Why do stars twinkle ? Explain.

Explanation

Due to atmospheric refraction
The density of different layers of air keeps on changing due to which the apparent image of the Stars keeps on changing. This changing position of stars appears as twinkling of stars.
Detailed Answer:
Stars are very distant and approximately point-sized sources. Path of star light changes continuously due to gradual changing of refractive index of the layers of earth.s atmosphere. Thus, the apparent position of the stars fluctuates and the amount of star light entering the eye flickers giving the twinkling effect.