At night satellites twinkle like stars

Why do stars twinkle?

Twinkling stars on a mild summer night are extremely romantic. In the universe, the stars shine very calmly, they only sparkle because bubbles of air of different temperatures in the earth's atmosphere repeatedly deflect the light beam of a star differently.

Stars in the southern sky - perfectly sharp thanks to adaptive optics

The light of the stars in the night sky has to cover distances of tens of trillion kilometers to us and runs through the space between the stars and planets practically unhindered. But when the light reaches the earth's atmosphere, the quiet run is over: air currents and bubbles of air of different temperatures deflect the light beam. Since the bubbles are often only a few meters in size and move quickly, the light is deflected differently within a fraction of a second. The star sparkles, it seems to shine irregularly, while for astronauts on the International Space Station it shines very evenly.

Stars twinkle, planets don't

The air bubbles not only change the brightness of the star, but also its position in the sky: the image of the star in the sky dances back and forth. Stars are therefore smeared into small discs on long-exposure photos. Since far more light comes from the bright planets in the solar system, the sparkle is only noticeable when the air is extremely turbulent. Stars and planets can usually be easily distinguished in the sky: stars twinkle, planets do not.

Switch off the sparkle with "rubber" mirrors

Even in the best locations on earth, the twinkling of the stars is so strong that it ultimately limits the performance of the telescopes. The top telescopes on earth could theoretically all see much more clearly. But the billowing atmosphere does not allow this. Only telescopes in space (such as the Hubble telescope) or future telescopes on the moon will always see extremely sharply, as the starlight does not have to pass through an annoying atmosphere.

In order to eliminate the disturbances from the atmosphere, astronomers have been using the so-called adaptive optics technology for several years. The telescope analyzes the dancing image of a star in the field of view. Based on this data, the shape of a very movable mirror in the beam path is changed up to 1000 times per second in order to correct the disturbances in the air envelope. This technology is used, for example, in the Very Large Telescope (VLT) of the European astronomy organization ESO in Chile. With a lot of technical effort, the telescope in the Atacama Desert sees almost as sharply as if it were in space.