Why is easier to see faint star in the sky if you look slightly to the side of the star instead of straight at it?

Answer 1

This is because the 'edges' of your eyes, i.e. the parts which are involved in peripheral vision, are highly populated by photoreceptor cells known as 'rods', which have greater visual acuity and are responsible for vision at low light level. This is also why you can see better out of the corner of your eye in darkness. The centre of your eye, or fovea, is more exclusively populated by 'cones', another kind of photoreceptor responsible for vision in higher light levels.

Answer 2

When you look slightly to the side of a faint star, you utilize your peripheral vision which is more sensitive to light and motion. This allows your eyes to pick up on the faint light of the star more easily than if you were looking directly at it with your foveal vision, which is less sensitive to dim light.