Question

a star sometimes appears brighter and some other time fanta what is this effect called state the reason for this effect

Answer 1

Stars are hundreds or thousands of light years away from us, so they can be considered as point sources of lights. The light rays emitted by these stars enter into the earth's atmosphere and undergoes continuous refraction, because each layer of earth's atmosphere has different density. They go on refracting and the light rays reaching our eyes will be bended like an arc. Thus we see the stars in an apparent position, which is slightly different from the actual position. Due to continuous refraction the amount of light gets fluctuated. We see that the position is slightly altered. This causes the Twinkling effect. And this phenomenon is called scintillation.
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Answer 2

A star sometimes appears brighter and some other time faint. This effect is called Twinkling of Star. This occurs due to the atmospheric refraction. light which travels through different layers of the atmosphere bents in different direction thus making the stars twinkle.
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