Canis Major

by Shaphan Shank | Mar 1, 2023 | 0 comments

Canis Major star map

Canis Major, the Great Dog, lies in the southern sky just southeast of Orion. Like Orion, many of the stars that make up Canis Major are bright. The constellation’s brightest star, Sirius, is the brightest star in our night sky. Although Canis Major is only a mid-sized constellation, it contains a wide variety of excellent telescopic targets.

Canis Major’s brightest star is also one of the constellation’s most interesting and challenging telescopic targets. Sirius is a double star with a separation that varies between 3” and 11” over a period of 50 years. Sirius A and B are currently around their maximum separation of 11”. Splitting the pair would be a breeze except for one thing—Sirius B is an Earth-sized white dwarf star that is 10 magnitudes (~10,000×) fainter than Sirius A. As a result, picking Sirius B out of the glare of Sirius A can be extremely difficult. However, it can be done with all but the smallest good-quality telescopes.

To see Sirius B, it is important to eliminate as much glare as possible from Sirius A. This means that the telescope should be collimated well, and that both the telescope and eyepiece should have clean optics. (Don’t grab glass cleaner and scrub your telescope, as you could easily damage it. Your first goal is to keep the scope clean. Secondly, if the mirror must be cleaned, learn the proper procedure prior to cleaning.) Most importantly, the atmosphere must be stable, with no turbulence. (In astronomers’ terms, the seeing must be excellent.) Use as much magnification as the seeing allows, at least 200×.

Sirius B is east-northeast of Sirius A, so it will drift across the field of view almost directly behind Sirius A if you are not using a telescope that tracks with the stars. It may be helpful to observe Sirius before the sky is completely dark. This will decrease some of the glare from Sirius A.

In the western part of Canis Major lies another beautiful double star that is quite the opposite of Sirius. Unfortunately, this overlooked double lacks a name, so it goes by its catalog designations of h 3945 or 145 Canis Majoris. Much like the summertime double Albireo, the component stars of h 3945 are a contrasting gold and blue. The components are separated by about 27”, so they are easy to split at low magnification. To find h 3945, first find the bright stars Adhara and Wezen in the southern part of Canis Major. Go from Adhara to Wezen, then go the same distance again to get to h 3945.

A gorgeous little open star cluster called NGC 2362 lies about 1.5° south-southwest of h 3945, or about 2.5° northeast of Wezen. NGC 2362 consists of a small spray of stars surrounding a bright triple star. Moderate magnifications (~100×) will probably give the best views of the cluster. Using averted vision may reveal a host of faint stars amid the cluster’s brighter members.

Two more noteworthy open clusters, M41 and NGC 2360, occupy the northern part of Canis Major. M41 is the brighter of these two. This cluster is located 4° south of Sirius. M41 is bright enough to be visible to the unaided eye under dark skies. Binoculars will resolve a few of the cluster’s stars, and telescopes at low magnification will give the best views of the cluster.

NGC 2360 lies about 3.5° east of the star Muliphein. This cluster is dimmer and smaller than M41, but it is still bright enough to be visible with binoculars as a fuzzy spot of light. Telescopes at low to moderate magnification will give the best views of NGC 2360.

Canis Major’s most impressive nebula is NGC 2359, also known as the Duck Nebula or Thor’s Helmet. To find this nebula, imagine a line connecting Sirius to Muliphein, 4.5° to the east-northeast of Sirius. Continue the line past Muliphein another 4.5°, but angle slightly to the north instead of staying in line with Muliphein and Sirius. This will bring you to the Duck Nebula.

NGC 2359 is not very bright, but it is relatively easy to observe under dark, clear skies. Light pollution will make it much harder to see. The Duck Nebula consists of a round “bubble” of nebulosity, with streamers of nebulosity extending in several directions from the bubble. The easiest parts of the nebula to observe are the central bubble and the brightest extension. With larger telescopes and dark skies, you may be able to see wispy detail within the central bubble, as well as three long extensions leaving the bubble. OIII and UHC filters greatly enhance the view of this nebula, increasing the amount of visible detail within it. Experiment with different magnifications when observing the Duck. Low magnifications may reveal more faint nebulosity surrounding the brightest area, while slightly higher magnifications will show the most detail in the main part of the nebula.

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