आकाश में रात के समय तारे टिमटिमाते क्यों दिखाई देते हैं?

Stars twinkle because of atmospheric effects. If you were on the moon, where there is no atmosphere you would not see any twinkling.

The main effect on starlight due to atmosphere is because of refractive index. Refractive index is not constant through the various layers of atmosphere. It is constantly changing because of winds, density and temperature. Stars can be treated as a point source because they are really far away. As star light passes through the earth's atmosphere it deflects in various ways because of constantly changing refractive index. Because of this variation sometimes more light enters the eye and sometimes less, which causes the twinkling of stars.

Planets don't twinkle because they are much closer to the earth and you can make out details of planet though a telescope. Because of the larger apparent size the effects of the atmosphere are averaged out.