Twinkling of a star is due to :

The twinkling of a star, scientifically known as astronomical scintillation, is primarily caused by the atmospheric refraction of starlight [1]. As starlight enters the Earth's atmosphere, it passes through various layers of air with different densities and temperatures. These layers act as a medium with a continuously changing refractive index, causing the light to bend or refract multiple times before reaching the observer [1]. Because the physical conditions of the atmosphere are turbulent and non-stationary, the apparent position and brightness of the star fluctuate rapidly [3]. This random focusing and defocusing of light, similar to patterns at the bottom of a swimming pool, creates the flickering effect perceived as twinkling. While interference can occur as distorted wavefronts overlap, the fundamental mechanism driving the phenomenon is the bending of light through atmospheric irregularities [2].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 10: The Human Eye and the Colourful World > Twinkling of stars > p. 168
2. [3] https://en.wikipedia.org/wiki/Twinkling
3. [2] https://ntrs.nasa.gov/api/citations/19720017702/downloads/19720017702.pdf