If atmospheric refraction did not occur, which change would be most noticeable during the observation of stars from Earth?

The correct option is: Stars would not twinkle; their light would remain steady.

To understand why this is the case, let us break down the physical process step-by-step:

1. The cause of twinkling (Scintillation):
Stars are extremely far away from Earth, which makes them appear to us as microscopic point sources of light. As this starlight travels through Earth's atmosphere, it must pass through multiple layers of air. These air layers are constantly moving and have differing temperatures and densities, which means they have continuously changing refractive indices.

2. The mechanism of atmospheric refraction:
Because the refractive index of these atmospheric layers is constantly fluctuating, the path of the starlight is bent (refracted) rapidly and randomly in different directions. This continuous shifting of the light path causes the amount of light reaching our eyes to fluctuate quickly in both intensity and apparent position. This rapid variation is perceived by us as twinkling.

3. Observation without refraction:
If atmospheric refraction did not occur (for example, if Earth had no atmosphere, similar to the viewing conditions in space), the light from the stars would travel in a perfectly straight, uninterrupted line to the observer. Without the turbulent bending of light rays, the starlight would reach our eyes at a constant intensity and from a fixed direction. Therefore, the stars would shine with a steady, stable brightness and would not twinkle.