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Winter star clusters

Two rich star groups offer dazzling views.
Harrington
As the stream of the winter Milky Way flows southward toward the horizon, it passes through eastern Orion and Canis Major, and continues through a region that looks nearly starless to the naked eye. Much of this empty patch of sky belongs to the constellation Puppis.

Puppis is one of three constellations carved from the old star-picture Argo Navis. Dating from ancient Greek mythology, the Argo was the mythical ship Jason and the Argonauts used to search for the Golden Fleece. When viewed from Greece, the stars of Argo skimmed the southern horizon, as if the ship were “sailing” across the Mediterranean Sea.

French astronomer Nicolas-Louis de Lacaille (1713–1762) was the first to dissect Argo Navis into several components. In his 1763 catalog of the southern sky, Coelum Australe Stelliferum, he introduced Puppis the Stern (or Poop Deck), Vela the Sails, and Carina the Keel. Two creatures were also drawn to accompany the trio: Columba the Dove and Volans the Flying Fish.

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The bright but sparse cluster NGC 2362 surrounds 4th-magnitude Tau Canis Majoris.
Adam Block/Mount Lemmon Skycenter/University of Arizona
For us living in the Northern Hemisphere, Puppis is the most familiar part of Argo Navis. Truth be told, it offers little to the eye alone. But thanks to the Milky Way running through, the region is rich in deep-sky treasures. This month, we are going to seek out an often ignored winter star cluster, M93.

Unfortunately, since there are no nearby bright stars to guide our way, finding M93 can be a challenge. Because we will need to bounce from point to point to point, it will make your life easier if you can mount your binoculars to a tripod or other support. That way, you won’t need to re-aim each time you pause along the way.

Begin at Canis Major and brilliant Sirius (Alpha [α] Canis Majoris). Trace the Large Dog’s body southeastward for about 11°, or two binocular fields, until you get to Wezen (Delta [δ] CMa). Once centered on Wezen, look for two 4th-magnitude stars in the same field. They are Omega (ω) CMa to its southeast and Tau (τ) CMa to its northeast. Tau is a type O blue supergiant star, one of the most luminous stars in tonight’s sky. Studies show that it is producing 280,000 times more energy than our Sun. It lies some 50,000 light-years away.

Take a close look at Tau. Does it look a little fuzzy to you? It might, especially if you are using 15x or more. That’s because Tau is the brightest of several hundred stars in the tight open cluster NGC 2362. Trying to make out the rest of the bunch is made difficult by Tau’s overwhelming brightness. My 10x50s fail, but I can see a few points through my 16x70s. Aside from Tau, the brightest cluster stars shine at 7th magnitude.

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Open cluster M93 is a rich, tight grouping of stars that jumps out in a binocular view.
Dan Crowson
From Tau and NGC 2362, look about a binocular field due east for 3rd-magnitude Xi (ξ) Puppis and its 5th-magnitude neighbor, SAO 174592. Both are spectral type G stars like our Sun, although they are significantly more massive. Their subtle yellowish tints contrast nicely against a field of blue-white stardust, especially if you slightly defocus the view. But while they may look close together, it turns out that this pairing is just an illusion. Xi is about 1,350 light-years from Earth, while its “companion” is only 300 light-years away.

While admiring Xi, you should also notice a dim glow in the same field, just 1.5° to its northwest. That’s our quest, open cluster M93. Charles Messier discovered M93 in March 1781. He described it as a “cluster of small stars without nebulosity.”

Through 50mm binoculars, however, M93 does look nebulous, with about a half-dozen or so faint points peeking out. Through my 10x50s, M93 impresses me as triangular in shape. Switching to my 16x70 and 25x100 giants, I can also imagine what the famous 19th-century astronomer William Henry Smyth (1788–1865) described in his 1844 classic Bedford Catalogue as a starfish when he looked this way. Still others envision a spider or a butterfly.

About 80 stars comprise M93. The brightest are type B blue supergiants and shine at 8th magnitude. Buried within are no fewer than eight orange and red giant stars. Can you spot any of them? The brightest, at 8th magnitude, is just west of center.

But be forewarned that to see M93, you’ll need to plan your visit. Its southerly position in our sky, no more than 27° above my horizon at 40° north latitude, means that the best view comes when it’s on or near the meridian. On February 1, it culminates at about 11 p.m. local time. By month’s end, that will occur about two hours earlier.

Have a favorite binocular target that you’d like to share with everyone? Tell me about it. Contact me through my website, philharrington.net.

Until next month, remember that two eyes are better than one.

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