Question

why did the stars twinkle at night........???????

Answer 1

Stars twinkle not because of stars. But because of the atmosphere.

     Stars are very far away from Earth. Stars radiate white light (& other electromagnetic radiation) towards Earth.  During the night the air layers are very turbulent and mobile.  The light rays get refracted a little this way and that way and so their image position changes on Earth.

    The refractive indices between the different layers of air are slightly different. So the refraction is very small.  But it keeps changing due to movement of air, change in pressure and temperature.  So refracting angles change.  So the image position  changes continuously in time.  Sometimes it changes a lot, so we don't see the star. It is like disappearance.

   This is the twinkling or scintillating effect of stars.

Answer 2

The movement of air in the atmosphere of Earth causes the starlight to get slightly bent as it travels from the distant star through the atmosphere down to us on the ground.
This happens due to REFRACTION of light in earth's atmosphere.This makes the star seem to twinkle.