YOU CAN SEE THE GREAT SHIP Argo Navis sinking into the southern horizon on this month's all-sky chart facing page 82. This celestial ship was one of the 48 constellations in Ptolemy's Almagest, representing the famous vessel of the Argonauts. But modern astronomers have dismantled Argo Navis, or simply Argo, into smaller constellations outlining its keel, sails, and stern (see page 90).
For those of us living at midnorthern latitudes it looks as if a disaster is occurring. Most of the great ship is out of sight below the southern horizon, with only the stern, Puppis, sticking up. Canis Major, the Larger Dog, appears to have jumped off the back of the doomed craft, and we see him in mid-leap to the northwest of Puppis.
In this area of the sky there are dozens of deep-sky delights within the grasp of a small telescope. This month we'll visit a few that are conveniently placed near naked-eye stars, so let's start our voyage in Canis Major at Sirius, the brightest star in the night sky.
Sirius is a double star, its white-dwarf companion having a diameter comparable to Earth's. Since Sirius is often called the Dog Star, its tiny companion has been nicknamed the Pup. The pair is not generally considered game for small scopes. Although the Pup is fairly bright at magnitude 8.5, it is usually lost in the intense glare of brilliant Sirius. The apparent separation of the two stars is increasing, however, and last year I was finally able to catch a glimpse of the Pup with my 4.1-inch (105-mm) refractor on one night of exceptionally fine seeing. Currently the Pup is 5.5" southeast of Sirius, and in 2022 it will reach its maximum separation of 11.3" toward the east-northeast. What's the first year when you'll spot the Pup? This definitely belongs on your list of greatest challenges.
Split or not, Sirius is the beacon that will guide us to the large open cluster M41. It lies 4[degrees] south of Sirius and is visible to the unaided eye from a dark observing site. Binoculars or finderscopes show M41 as a good-size hazy patch and may unveil some of its stars. Massachusetts amateur Lew Gramer calls this cluster the Dog's Spot. My 4.1-inch refractor at 17x shows 50 fairly bright to faint stars within 40'. The two brightest dwell in the heart of the cluster and form a wide, nearly matched pair with the western one displaying a golden hue. Some skygazers describe this star as orange or even reddish. What color do you see? Increasing the power to 47x helps bring out the fainter members, swelling the crowd to 80. Many of the stars seem to be arranged in chains radiating from the center, and a 6th-magnitude star (12 Canis Majoris) lies just off the southeast edge.
Our next guidepost is the 3.9-magnitude star [Omicron.sup.1] ([[omicron].sup.1]) Canis Majoris, which dwells within the boundaries of Collinder 121. While a little bigger and brighter than M41, Cr 121 looks far less like a cluster. Its stars are loosely scattered and show no central concentration. Through my 4.1-inch scope at 17x, [Omicron.sup.1] looks deep yellow. A few dozen stars of 6th magnitude and fainter are gathered around it in a group with illdefined borders. A bright star occupies the southern reaches of the cluster, and a slightly dimmer one sits in the north. A kite-shaped group of 7th- and 8th-magnitude stars is conspicuous to the west of [Omicron.sup.1] along with a triangle of fainter stars to its east-southeast.
To locate our next cluster, look for the stars Delta (delta]) and Tau ([tau]) Canis Majoris shining at magnitude 1.8 and 4.4, respectively. NGC 2354 lies about halfway between them. At 47x, my little refractor shows 30 stars, faint to very faint, in a patch about 20' across. This is not an impressive group, but its knots and curving lines of stars invite dot-to-dot games. To me it looks like a slug executing a sharp turn, while others claim to see a pattern like the constellation Scorpius.
A pretty chest of starry gems surrounds the star [tau] CMa itself. Tau rests like a bright sapphire amid a tiny bed of lesser jewels. The collection is generally known as the Tau Canis Majoris Cluster and bears the designation NGC 2362. Use medium to high powers to separate blue-white Tau from its host of close companions. My 4.1-inch scope shows about 25 stars crowded into a mere 6'.
With an age of about 5 million years, NGC 2362 is one of the youngest star clusters known. Tau, a blue supergiant of spectral class O9, is thought to be a true member of the group. At the cluster's distance of 4,800 light-years, Tau must shine with the light of 50,000 Suns.
Finally, we leave the Big Dog behind and jump onto the sinking stern of Argo. The yellow star Xi ([xi]) Puppis, at magnitude 3.3, anchors the northeast end of a small curve of three stars. The other two shine at magnitude 5.3 and 7.9 and are dressed in a paler yellow light.
Our last cluster, M93, lies 1.5[degrees] northwest of Xi. M93 is a misty blur when viewed in binoculars or a finder. The cluster looks very nice through my little refractor at 47x, appearing rich in 8thmagnitude and fainter stars. At 87x I count 33 stars in a 7'-by-10' core, which is surrounded by a sparser, ill-defined halo containing about 50 stars of 10th magnitude and dimmer. The cluster is very patchy, with many irregular bunches of stars. The core looks roughly like a notched arrowhead, its two brightest stars being slightly orange. Other observers have imagined these stars arranged in the shape of a starfish or a butterfly. What does M93's pattern remind you of?
This month's clusters span a wide range of size, richness, and brightness. We can delight in the diversity that makes this class of objects so much fun to explore. Be sure to enjoy the sights of Canis Major and Puppis before they completely submerge below the horizon in late spring.
Stellar Sights in and near Canis Major Object Type Magnitude(s) Size/Sep. Sirius Double star -1.46,8.5 5.5" M41 Open cluster 4.5 38' Cr 121 Open cluster 2.6 50' NGC 2354 Open cluster 6.5 19' NGC 2362 Open cluster 4.1 7' M93 Open cluster 6.2 21' Object Distance (l-y) R.A. (2000.0) Dec. (2000.0) Sirius 8.6 6h 45.2m -16[degrees] 43' M41 2,300 6h 46.0m -20[degrees] 44' Cr 121 1,500 6h 54.1m -24[degrees] 11' NGC 2354 13,000 7h 14.3m -25[degrees] 44' NGC 2362 4,800 7h 18.8m -24[degrees] 57' M93 3,400 7h 44.6m -23[degrees] 52'
SUE FRENCH can be contacted by e-mail at firstname.lastname@example.org.
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|Title Annotation:||Dog Star's companion star|
|Publication:||Sky & Telescope|
|Date:||Mar 1, 2002|
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