Professor Henning, why do stars twinkle?

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“Stars twinkle for reasons that are entirely earthly. Most of the light that reaches us from the stars is able to make its long journey through the cosmos to us virtually unhindered. And we are talking about some very considerable distances here: even light from Proxima Centauri – the star nearest to the Earth – takes more than four years to reach us. Whereas light from the opposite edge of our home galaxy, the Milky Way, even takes around 70,000 years to get here!

Why stars do not really twinkle at all

Stars twinkle when light encounters the Earth’s atmosphere

On its final few kilometres, however, the light from the cosmos does encounter an obstacle after all: namely our Earth’s atmosphere. It contains some fairly turbulent layers in which part of the light is diverted in one direction and another part is diverted in the other direction. Using a suitable camera, it is possible to observe through a telescope how the image of a star leaps back and forth, and how several images of the same star can even be seen at the same time. Without the telescope, that is to say with the naked eye, this gives the impression that the star is twinkling.

Why astronomers are happy to do without the romance

We may find the twinkling of stars romantic on a mild summer’s night. For astronomers, on the other hand, this twinkling is anything but romantic – it is a sign of dramatically deteriorated image quality. They therefore invest a great deal in technologies designed to correct these image distortions. The problem is neatly avoided by space telescopes that carry out their observations outside the Earth’s atmosphere.

Crystal clear images with no twinkling

There is also a solution for the large ground-based telescopes, however: using a method known as adaptive optics, a precise observation is carried out to establish how exactly a test star is distorted and diverted by atmospheric turbulence. In real time, a computer then adjusts a deformable mirror in the telescope’s optical path in just the right way to correct the image distortions for the most part. The result is crystal clear images of astronomical objects. In this sense the honest astronomical answer to why stars twinkle would be: if all goes well, the stars do not twinkle at all!”