The twinkling of a star is due to :

The twinkling of a star, scientifically known as stellar scintillation, is primarily caused by the atmospheric refraction of starlight [1]. As starlight enters the Earth's atmosphere, it passes through various layers with different densities and temperatures, which act as a medium of gradually changing refractive indices [1]. These atmospheric pockets of hot and cold air are in constant motion due to turbulence, causing the light rays to bend or refract continuously [2]. This process results in random focusing and defocusing of the light, making the star appear to flicker or change in brightness and position. While the star's apparent position shifts slightly, the fundamental physical mechanism driving this visual effect is the refraction of light as it traverses the non-stationary physical conditions of the atmosphere [1].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 10: The Human Eye and the Colourful World > Twinkling of stars > p. 168
2. [2] https://ntrs.nasa.gov/api/citations/19890017910/downloads/19890017910.pdf