Twinkling of stars is due to

The twinkling of stars, scientifically known as stellar scintillation, is primarily caused by the atmospheric refraction of starlight as it passes through the Earth's atmosphere [1]. As starlight enters the atmosphere, it undergoes continuous refraction because it travels through a medium with a gradually changing refractive index [1]. This variation is driven by atmospheric turbulence, which creates fluctuations in air density, temperature, and pressure. These turbulent layers act like a series of refracting lenses that bend the light in different directions. Consequently, the apparent position of the star shifts slightly and the amount of light reaching the observer's eye fluctuates, making the star appear to change in brightness and color [1]. Unlike stars, which are distant point sources, planets are closer and appear as extended disks, which averages out these refractive effects and prevents them from twinkling.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 10: The Human Eye and the Colourful World > Twinkling of stars > p. 168