Twinkling stars always catch our attention. People will often report a bright object flashing or pulsating in the different colors and swear it’s a UFO. When I ask the time and location of their sighting, it almost always turns out to be a bright star like Sirius, Capella or Arcturus seen near the horizon.
Dim stars twinkle too, but we don’t usually notice it, because our vision isn’t sensitive enough to detect the bouncing about of a star’s image that creates the twinkling effect. Bright stars can scintillate like crazy especially when low in the sky. To my eye they look like nonstop shimmers and flashes in every color of the spectrum.
Our atmosphere’s to blame for all this frenetic activity. Starlight passes through air pockets of different density and humidity on its way to our eyes. Each pocket act like an individual lens that focuses its own image of the star. As the air churns and the winds blow, the number and positions of all those individual images are constantly changing.
We perceive these nonstop, tiny shifts as sputtering light or twinkling. The scientific description of the cause is atmospheric refraction. Refraction by moving air causes star images to constantly jump about and change brightness.
Planets generally don’t twinkle, because they appear much bigger than stars (they’re close) while stars are essentially point sources. The air bundles are too small to “knock” planets around and create multiple, separate images, so they appear steady to the eye. Most of the time. During high turbulence, I’ve seen planets jump about, too. On the next clear night, compare the bright stars shown above with Jupiter and you’ll see the difference.
The colors come from the air, too. Just as white light is composed of a rainbow or spectrum of individual colors from indigo to green to red, so is starlight. When a star is near the horizon, refraction is strong enough to create images of the star in every color of the rainbow and cast them about in different directions. To our eye, the star looks like a continuous sparkle of varying colored light as split-second variations in moving air pockets make it dance about. Wonderful!
Pretty to the eye, twinkling drives astronomers crazy, since images won’t sit still long enough to get a sharp view or picture. To compensate, many professional observatory telescopes use a recently developed technology called adaptive optics. Here, a bit of a star’s quivering light is analyzed live by a computer, which calculates how the telescope’s deformable mirror must be shaped to compensate for the distortion. Machinery attached to the mirror tips, tilts and otherwise slightly changes the mirror’s shape every few milliseconds to keep the star images “calm”.
Amateur astronomers aren’t so lucky. Most nights, the stars jump about or look blurry in our eyepieces at medium and high magnifications. We persevere in hopes of striking gold on those occasional nights of little turbulence and fine air when star images are pinpoint and still – something called ‘steady seeing’. Ah, they’re to die for!