Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why do I see stars as a point in the sky?

  1. May 15, 2014 #1
    The light from stars radiates out from them as a sphere. When a part of the surface of the light sphere reaches the earth, it is far enough away from the star so that each point on that part of the surface of the sphere near the earth has about the same light energy. But, I only see a point, not what I presume that part of a sphere would look like (filling the entire sky.) Why is this?

    Thank you
     
  2. jcsd
  3. May 15, 2014 #2

    phyzguy

    User Avatar
    Science Advisor

    Only the photons pointed directly at you enter your eye where you can see them. You're correct in that there is an expanding sphere of photons from the star spread out over the whole Earth (and beyond). But the photons from the same star that land in your neighbor's back yard or across town don't enter your eye so you don't see them.
     
  4. May 15, 2014 #3

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    Why do you presume that?
     
  5. May 15, 2014 #4
    Thank you
     
  6. May 15, 2014 #5

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    The reason that you see all stars as 'points' is that your eye has only a finite resolution (much smaller pupil than even cheap bino's). There is no way that your eye can distinguish which side of a distant star the light is coming from so you just see a tiny blob of light.
    You can see many of the planets as actual discs because they subtend a big enough angle - even Venus looks like a 'little Moon' because it is a big enough object.

    Any optical system (telescope) has a finite resolution and even the nearest stars are still registered as points, afaik. The advantages of a larger aperture are twofold - firstly, the light gathering is much greater so you can see fainter objects and secondly, the larger the aperture, the better you can resolve what looks to the eye as a bright blob / point into two or more objects. Because of the resolution of your eye, a brighter star may look bigger, because more of your receptors get enough light to register that something is there.
    PS @oneamp You need to distinguish between thinking about light spreading out from a source (On a cone or over a sphere) and the cone of light that your eye accepts. Your eye only intercepts a tiny proportion of the light output from a star (yes - that's obvious) and the star only occupies a small proportion of your total field of view (that's obvious too - but some of your question seems to be conflating the two ).
     
  7. May 16, 2014 #6

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    Google suggests that you can see stars that are perhaps 1500 light years away with the naked eye. So the light from these stars is spread out over an enormous sphere of that radius...yet still enough photons arrive at your eye.
     
  8. May 17, 2014 #7

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    The fact is that our visual perception is pretty well logarithmic and we perceive an amazing range of contrast - from individual photons to almost direct sunlight (1kW per sq m - but that will harm your retina, of course, if sustained). This Wiki article discusses the factors governing the visibility of stars. If you bear in mind that only the very biggest / brightest stars at great distances are visible to the naked eye and that, to see them, you need to be young and fully dark adapted (no light pollution), the actual numbers are likely to be in the right ball park. (I am ignoring red shift, here but 1500LY is really pretty close in cosmological terms).
    If you double the distance of a source, you are 'only' reducing the perceived brightness by a factor of four. This is the same, whether going from 1 km to 2 km or from 750LY to 1500LY. This "6dB reduction by doubling the distance" is a bit counter intuitive but it accounts for how the feeble radio transmissions (a very few Watts) from the deepest space probes can still be detected. Compare this with the sort of losses you get through a medium like glass (or RF cables) where the loss (in dB) is proportional to the distance. A cable to the Moon would never work - let alone a cable to a nearby star.
     
  9. May 18, 2014 #8
    This should put it into perspective.

    Alpha Centauri A is 4.37 LY or 4.13x1016 meters from us. I'll assume that the diameter of Alpha Centauri A is similar to our sun (1.39x109 meters). Using similar triangles to calculate the diameter of a disc that is one meter away yields

    X / 1 = 1.39x109 / 4.13x1016 meters

    X = 3.36x10-8 meters

    The best that the human eye can discern is a fraction of a millimeter, say 1x10-4 meters. Alpha Centauri A is four orders of magnitude smaller than what our eyes can pick out, relative to the disc that is one meter from our eyes.

    Space is VERY VERY big.
     
  10. May 19, 2014 #9

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    That figure refers to 'resolving power' or ability to separate a blob into two distant objects. Actually 'seeing' the star just depends upon the amount of light power entering the eye. What you actually see could be a whole bunch of stars, close together.
     
  11. May 19, 2014 #10
    But, light from the distance greater than 1500 light years can reach us. Isn't it?
     
  12. May 19, 2014 #11

    UltrafastPED

    User Avatar
    Science Advisor
    Gold Member

    Larger objects, such as the galaxy in Andromeda, are much further away - yet we can still see them. This is because they contain many stars - perhaps 100 billion stars, and 2 million light years away.

    So it is about the amount of light; the light from a single star is too faint for the human eye to see; the ancients documented about 3,000 stars - that's about how many you can see from one point on the earth. Double that if you travel a lot!

    Hence telescopes that track the motion, forming an image over time.
     
  13. May 19, 2014 #12
    Consider the following picture, in which laser is placed in a dark room. Though, laser is unidirectional, one can see it. The reason I might assume must be because of the scattering of light by the dust particles coming in its way (related to tyndall effect).

    uHxJx.jpg

    One must observe here that the intensity of light reaching us, is the least of the total intensity. From this, I prefer saying that even a mere intensity of light coming from the star is enough to see it.
     
  14. May 19, 2014 #13

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Of course you are right. It was just an example, involving a large distance. A star, four times brighter, would be visible at 3000LY.
     
  15. May 19, 2014 #14
    I am under the impression that, even a star which is not brighter as four times the original, will be visible to us. Read my last post in this thread, to know why I think so.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Why do I see stars as a point in the sky?
  1. Why do stars twinkle? (Replies: 8)

Loading...