Why do Stars Twinkle at Night?

Posted by David Wetzel

Twinkling Stars

Stars twinkle because of turbulence in the Earth’s atmosphere. Light from distant stars passes through various layers in the Earth’s atmosphere, and it gets refracted depending on the temperature and density of the air at that point.

So light will pass through one layer, be refracted at one angle, and then pass through a different layer and be refracted at a different angle.

When you see a twinkling star, you’re seeing the accumulated refractions from all those layers, which change the position and size of the star - many times a second.

Because of this “chromatic aberration,” stars can appear to change colors when they are twinkling strongly.

On a clear, dark night, our eyes can see about 6,000 or so stars in the sky. They seem to twinkle, or change their brightness, all the time. In fact, most of the stars are shining with a steady light. This means that some of the light reaches us directly and some gets bent slightly away. To our eyes, this makes the star seem to twinkle.

You will notice that stars closer to the horizon appear to twinkle more than other stars. This is because there is a lot more atmosphere between you and a star near the horizon than between you and a star higher in the sky. Go out some night soon and have a look!

In outer space, where there is no atmosphere, stars do not twinkle. This is why the Hubble Space Telescope can produce the brilliant and crisp images of the universe that we have come to know. At our Earthbound observatories, we are learning how to compensate for the twinkling effect by adapting the optics of our large telescopes as fast as it occurs. As a result, we should soon be able to produce much sharper images from here on the ground.

For example, the Pleiades Star Cluster taken from the Hubble Space Telescope: