Stars twinkle in the night sky due to a phenomenon called atmospheric scintillation. The twinkling effect is caused by the Earth's atmosphere and its interaction with the light emitted by stars.
As starlight travels through the Earth's atmosphere, it encounters various layers of air with different densities and temperatures. These layers act as a medium through which the starlight must pass, and they have different refractive properties. The refraction of light occurs when it changes direction as it passes through a medium of different density.
The air in the Earth's atmosphere is not completely uniform, and it contains pockets of air with slightly different temperatures and densities. When starlight passes through these pockets, it undergoes multiple refractions and bends in different directions. This bending of light causes the apparent position of the star to shift slightly.
As a result of the continuous bending and shifting of starlight, the brightness and position of the star appear to flicker or twinkle when observed from Earth. This effect is more pronounced when the star is near the horizon because the light has to pass through a greater thickness of the atmosphere.
In contrast, planets, which are closer to us and have a larger apparent size, tend to twinkle less because their light is spread out over a larger area. They appear as relatively steady points of light in the night sky.
It's worth noting that the twinkling effect is also influenced by other factors such as atmospheric turbulence, temperature gradients, and local weather conditions. However, the primary cause of the twinkling phenomenon is the refraction of starlight as it passes through Earth's atmosphere, resulting in the apparent fluctuation in brightness and position of stars.