northern sky constellation that lies on the celestial equator, and can be seen by observers located between +75° and -90° of latitude. It is the 35th largest constellation, but is faint and difficult to recognise, with its brightest star, Alpha Monocerotis, having a visual magnitude of just 3.94. Monoceros, nevertheless, contains a number of notable stellar objects, including 16 stars with confirmed planets, and a massive binary system called Plaskett’s Star.


Monoceros was invented in the 17th century by Petrus Plancius, a Dutch astronomer and cartographer, based upon the observations made by Dutch navigators in South east Asia who filled a perceived gap in the night sky between the constellations of Orion and Hydra. This faint area had been left blank since ancient Greek times, and Plancius’s motivation for creating a unicorn themed constellation where none had existed before is said to have been because the mythical creature appears several times in the Old Testament of the Bible. Whatever the reason behind the creation of Monoceros, it is was included in the 88 modern constellations recognised by the International Astronomical Union (IAU) in 1922.

Orion Family of Constellations

Monoceros borders the constellations of Canis Major, Canis Minor, Gemini, Hydra, Lepus, Orion and Puppis, and also forms part of the small ‘Orion Family of Constellations’ along with Canis Major, Canis Minor, Lepus and Orion.

Meteor Showers

Two meteor showers emanate from Monoceros- the Alpha Monocerids, and the December Monocerids.

– Alpha Monocerids, also known as the November Monocerotids, is a meteor shower that occurs from November 13th to December 2nd, with a peak on the 21st when up to 12 meteors per hour can be seen. However, peaks of more than 100 meteors per hour is said to occur every ten years, or so.

– December Monocerids runs from November 9th to December 11th, and peaks on the 8th of December each year, although visual observations rarely exceed two meteors per hour.

Notable Stars

Star Constellation Facts: Monoceros

– Alpha Monocerotis, the constellation’s brightest star, is an orange giant (K0 III) located 144 light years away with an apparent visual magnitude of 3.94. It has about 10.1 times the Sun’s radius, but just twice its mass.

– Gamma Monocerotis, the second brightest star in Monoceros, is an orange giant (K1.5III) found 645 light years from our solar system of magnitude 3.98.

– Delta Monocerotis, the constellation’s third brightest star, is a white main sequence star (A2V) situated 375 light years from the Sun with a visual magnitude of 4.15.

While Monoceros has only a few stars brighter than magnitude 4.0, what the constellation lacks in bright stars, it makes up for with the many massive stars it contains, some of which are noted below:

– Zeta Monocerotis is a massive, yellow supergiant (G2IB)
1,852 light years distant with an apparent magnitude of only 4.37. It has 62 times our sun’s radius, and a
spectacular 2,535 times its luminosity. Look for this star about three-quarters of a degree from the border between Hydra and Monoceros.

– 13 Monocerotis is a white supergiant (A0Ib) about 1,500 light years distant of magnitude 4.47. It has 37 times the Sun’s radius, 9 times its mass, and shines 10,800 times brighter. It is thought to have been ejected from the star cluster NGC 2264, which is located only 3.5 degrees to the north, and together with Epsilon Monoceros, it forms the “gateway” to the Rosette Nebula, which is located a few degrees southward of the pair. One other notable feature of 13 Monocerotis is that it appears to be enveloped by a dim reflection nebula, as a result of its light being scattered over a 10-light year area by a relatively dense cloud of interstellar dust.

– HD 48099, as bright stars go, is in a league of its own. Due to its distance of nearly 41,000 light years from Earth, its -9.11 absolute magnitude is dimmed to only an apparent magnitude of 6.37, which means that this star must be at least 350,000 times as bright as the Sun. HD 48099 is in fact a very close binary system with an orbital period of only 3.078 days, and both components are thought to be hot O-class stars.

V838 Monocerotis– V838 Monocerotis, a red supergiant about 20,000 light years away, is arguably one of the most famous stars in the entire sky. The star was almost invisible to astronomers until its sudden outburst in 2002 started lighting up V838’s surroundings. The visible matter in the image does not form part of the star, but is instead matter surrounding the star that has been made visible by the spreading light echo from the outburst. One strange feature of the supergiant is the fact that it did not blow off its outer layers, which is what normally happens when stars explode, or suffer an outburst. Nonetheless, even though V838’s temperature has increased, it’s diameter has decreased, and it is now about 370 times bigger, and 15,000 times brighter than the Sun.

Deep Sky Objects

Monoceros contains one Messier object, namely a beautiful open cluster called M50 (NGC 2323), with other deep-sky objects in the constellation including the Rosette Nebula, the Christmas Tree Nebula, and the Cone Nebula. Below are details of some of the more spectacular deep sky objects in The Unicorn.

– Messier 50 (NGC 2323) is an open cluster located about 3,200 light yearsfrom Earth with an apparent visual magnitude of 5.9. It was discovered by Charles Messier in 1772, and is famous for the arrangement of its constituent stars, which is said to form the popular view of a human heart. However, the shape of a heart is not readily apparent.

Rosette Nebula Rosette Nebula (Caldwell 49) is a vast gas and dust cloud that is around 65 light years across, 5000 light years distant, and has an apparent visual magnitude of 9.0. The Rosette Nebula supplied the material from which NGC 2244, the star cluster at the centre of the nebula, was formed.

– Hubble’s Variable Nebula (NGC 2261, Caldwell 46) is a reflection nebula located about 2,500 light years away with an apparent visual magnitude of 9.0. Its variability is thought to be the result of dust and gas in the nebula that periodically obscures the light from the star R Monocerotis, which is buried deep within it, and therefore not directly visible.

Red Rectangle Nebula– Red Rectangle Nebula (HD 44179) is a spectacular protoplanetary nebula located about 2,300 light years away with an apparent magnitude of 9.02. Before the advent of the Hubble Space Telescope, the nebula appeared to be rectangular, but from this image taken by Hubble, it seems that it is more shaped like an X. When the relatively cool star in the centre of the structure evolves into a hot white dwarf sometime in the next several thousand years, the structure is expected to develop fully into a planetary nebula on the lines of M57 in Libra.