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Monoceros and the Christmas Tree Cluster

The Backyard Astronomer by Adam England


The hollow between Orion, Canis Major (the Great Dog) and Gemini (the Twins) is a relatively dark area. Though the stars are difficult to see with the naked eye due to modern light pollution, in this void is a less-known and relatively modern constellation.

Image courtesy Joel Cohen, Prescott Valley 2021

Monoceros – ‘mono’ meaning ’one’ and ‘ceros’ meaning ‘horn,’ or the Unicorn — is a simple seven-point constellation, though only two of the stars are usually discernible without the aid of binoculars or telescope. Dutch mapmak


er Petrus Plancius first noted the Unicorn in 1612, with others following through the 17th century. With better telescopes William Herschel studied the area in 1781, becoming the first to discover that Beta Monocerotis, the brightest of the constellation’s stars, is actually three separate entities, observed as a curved line of pale blue-yellow stars. He later described this triple-star system as “one of the most beautiful sights in the heavens.”


Using a telescope to starhop around the dark areas among these constellations, you can find a slew of other heavenly surprises. The Rosette Nebula (NGC 2244) is known for its ring shape and dense nebulosity. The open cluster Messier 50 and NGC 2506 will appear as fuzzy cotton balls in the smallest apertures, and larger scopes can begin to define many close stars.The real highlight of Monoceros, though, is NGC 2264, the Cone Nebula. Appearing as though a celestial bowling ball was thrust down a foggy lane, parting the fog around it, a dark cone imparts densely defined edges to a large diffusion of nebulous clouds. At 2,600 light-years distant it is one of the closer nebulae to Earth, and covers a large swath of space with star-forming elements to create its stellar nursery. Along with the Fox Fur Nebula, this diffuse nebula is often referred to as the Christmas Tree Cluster due to its triangular shape with a star at the apex.


Take your binoculars or telescope out and start at the bright star Betelgeuse in Orion, exploring the area just below and down to the Hunter’s belt. As you scan the sky, moving downward toward the Beta Monocerotis triple-star system, you’ll be surprised at what you find.


To learn more about the sky, telescopes, or socialize with other amateur astronomers, visit us at prescottastronomyclub.org or Facebook @PrescottAstronomyClub to find the next star party, Star Talk or event.


Adam England is the owner of Manzanita Financial and moonlights as an amateur astronomer, writer, and interplanetary conquest consultant. Follow his rants andexploits on Twitter@AZSalesman or Facebook. com/insuredbyadam.


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