We are taught that stars twinkle from a very young age when we hear “twinkle, twinkle, little star” — but why do stars twinkle and planets do not?
Astronomers call it stellar scintillation, a big name for the apparent twinkling of stars in the sky as we view them from down here on Earth. In fact, stars don’t twinkle at all, they shine with constant brightness.
When the light from a star reaches our planet, it passes through our atmosphere, and this has an effect on how that light travels. When rays of light travel from one medium to another, or from one area of density to another, the path of each ray of light is bent. We call this refraction.
Our atmosphere contains air, but space does not, so the light from a star changes from one medium to another at this point. As the star’s light enters our atmosphere, the rays of light are bent, but this alone does not account for the twinkling effect.
The air in our atmosphere is not all the same — there are areas of greater density and different temperatures, and each of these pockets or layers of air act as a different medium. So every time rays of starlight travel through these areas, they get refracted.
This leads to the light travelling in a type of zig-zag fashion.
At any given moment, some of the light from that star will reach our eyes and some will be refracted away. However, the path of these rays of light keeps changing, so the light reaching our eyes is constantly changing. This makes the star appear brighter and dimmer repeatedly; it makes the star appear to be twinkling.
Astronauts in space have confirmed that stars appear to shine constantly from their viewpoint. So once there is no air to distort the path the light travels in, then the stars really do appear to shine constantly.
Planets are much closer to Earth than stars, so they appear as large discs in space rather than distant dots.
In fact, the light reaching our eyes from planets is refracted as it passes through our atmosphere, in just the same way as with starlight.
The difference is that there is a much wider field of light rays reaching our eyes from the broad disc shape of a planet, so while some may be bent out of vision, many won’t. The refracting of different light rays tend to cancel each other out; the overall effect is that we see the light as constant.
Knowing that stars twinkle and planets don’t can help the amateur observer to determine a star from a planet — a handy tip to remember the next time you observe a dark starry night.
Maybe the song should be changed to ‘twinkle, twinkle little star, now I know just what you are’.