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Star clusters in Monoceros

Little-known star groups offer gemmy views.
Harrington
This month, three of the season’s brightest stars — Betelgeuse, Sirius, and Procyon — form a large equilateral triangle that many know as the Winter Triangle. Although they blaze brightly on clear nights, they frame a surprisingly barren part of the late winter sky. Much of this vast, starless region belongs to Monoceros the Unicorn.

Although hapless Monoceros holds little to grab the attention of constellation hunters, it overflows with open star clusters for us binocularists. Yet, while many of those clusters are visible in binoculars, only one found its way into Charles Messier’s catalog. That’s our first stop this month, M50.

M50’s story is an interesting one. The Italian-born French astronomer Giovanni Cassini is often credited with its discovery. He is said to have spotted a “nebula” between Canis Major and Canis Minor sometime before 1711. Although Messier searched for Cassini’s nebula in 1771, he never found it. Only a year later did he chance upon an errant open cluster in the same vicinity; it became M50. Most assume that this was Cassini’s “nebula,” but we will probably never really know for sure.

One thing we do know for sure is that even though M50 lies in the middle of nowhere, it’s surprisingly easy to find with binoculars. Begin your hunt at Sirius and slowly scan northeastward toward Procyon. Along the way, you’ll pass Theta (θ) Canis Majoris about a binocular field from Sirius. Another field-hop in the same direction should bring a tiny blur of starlight into view. That’s M50. 

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Pakan’s 3 is an asterism that lies close to M50. This zigzag clump was named by amateur astronomer Randy Pakan from Edmonton, Alberta.
Tony Hallas
Through my 10x50 binoculars, M50 offers a soft blur of light peppered by a few faint points. The brightest cluster star, at 8th magnitude, lies just south of center. When I use my 16x70 binoculars, more stars emerge from the fog, while under dark skies my 25x100s reveal that those stars seem to fall into curved lines and arcs that some see as heart-shaped or possibly like an arrowhead oriented east-west. If your eyes are especially color-sensitive, you may notice that M50’s brightest star shows a subtle golden tint. All other cluster members appear white.

A trio of open star clusters lies southeast of M50. The most obvious is NGC 2353. With M50 still in view, shift your attention about half a field to the southeast. There, you’ll spot three 6th-magnitude stars in a gentle arc 3/4° long. Look carefully, and you may see that the center star in the arc looks a little fuzzy and is accompanied by some fainter companions. That’s the cluster. All told, some 100 stars belong to NGC 2353, although most fall below the reach of binoculars. The other two clusters, NGC 2335 and NGC 2343 are fainter still.

ASYPH0318_01copy

The sparse open star cluster M50 makes a good sight in binoculars. It is one of the highlights of Monoceros.

Richard McCoy
As you approach M50 from Theta Canis Major to the south, you may notice a tiny zigzag clump of faint stars a little west of the line connecting those two. That asterism is best known as Pakan’s 3, named after amateur astronomer Randy Pakan from Edmonton, Alberta, who first noted its shape. Depending on your binoculars, it should be in the same field as M50, since both are separated by only 3°. Put M50 in the northeastern part of your view, then look to the southwest for a collection of eight faint stars that together form an angled number 3. None of those stars shines brighter than 9th magnitude, however, so you’ll need a very clear March night to see them.

Finally, let’s visit another fun asterism that I first bumped into more than three decades ago while researching for my book Touring the Universe Through Binoculars. Look about 5.5° west of M50, and less than half a degree northwest of the 5th-magnitude red giant SAO 133585. There, you’ll find a compact clump of seven faint stars forming an inverted letter V. Given the constellation they lie within, the group has since become known as the Unicorn’s Horn. All seven stars span a compact 7', so they may look nebulous. Since none shines brighter than 9th magnitude, 10x70 binoculars are probably the smallest that will show them well. The group is clear in my 16x70s and striking in my 25x100s.

Let me know how you do in sighting the Unicorn’s Horn, as well as the other targets mentioned this month. Contact me through my website, philharrington.net. Until next month, remember that two eyes are better than one.

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