For the week including January 14, 2011

Sirus Illustration M41 Tau canis Majoris M41 Location Larger Illustration

THE BIG DOG

If you look low in the southeast around nine o’clock, you will see a very bright star just above the horizon. This star is Sirius, the Dog Star, which is the brightest star visible in the night sky. The constellation it’s a part of is Canis Major, the Greater Dog. Over the centuries, ancient people have connected this constellation to various dogs that appear in Greek mythology, but the prevailing story links it to one of the dogs that accompanies Orion, the Hunter.

The brilliant Sirius has a long history in astronomy. The ancient Egyptians waited every year for this star to climb above the horizon just before sunrise. When this happened, it was their signal to bring in the lawn furniture because the Nile River always flooded right afterward. In the summer, when Sirius and the Sun were closest together in the sky, it was thought that the Dog Star brought additional warmth. This time became known as the Days of the Dog, or Dog Days of summer.

Studying Sirius brought forth a compelling discovery in the 1830’s. A European astronomer noted that Sirius moved through the sky in a weird oscillating path as if it was twirling around some unseen anchor point. It was another thirty years before an American astronomer, Alvan Graham Clark, found that the Dog Star had a faint companion whose intense gravity was responsible for Sirius’ unusual behavior. Nicknamed “The Pup”, it can be seen as the tiny blue star accompanying Sirius in the inset NASA illustration at upper left. This little star is about twice the size of the Earth, but weighs about as much as our Sun. That means a handful of material from this star would weigh more than the combined weight of everyone who attends the Super Bowl.

Over two thousand years ago, Aristotle wrote about seeing a glow of light just underneath Sirius. On moonless nights, you can still see this glow and if you look at it with a set of binoculars you’ll see M41, a cluster of some one hundred stars having the shape of a large butterfly. Shown in the National Optical Astronomy Observatory photograph at lower right in our illustration, this cluster may be the faintest astronomical object recorded in antiquity.

If you can get a hold of a telescope, see if you can locate the dense group of stars called NGC 2362. Shown in the Spitzer Space Telescope infrared image at upper right in our illustration, there are some sixty stars clumped together in this cluster. NGC 2362’s brightest member is the supergiant star designated Tau (τ) Canis Majoris. There’s some uncertainty about whether Tau is actually a member of the cluster, or is just sitting in our foreground view. If it were part NGC 2362, at a distance of some 5,000 light years, it would be one of the Milky Way galaxy’s brightest stars.

NASA illustration credit: NASA, ESA and G. Bacon (STScI)

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