Cluster Hunting with the “Big Dog”
As the weather moderates and northern observers emerge from hibernation this time of year, the constellation Canis Major, the Big Dog, is ideal for exploring an hour or two after sunset. In this part of the sky, as you look along the Orion Arm of the Milky Way, you’ll find many fine open clusters and star fields. It’s well worth a look with binoculars or a small telescope, or even with your unaided eye on a dark early spring night.
Located just southeast of Orion, Canis Major is smaller than the great hunter but hardly less striking. Here’s an image of the constellation, snapped with a low-end digital SLR, a 50 mm lens, and the help of this fine guidebook. Even in this 30-second exposure, taken recently from a light-clogged hotel parking lot, you can see the richness of the star fields of this constellation.
A DSLR image of Canis Major, shot with the help of this fine guidebook.
Starting from Sirius, you can sweep south to the famous open clusters M41 and NGC 2362, which we’ve looked at before.
But if you can, keep moving south to find four lesser-known clusters in Canis Major, Collinder 121, 132, and 140, along with Collinder 135 just over the border in the constellation Puppis (see map below). Each has a distinct shape and density, and you’ll have fun tracing out shapes among the constituents of these sparse galactic clusters.
Let’s start with Collinder 121. It’s a small group of associated stars centered on the bright star sigma-1 Canis Majoris. In binoculars, it looks like a little knot of stars spread-out stars; in a telescope, it hardly looks like a cluster at all compared to its larger and more compact brethren like M41. Some astronomers believe Cr121 is an older cluster that’s slowly losing its constituent members. Others believe it’s a young cluster with a number of low-mass stars. It’s not clear which view is correct: the cluster is somewhat hard to study because of its small size and great distance of 3300 light years.
A map of Canis Major and northern Puppis, showing the star clusters Cr 121, Cr132, Cr 140, and Cr135.
Now to Cr132, just south of the bright stars Aludra and Adhara and roughly equidistant from each. It’s a group of perhaps a dozen stars spread out over a full degree of sky. Binoculars give you a better view of this cluster, which lies some 1500 light-years from Earth.
Just 2 degrees to the southeast of Cr 140, you’ll find a slightly tighter but still sprawling group of 20-30 stars. This is Collinder 140 (see the top image on this page). This cluster is just 25 million years old and lies about 1000 light years away. It was likely observed by the diligent Lacaille in the late 18th century, long before Per Collinder logged it. Like the first two clusters on our tour, Cr 140 is best observed at low magnification. The brightest stars are of magnitude 5.4 or so.
Finally, drop 7-8 degrees due south of Cr 132 to find the the 2.7-magnitude star pi Puppis. It’s surrounded by the 10-15 dimmer stars of Collinder 135. Pi Pup belongs to the cluster. For decades, astronomers believed Cr 135 wasn’t a cluster at all. But ten years ago, the Hipparcos satellite confirmed the stars are all about the same distance away, so it may be a true open cluster after all. Pi Pup is the first star in the cluster to evolve into K-class supergiant. It’s destined to explode as a supernova one day.
In a future sky tour, we’ll continue south from here into Puppis and Carina, examining more delights of the Orion and Carina Arms of our galaxy. But for today, that’s it.
