The twinkling of a star is due to the atmospheric

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 multiple layers of air with varying densities and temperatures, which results in a continuously changing refractive index [2]. This causes the light to bend or refract multiple times before reaching the observer's eye [2]. Because the physical conditions of the atmosphere are turbulent and non-stationary, the apparent position and brightness of the star fluctuate rapidly [1]. When the atmosphere refracts more light toward the observer, the star appears bright; when it refracts less, the star appears dim, creating the twinkling effect [1]. Other optical phenomena like diffraction, reflection, or dispersion do not cause this specific effect [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://en.wikipedia.org/wiki/Twinkling