Stars twinkle because of a phenomenon known as atmospheric turbulence. When starlight passes through Earth's atmosphere, it encounters various layers of air with different densities and temperatures. These variations in the atmosphere cause the light to refract or bend in different directions.
The movement of air pockets and temperature fluctuations in the atmosphere create a constantly changing pattern of refraction. As a result, the starlight reaching our eyes appears to flicker or twinkle. This effect is more pronounced when the star is closer to the horizon, as the light passes through a larger portion of Earth's atmosphere.
In contrast, when observing stars from space or using adaptive optics systems that compensate for atmospheric distortion, stars appear steady and don't twinkle.
It's worth noting that the twinkling of stars is more noticeable for point-like sources of light, like stars, compared to larger objects like planets. This is because the light from planets, such as Venus or Jupiter, appears to emanate from a small disk rather than a single point, which reduces the visible effects of atmospheric turbulence.