The atmosphere is responsible for the twinkling. As the light from the stars filters down to us through the atmosphere, the turbulence causes it to distort slightly in different directions. We see this as “twinkling”.
Yes, stars twinkle due to the atmosphere. But have you ever noticed that planets do not twinkle. Try looking at Mars or Venus the next night they are visible. You will see there is no twinkling. This can be explained using optics. The stars are so far away that they can be considered as point sources of light. All of the waves in a point source line up together which makes them very pure sources of light. But if they waves are interrupted in any way we get a mixing of the waves called interference. Sometimes the interference can cause the light to be stronger than usual, but sometimes it can cause the light to be dimmer. The brighter/dimmer switching happens very quickly making the stars twinkle.
This does not happen with an extended source of light (the planets) because the waves are already randomly mixed up leaving the average brightness the same all the time, so the atmosphere has no real effect on the waves. Therefore we see no twinkling on planets.
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