Why do stars seem to get more dim if I look directly at them?
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Night vision is based on your black and white (rod cells in your eyes) and not your color vision (cone cells in your eyes). You have way fewer rod cells in the center of your vision and more on the sides of your vision. So, you are more sensitive to faint light when you actually look away from the direct location of the Star.
Cone cells dominate the center of your vision, but aren’t as sensitive to faint light.
Because that makes the light hit your retina where the rods happen to be in greater number. Those are the low light receptors.
Our eyes detect light through two different photoreceptors: rods and cones. Cones are used to detect color and are more concentrated at the center of our vision. Rods do not detect color but are more sensitive to light, so they pick up dim levels of light more easily. Since rods are more concentrated in your peripherals, or the corners of your eyes, stars appear brighter when looking out the corners of your eyes or slightly away from directly at them.
The center of your vision is called your *fovea*, which is a tiny depression in your retina with the most cells packed in. However, most of the cells there are the color-sensing cone cells. Those cells are sensitive to specific wavelengths, which is how you see color. As a consequence, though, they are less sensitive to light overall.
Your retinas around the foveas have fewer cells overall, but they’re predominately rod cells which are sensitive to a wide range so they can’t be used to detect color, but they are more sensitive to any light.
Stars are very dim. When you’re not quite looking directly at them they appear a little bit brighter to the more sensitive rod cells.
The others have explained the main reason (rods vs cones in the centre spot), but I’ll add that ‘peripheral viewing’ is a common technique in amateur astronomy. It allows you to see faint objects better in smaller telescopes by centering it in the view, then looking off the side of it to abuse those rods.
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