Stars twinkle in the sky at night because

The twinkling of stars, scientifically known as stellar scintillation, is caused by the atmospheric refraction of starlight. As light from a star enters the Earth's atmosphere, it passes through multiple layers of air with varying densities and temperatures. The refractive index of these layers changes continuously because the atmosphere is dynamic and turbulent. Consequently, the path of the light bends randomly, causing the star's apparent position and brightness to fluctuate rapidly. Since stars are so distant that they appear as point sources of light, even slight atmospheric disturbances cause a noticeable twinkling effect. In contrast, planets are closer and appear as extended disks; the refractive variations from different points on the disk cancel each other out, which is why planets generally do not twinkle.