The stars appear to twinkle at night. Even the star closest to earth (sun) is about 150 million kilometers from the earth. There are more than about 6000 stars visible to us on a clear night sky. These stars emit light. While some shine with steady light others appear to have varying brightness. This is what we perceive as twinkling.
The twinkling of stars is scientifically termed as ‘stellar scintillation’. Between us and the stars is lots of air which is our atmosphere. The light from the star travels to earth via our atmosphere. The earth’s atmosphere has lot of turbulence due to the moving warm and cold wind. Thus the light that passes through this air turbulence is refracted at different angles at different moments.
The refractive index of the different layers of the earth’s atmosphere changes continuously and so the position of the image of a start changes with time. This refraction or bending of light coming from the stars makes the star to appear with varying brightness which the human eye perceives as ‘twinkling’.
Planets do not scintillate. Stars are extremely distant from earth compared to the planets. Because they are nearer to earth, we receive a greater amount of light and therefore minor variations in the intensity are not noticeable.