Why can we see the stars? And why do they twinkle? On a clear night, when the moon is setting and the lights of the cities are far enough away, you can observe an infinite number of stars in the sky.
On our half of the earth, you can see about 3,000 stars at night with the naked eye.
Several thousand stars twinkle in the sky on a clear night! And one shines even during the day: the sun.
It is the star that is closest to our earth.
Stars are huge balls of gas. Most of them were formed many billions of years ago.
They are held together by their own gravity. In their interior, it is so hot that atomic nuclei fuse together. This produces energy that makes the star shine.
Almost all stars that we see as tiny dots in the sky are suns light-years away. Many of them are much larger than our sun.
By the way: Only stars themselves shine. Our “evening star”, Venus, is not a star at all, but a planet. It reflects only the light of the sun.
Why do the stars twinkle?
The light of the stars in the night sky must travel distances of tens of trillions of kilometers to reach us and pass 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 every fraction of a second.
The star twinkles, it seems to shine irregularly, while for astronauts on the International Space Station it shines very evenly.
Stars twinkle, planets do not
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.
In long exposures, stars are therefore smeared into slices.
Since far more light comes from the bright planets in the solar system, the twinkling is only noticeable when the air is extremely turbulent.
So stars and planets can usually be easily distinguished in the sky: Stars twinkle, planets do not.
How scientists stop the “twinkle”
Even in the best locations on earth, the twinkling of the stars is so strong that it ultimately limits the performance of telescopes.
In theory, the top telescopes on Earth could all see much sharper. But the swirling envelope of air does not allow this.
Only telescopes in space (such as the Hubble telescope) or future telescopes on the moon always see extremely sharply, since the starlight does not have to pass through a disturbing envelope of air.
To eliminate the disturbances caused by the atmosphere, astronomers have been using the technique known as adaptive optics for several years.
Here, the telescope analyzes the dancing image of a star in the field of view.
Based on this data, the shape of a very moving mirror in the path of the rays is changed up to 1000 times per second to correct the disturbances of the air envelope.
This technique is used, for example, in the Very Large Telescope (VLT) of the European Astronomy Organization ESO in Chile.
With a great deal of technical effort, the telescope in the Atacama Desert then sees almost as sharply as if it were in space.
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