Monoceros the Unicorn is one of the most overlooked winter constellations. This is probably because the constellation is sprawling and faint, without an eye-catching shape. Fortunately, Monoceros is surrounded by familiar constellations, which makes it easier to locate. Monoceros lies right in the void between Orion and his two dogs, Canis Major and Canis Minor. Despite its underwhelming appearance, Monoceros contains a good number of star clusters and nebulas.
Beta Monocerotis, in the southwest corner of the constellation, is one of the finest triple stars in the sky. All three component stars are white, and all are of similar brightness. With a separation just under 3 arcseconds, the B and C component stars are pretty tight and will require moderate magnification (100-150x) to split. The whole triple system is compact, bright, and beautiful.
M50, the only Messier object in Monoceros, is an open star cluster about 8½° east-southeast of Beta Mon. It is almost exactly halfway between Alpha and Beta Mon. This is a nice bright cluster, with a bright orange star at its southern edge. A binocular will easily show this cluster and may resolve a few stars. A telescope at low magnification will give the best view.
Epsilon Monocerotis is the starting point for finding our next targets. This star is about 7½° southeast of Betelgeuse in Orion.
The Rosette Nebula (NGC 2237-39, 2246) is located 2° east of Epsilon Mon. The Rosette is a large emission nebula surrounding NGC 2244, an open star cluster. This nebula is beautiful, but it can be a very challenging target because of its 1° size and resulting low surface brightness. The keys to successfully observing the Rosette are to observe from a site with little light pollution and to use low magnification (50x or less). A UHC or OIII filter will boost the nebula’s contrast and make it easier to observe. A large aperture will help when observing the Rosette, but it can be seen in smaller telescopes if the other conditions are met.
The Rosette Nebula looks a little like a puffy doughnut, with one side appearing brighter and more filled out than the other. NGC 2244, the open cluster associated with the Rosette, lies in the hole in the middle of the nebula. This cluster is bright and can be seen with binoculars and even with the unaided eye as a brighter spot in the Milky Way.
(Note: Because the Rosette Nebula has 4 NGC numbers, the map labels the nebula/cluster simply as NGC 2244.)
The star 15 Monocerotis lies about 7° northeast of Epsilon Mon. This star itself is rather unremarkable, but it is the brightest member of NGC 2264, a beautiful open star cluster called the Christmas Tree Cluster. This cluster really resembles an evergreen tree with the top pointing south; 15 Mon marks the trunk, and dimmer stars outline the rest of the tree. A telescope at low magnification will probably give the best view of this cluster, but you should be able to find the cluster and see its shape with a binocular.
NGC 2261, also known as Hubble’s Variable Nebula, is located just over 1° southwest of 15 Mon. Quite the opposite of the Rosette, this nebula is small and bright. It is triangular and strongly resembles a small comet, complete with a fan-shaped “tail” and a star making a bright “nucleus.” Once you’ve found this nebula, boost the magnification to look for details such as light and dark structure.
While most nebulas appear the same from one year to the next, this nebula changes appearance slightly as dark and bright areas shift around. Astronomers believe that the variation in appearance is caused by shadows from fast-moving clouds of dust near the star at the brightest corner of the nebula. The variations are easy to see in photos, but detecting them visually is more difficult because the changes are not drastic and occur over a period of weeks or months.
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Measuring Degrees Made Simple
You can use your hand to approximate distances in the sky. At arm’s length, palm away:
1° – Width of little finger
5° – Three middle fingers
10° – Clenched fist
20° – Outstretched thumb to outstretched little finger
Reading a Star Map
To read a star map, hold overhead with the side of the map labeled “N” toward the north.