Twinkling of stars

Why do stars twinkle and how does it affect astronomy and telecommunications?

Stars are twinkling. Any of us can observe this phenomenon by looking at a clear night sky: stars seem to quickly change of apparent brightness, colour or even position. This is what is described as twinkling. In fact, the origin of this phenomenon is not related to the stars themselves but rather to the medium in which their light propagates at the end of their long journey through space: our atmosphere.


Origin: scintillation

As a matter of fact, our atmosphere is made of air that does not have the same properties everywhere and at all instants. Indeed, a parcel of air can be described by several quantities, such as its pressure or its temperature, and variations of these quantities over large areas lead to motions of air (i.e. wind). However, fluctuations of pressure or temperature (and thus air motions) can also happen at small scale, arising from turbulence in the atmosphere. This implies that characteristics of the medium in which the light is travelling are locally varying, modifying the light trajectory.

Compare our atmosphere to a swimming pool in which there is a small coin (depicting the star): looking through the water, the coin position seems to wobble from side to side. This is because the water of the pool bends the light coming from the coin (this phenomenon is called refraction) and the wobbling originates from water motion. In fact, this is exactly what happens to the light of stars in our atmosphere. Scientists have named “scintillation” this physical phenomenon behind the twinkling of stars.

Scientific interests

Astronomers do not like scintillation as this reduces the quality of ground-based observations. However, astronomers are not the only one to suffer from scintillation, electrical and optical engineers also struggle with this phenomenon, especially for ground to satellite communications. The situation is similar: think of the satellite as a star that is sending data towards Earth and that can also receive data sent from Earth. In a general sense, the information is encoded on electromagnetic waves (such as the light or the waves used for mobile communications) that also travel through our challenging atmosphere. Scintillation damages the received waves, making it harder or even impossible to recover the sent information. This comparison is even more true today since optical waves (i.e. light) are getting interest for ground to satellite communications. Nevertheless, scintillation influences any electromagnetic waves and the study of its effects for telecommunication is part of a broader goal: the characterization of communication channels (i.e. the medium in which the waves are propagating). Based on observations, measurements and models, the main interest of astronomers and engineers is to predict and measure scintillation effects, either to compensate them (if possible) or to avoid them by delaying the communication or the astronomical observation period.

References and further reading