Stars twinkle because their light passes through moving layers of Earth's atmosphere. Warm and cool pockets of air have slightly different densities, so they bend the light in slightly different ways from moment to moment, making a star seem to brighten and dim.
Explanation
The atmosphere is never perfectly still. It always contains turbulence, wind shear, and small moving streams of air. Because of that, starlight keeps shifting direction a little as it comes down to us, and to our eyes the source seems to shimmer. The effect is especially noticeable because stars are so far away that they look almost like mathematical points of light.
Details
The lower a star sits above the horizon, the stronger the twinkling usually becomes because its light has to travel through a longer and more disturbed path in the atmosphere. That is also why stars near the horizon may appear to change color. Planets usually twinkle less because they show a tiny visible disk, so the light from different parts of that disk averages out some of the distortion. For astronomers, this is a serious issue because atmospheric shimmer makes telescope images less sharp.
Good to know
- Objects are best observed high above the horizon: there is less atmosphere there to distort their light.
- Dry calm air gives steadier skies: that is one reason observatories are often built in mountains or desert regions.
- Space telescopes avoid this problem: above the atmosphere, stars do not twinkle and look much sharper.
