Twinkle Twinkle Little Star

It's semi-common knowledge that stars twinkle and planets don't. By semi-common, I mean that a lot of folks know that, but I also mean it isn't strictly true.

Stars twinkle because we see them from the bottom of a sea of air. Little cells of air, which are about ten centimeters across and located many kilometers high, move across our vision as we watch the sky. These small bundles of air act like little lenses, bending light as it passes through them. This bending, called refraction, is familiar to anyone who drives on a hot day: hot air just above the road surface bends light more than the cooler air slightly above it. That's why you can sometimes see that shimmery veil of what looks like water on the road; it's really the air bending the light above it. Sometimes you can even see cars reflected in the road!

Anyway, these parcels of air up high in the atmosphere travel to and fro, bending the light from a star in more or less random directions. Stars are big, but they are so far away that they appear to be very small, much smaller to our eyes than each of these air bundles. So when the light gets bent, the apparent movement of the star is larger than the size of the star in the sky, and we see the star shifting around. Our eye can't really detect that motion, because it's too small. What we see is the light from the star flickering. That's why stars twinkle!

So why don't planets twinkle? It's because planets are bigger. Well, really, they're smaller than stars, but they are so much closer they appear bigger to us. They are much bigger in apparent size than the air bundles, so the smearing out of their light is much less relative to the size of the planet itself. Since the image doesn't jump around, they don't appear to twinkle.

There's always an exception though. In very turbulent air, even planets can appear to twinkle. The air is moving so rapidly and so randomly that even something as large as a planet can twinkle.

This effect also plays with a star's color. Blue and green light get bent more than orange and red, so sometimes in very turbulent seeing a star's colors will rapidly change. This usually happens when the star is low on the horizon (so there's more air for it to pass through). The brighter the star, the easier it is to see; Sirius, the brightest nighttime star, is often seen changing from green to red to orange and back, very rapidly. I've seen it myself and it's quite lovely. If you're not prepared for it it's quite surprising; many people report a UFO when they see it! If they're driving, the star appears to follow them too, just like any distant object appears to follow you while you're driving (it's due to parallax). So if a friend says they saw a bright UFO, low to the ground, rapidly changing color and following them gently point out that most likely it wasn't a spaceship, but it was almost certainly extraterrestrial!

Twinkling is actually a serious problem for astronomers. Next week's Snack will deal with how it affects astronomers and what they do about it.

[Note added August 29, 2002: The above text has been slightly changed: I originally said the cells of air were "centimeters across and kilometers high". I meant to say that they are located that high, not that they are like long, thin columns of air! It's been fixed, and I thank the folks at Ask Yahoo! for bringing this to my attention.]