The apparent path of light from distant stars is not always a straight line due to a phenomenon called atmospheric refraction. When light passes through Earth's atmosphere, it encounters variations in the density of the air, which causes the path of light to bend or deviate from a straight line.
Atmospheric refraction is caused by the fact that the speed of light changes as it travels through different mediums, such as air. The refractive index of air is slightly greater than 1, which means that light slows down when it passes through the atmosphere compared to when it is in a vacuum. This change in speed causes the direction of the light to change.
The bending of light due to atmospheric refraction is more noticeable when stars are close to the horizon. When a star is low in the sky, its light has to pass through a larger portion of the Earth's atmosphere, which results in a more significant bending effect. This can cause the star to appear slightly higher in the sky than its actual position.
However, when a star is directly overhead (at the zenith), the amount of atmosphere it passes through is minimized, and the bending effect is significantly reduced. As a result, stars closer to the zenith appear closer to a straight line.
It's worth noting that atmospheric refraction is not the only factor that can affect the apparent position of stars. Other factors, such as atmospheric turbulence, can also cause the light from stars to appear to twinkle or shimmer.