Biology Stack Exchange is a question and answer site for biology researchers, academics, and students. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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.

share|improve this question
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
up vote 22 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.

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

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.