Posted on February 01 2017
For stargazers older than first-graders, it’s no news that stars are giant balls of flaming gas, just like our Mr. Sun. That they’re so tiny to our eyes only clarifies how very far away they are. We group them into convenient constellations for easy reference (and to illustrate great stories of the gods or creation myths), but of course, the constituent stars of any constellation are at widely varying distances from our eyes. If you moved far enough off our planet, their connect-the-dot appearance would distort until they were unrecognizable. You’d have to go quite a ways, however. And although the universe is ever expanding, the arrangement of the stars in the sky will appear more constant than any other vista you see over your entire lifetime. But as you look up at the distant stars, as opposed to say, the moon or sun or the other planets, the stars can appear to dance in place, jittering about in the movement known as twinkling.
The twinkling, or “scintillation” as it is called in scholarly circles, is a result of our viewing the starlight as it comes through our atmosphere. In space, nobody can hear you scream, or see the twinkle, because there is no atmosphere. How does the atmosphere do that twinkly trick? First, as the white light of stars passes from space into air, it goes through dispersion, or the separation into the various colors of the spectrum. But the gases of the atmosphere themselves don’t create the twinkle; it’s the variation between cooler and warmer bodies of air that do it. The temperature variations caused by altitude and mixing winds create a myriad of small air pockets. Cooler air spaces are more dense than warmer ones, and these different pockets act like lenses which bend and shift light to different degrees as it comes down from the heavens. The dense cool air bends the starlight more than the warm does, in this process called refraction. A beam of light coming to your eye thus travels a herky-jerky path across these “lenses”, making microscopic variations in apparent location and breaking the point of light into its constituent colors. The colors of a bright star like Sirius flash from reds to blues to greens and yellows as the spectrum of incoming light constantly changes its position on your eye. The different frequencies of the different colors, combined with the variable pathways, causes the light stream to appear to blink out for tiny bits of time—creating the twinkle. This optical effect only works for the kind of pinpoint light sources that stars provide.
The much closer and larger-seeming moon and planets deliver a wider beam and appear as disks. Though their images are also distorted by the atmosphere, there is enough “volume” of light that despite the tiny jitters, they manage to deliver a continuous stream of photons to your eye. You can usually distinguish planets from stars by this lack of twitchy scintillation. Observing even pinpoint starlight from a viewpoint outside of our atmosphere allows that light to arrive unaltered, which is why the Hubble Space Telescope, orbiting the Earth, has delivered better quality images than any ever seen before. Here on Earth, twinkle is most pronounced on windy nights, or when stars are low in the sky, and their light is passing through the maximum amount of atmosphere between it and your eye. Stars directly overhead, especially if you are high on a mountaintop, have much less air to traverse, and therefore much less scintillation. This is one reason why so many observatories are built at high elevation, like Mauna Kea in Hawaii, to afford the clearest views from the Earth’s surface. The telescopes used by astronomers for ground-based photography have computer-driven systems that make micro-adjustments to help correct for the jiggling light, delivering sharper images.
Next time you’re out at night and it’s clear enough to see the stars above, consider the wiggly route that that light has traveled at the very end of its light-years-long journey, just before it impacts your retina with its beautiful sparkle.