Why do stars twinkle

Stars are the wonders of the night sky and have captivated people for centuries. Many throughout the course of history have wondered exactly why starts twinkle.

We see all stars – with the exception of the sun – as tiny dots of light in the night sky. The stars’ light must travel through the Earth’s multi-layered atmosphere, and as it does so it is refracted (bent) multiple times and in random directions. Refraction occurs when the light from the stars encounters a density change – for example, a pocket of either hot or cold air. The phenomenon of random refraction appears to make the star wink as though it is in motion. It is this action that our eyes translate as twinkling.

When the light itself enters the air – or atmosphere – it is scattered, with the scattering depending on the air temperature with warm air causing less bends and colder air causing more. The reason for this is that the molecules are further apart in warm air conditions, which results in less scattering.

All stars, with the exception of the sun, are so far away from us that they may only be sending single rays of light in our direction, and as these rays enter Earth’s atmosphere they are scattered differently (due to passing through different air layers of varying temperatures). When the light is scattered away from us it seems as though the star disappears for a fraction of a second, which seems to result in a twinkle effect. Stars that are nearer the horizon seem to twinkle more than overhead stars due to the fact that light from those stars nearer the horizon must travel through more air, thus subjecting it to greater levels and instances of refraction.

Additionally, planets do not appear to twinkle to us due to the fact that their proximity to us means that they appear large enough for twinkling to be unnoticeable. Also, with regards to the sun and the moon, they throw so many rays of light at us that it is immaterial that some of these are scattered away as long as those that remain come towards us. As a result our image of the sun, moon and other celestial bodies close by appears steady to us.

Interestingly, stars would not seem to twinkle at all if seen from wither outer space or from a planet or a moon devoid of an atmosphere. It is for this reason that the Hubble telescope has been particularly effective in monitoring the various celestial bodies around us – including thousands of galaxies as demonstrated in the Ultra Deep Field, a picture hailed as the most important photo taken by mankind to date – due to the fact that the pictures it takes doe not have the standard interference that other Earth-based observatories do by attempting to view locations through the atmosphere.

For further examples of how light is refracted due to heat look carefully near a stove top while the burner is on or a local sidewalk in the summer and pay careful attention to how the surface appears to shimmer. This, again, is due to heat distorting the light the same way stars are distorted, just in a much more immediate and tangible way.