Eyes, snowflakes, stars — I like things that twinkle. Snowflakes glint when struck by sunlight while twinkling stars are an indication of turbulent air. Bumpy air can turn pinpoint stars to fuzzballs and planets to mush when magnified through a telescope. That's why I always glance skyward first before an observing session. If the stars "sit still" it often means a good night ahead with sharp, steady images even at high magnification. If they flutter, I lower my expectations.
After packing up the scope one night last week I noticed the frenzied twinkle of Sirius, the brilliant, white star below Orion's Belt. The light show was so captivating I stood and stared that star in the eye for 15 minutes. On another night I watched again but this time with binoculars, the better to appreciate the rapid-fire color changes. Sirius flashed and quivered continuously as if it were having rapid heart palpitations.
Although I glimpsed all the colors of the rainbow in its pulsations, red and blue flashes were the most intense and colorful, while yellow flashes were weak and rare. At times, Sirius also appeared pure white, and its brightness varied dramatically from moment to moment. Frankly, it was hard to tear myself away from the sight.
All stars twinkle, but the bright ones are a lot more obvious. After my session with the Dog Star I moved on to fainter suns and found that I could detect colorless twinkles in stars as dim as 4th magnitude. Lack of color wasn't surprising since 3rd magnitude and fainter stars are too dim to activate the eye's color receptors.
Millions of eddies of air from a fraction of an inch to several feet across rush and tumble across the sky even on nights when the air at ground level is absolutely still. Eddies form when air masses of different pressure and temperature slide past one another or when warm air rises through cold air. The atmosphere's ceaseless movement create swarms of individual air pockets or cells, each with a slightly different temperature and density.
When a beam of starlight pierces Earth's atmosphere each cell acts like a lens that refracts or bends the star's light. When a cell (or cells) direct the light toward your eye the star appears a little brighter. When it bends light away from your line of sight the star appears a little fainter. Nothing is static. As the swarm of airy lenses crosses your field of vision the star brightens and fades rapidly and unpredictably. Our eyes perceive the silent fury as twinkling.
Twinkling is more intense the closer a star is to the horizon because its light traverses the bottom, thickest layer of the atmosphere. When viewed overhead we look through about 10 miles of dense air and then hundreds of miles of practically nothing. The more air, the more eddies and the greater the glimmer.
Incoming starlight is also affected by dispersion, where light of different colors or wavelengths is refracted by different amounts as it passes through the atmosphere. Dispersion is most evident the closer a star is to the horizon for the same reason twinkling is: the air is densest there and bends light more strongly.
Through a telescope, a low, bright star looks like an elongated, rainbow-colored streak. Blue and purple are refracted more than red and orange so the streak is red at the bottom and blue/purple along the top. You can see the same colors along the top and bottom edges of the rising full moon when viewed through binoculars. As the eddies whirl by they refract these colors towards and away from our eyes, the reason why bright stars below about 30° elevation twinkle in technicolor.
As the brightest star in the nighttime sky I'm going to officially crown Sirius as the Twinkle King. It helps that for mid-northern latitude observers the star keeps fairly low in the sky where it's more affected by atmospheric turbulence than similar bright stars.
January is one of the best times to see Sirius in a flickering fit because it hovers near the horizon during early evening hours. At midmonth Sirius rises around 6:30 p.m. in the southeastern sky. For a memorable experience stare it down between 6:30 and 8 p.m.
What colors are most obvious to you? Can you see other fainter, nearby stars twinkle? If you pay attention night to night you'll notice that Sirius twinkles more or less depending on the state of the atmosphere, whether that's local (where you happen to be standing) or higher up.
Stars are so remote they appear as mere points of light and make easy targets for the twitching air. Planets, being much closer, have disks and almost never twinkle. To understand why, picture a planet as a bunch of individual stars jammed together twinkling wildly. Some of those twinkles will be faint, others bright, but they average out, so the planet glows with what appears to be a steady light.
From the International Space Station or the moon no stars twinkle. Can you guess why? Exactly — no air up there!
Twinkling stars are pretty to look at but also act as probes of our atmosphere, literally shedding light on its otherwise invisible parts and movements.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.