Question

A star sometimes appears brighter and some other times fainter. What is this effect called? State the reason for this effect.

Answer 1

Hint: When we see a star, we actually see the light emitted by the star. This light first travels through the vacuum and then through the earth’s atmosphere to finally reach our eye. This light might be a few light years away or hundreds of light years away.

Complete answer:
This phenomenon is due to the atmospheric refraction of starlight. Earth's atmosphere has roughly five layers based on temperature. The varying temperature causes air to develop different refractive indices accordingly.
The gradual change in the refractive index of different layers of atmosphere causes atmospheric refraction and starlight undergoes multiple refraction.
This causes the light travelling from the stars to get bent in different directions. So, the fluctuation in the intensity of the starlight occurs continuously due to the changing amount of light entering the eye. The star sometimes appears brighter and at other times, it appears fainter. This causes the twinkling of stars.
This effect is called the Twinkling Effect. Atmospheric refraction is the causing agent of this phenomenon.

Note: Our earth's atmosphere is constantly moving. So sometimes, winds coming from different directions suddenly alter the temperatures within a layer itself. Also, the change in refractive index is caused partially due to temperature differences in the air, and partially due to the changing density of air. Air becomes less dense as we go towards outer space.