The Dog Star always has a twinkle in his eye

Orion climbs up over my neighbor’s house last night. Follow the Belt
down to the lower left of the frame and you’ll see Sirius between the branches
of the tree. Details: 24 mm lens, f/2.8, ISO 800 and 10" exposure. Photo: Bob King

Maybe it was decorating the Christmas tree yesterday but sparkles were on my mind last night when I went out to gather more wood for the fire. It was 10:30. Orion had sprung up in the southeastern sky; below him further east the Dog Star Sirius flashed a St. Vitus dance of light and color. All the bright stars twinkled. Sirius, being the brightest of them all and still low in the sky, flashed most vividly.

We’ve all seen twinkling stars before and I’ve written about the phenomenon several times in this blog. Twinkling or scintillation is caused by moving air currents of various temperatures and densities passing between your eye and the star. Each variation in the air bends or refracts the star’s light a little differently causing it to brighten and fade as well as briefly flash this and that color of the rainbow. You’ll recall white light is made of all different colors. Varying air cells act like lenses that focus and intensify the individual colors of white light.

These photos are all of the star Sirius and show how its color varies as it twinkles. I shot these slightly out of focus to spread the color out and show it more clearly otherwise the images aren’t manipulated. Not only are the hues striking but you can also see how the star changes in brightness, too. Details: 200mm lens at f/2.8, 1/30 to 1/50" exposure at ISO 1600. Photo: Bob King

If you could see Sirius above and beyond the atmosphere, it would glow a steady, pure untwinkling white. Back down on Earth, we see it constantly flickering, and if you pay close attention, you’ll see rapid color changes, too. Last night at first glance the star appeared white, but looking more closely I saw very brief flashes of blue, purple-magenta, yellow and even an occasional red. You could almost picture the air cells sliding by like thousands of pairs of different prescription eyeglasses, each one warping Sirius’ light for a split second.

Stars twinkle more when they’re lower in the sky than higher because of the greater amount of air their light must pass through. Illustration: Bob King

When we look at a star low in the sky, we’re peering through a far greater density of air than when looking straight up. The more air, the messier and crazier the twinkling is. That’s why December is a good month to get acquainted with Sirius. It’s bright enough to see color in its scintillations and very easy to find just by drawing a line through Orion’s Belt down toward the horizon. For the best twinkling, try to observe Sirius when it’s low in the sky. In early December that would be around 10:30-11 p.m.; mid December 10-10:30 and 9:30-10 near month’s end. Happy scintillations!

The waning moon will be near the planet Mars and the Leo the Lion’s brightest star Regulus tonight (Sunday). This map shows the sky around 11 p.m. Planets rarely twinkle like stars because they have round, measurable disks that are less affected by moving air currents. Because of their great distance, stars appear as pinpoints. Compare Mars to Sirius the next clear night and you’ll see what I mean. Created with Stellarium.