Due to the high temp of the star the light suffers continuous refraction and thus when it enters the earth's atmosphere the stars look as if they are twinkling.
In fact, most of the stars are shinning with a steady light.the movement of air in the atmosphere of the earth causes the starlight to get slightly bent as it travels from the distant star through the atmosphere down to us on ground.this means that some of light reaches us directly and some gets bent slightly are eyes, this makes the star shinning.
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