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I've noticed that when I'm out at night stars seem to vanish when I focus on them. I assume the reason is physiological and not astronomic. Why does this happen? I would expect that things would be more clear when I focus on them.

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You might also notice this phenomenon while lying in bed, e.g. the dim glow from the illumination of a digital clock or small LED is often visible only if you look away (using peripheral vision). –  Mechanical snail Sep 16 '12 at 4:38

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up vote 20 down vote accepted

When there is little light, the color-detecting cone cells are not sensitive enough, and all vision is done by rod cells. Cone cells are concentrated in the center of the eye, whereas rod cells are very rare in the center (image source):

Density of rod and cone cells

When you focus on the star, the light is projected close to the center of the retina, where it will hit few rod cells. Thus the star appears to vanish.

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citation for figure? –  Shep May 3 '12 at 6:49
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it's the link before the figure, I've added "image source" to the link –  Michael Kuhn May 3 '12 at 6:50

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