Mohawk Valley Astronomical Society

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Dog Nights of Winter

by Perry Pezzolanella, MVAS

The fiercest, coldest nights of the year glitter with some of the brightest stars. Sirius, the brightest of all the stars in the night sky, blazes a cold bluish-white in the constellation Canis Major, the Great Dog, located below mighty Orion. The sky around Sirius has a wealth of beautiful deep sky objects just waiting to be discovered by those willing to brave the long, cold nights.

Using the chart that follows, the most obvious star is Sirius, the Dog Star, shining at magnitude -1.46. It is a double star with a white dwarf nicknamed the Pup shining at magnitude +8.5. The Pup will become easier to see in the years ahead as its orbit takes it farther from Sirius. The white color of the Pup makes the cold, bluish tint of Sirius obvious. The bluish tint is not an optical illusion as the color indicates that Sirius is an extremely hot star. It is interesting to note that Sirius in moving slightly closer to Earth and will be slightly brighter in about 60,000 years, but will have moved into the southern constellation Columba, the Dove. Eventually it will disappear below the southern horizon for good as it moves away from Earth and slowly fades.

Canis Major is home to several open clusters. One of the best is M41 shining at magnitude +4.5 below Sirius. This author has described M41 as a “thick cluster of diamonds with a golden double star near the core and sprawling dimmer stars” through his 80mm (3.1”) Meade refractor. Other open clusters include NGC2354, a large and roundish scattered cluster with stars arranged like curving chains and NGC2362, which looks like a “beautiful sugary glow around a bright star for a sparkling effect”. The bright star, Tau Canis Majoris, along with NGC2362, are known as the Tau Canis Majoris Cluster. Often overlooked is Collinder 121 (Cr121) below M41. This is a large, scattered cluster with no concentrated center, but several groups of stars make neat patterns and asterisms.

In the nearby constellation, Puppis, but still within the realm of Sirius, is M93, an impressive open cluster shaped like a triangle of several bright stars all immersed in a halo of dimmer stars, which gives it depth. A pleasing open cluster is M47, which has several bright stars including a pair of doubles concentrated in a loose, irregular patch. The best open cluster is M46, which is just to the east of M47 and also in Puppis. It is a fuzzy grouping of delicate stars giving it a powdery look, but there is an added surprise. A tiny planetary nebula, NGC2438 can be seen within the cluster near the northeast edge appearing like a tiny soap bubble. This tiny nebula is probably not part of the cluster and simply lies along the same line of sight in front of it. A larger telescope fitted with an oxygen III filter will enhance the nebula for a dramatic sight.

The Sun is near Sirius in August and according to folklore the combined brilliance of both brings increased heat to the Earth during that time of year. The sweltering heat tends to cause dogs to become lazy, hence the “Dog Days of Summer”. However, it is during these frigid winter nights that the Great Dog appears to energetically leap into the heavens with Sirius and his wealth of deep sky wonders for all to appreciate.