Color of the sun and stars

  1. I've read previous threads that the color of our sun from outer space is white, but on earth it is yellow because oxygen and nitrogen scatter the blue light. First, I am wondering if this is accurate. Second, if this is true, why do we see stars in the night sky as white? I would think the oxygen and nitrogen in our atmosphere would scatter the blue light from the stars. But the stars appear white. What's going on?
     
  2. jcsd
  3. Simon Bridge

    Simon Bridge 15,274
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    http://www.badastronomy.com/bad/misc/starcolors.html
    TLDR: you can only see the color of bright stars.

    The reason most stars appear white to us is because we have two different kind of light sensors in our eyes. Sensors called "rods" detect brightness, while sensors called "cones" detect color. The cones are not very sensitive, so if a light is too dim they are not activated, and we perceive the color as white. So even a red star looks white if it is dim, and only brighter stars look like they have color to us!

    ... this is not entirely accurate, no.

    It is true that O and N scatter the blue light - making the sky blue - and that does favor a yellow end result and a striking yellow color for the Sun, but the Sun is also actually yellow - classified as a "yellow dwarf" star.
    http://en.wikipedia.org/wiki/Sun
    http://stellarplanet.blogspot.co.nz/2013/04/why-is-sun-yellow-sky-blue-and-clouds.html
    In fact, the more atmsophere the sunlight/starlight has to pass through, the redder it looks.

    It follows that all hot stars are a tad redder than they would be viewed from space.
    There is dust and gas in space too - so distant stars are redder than the same sort of star nearby.
     
    Last edited: May 14, 2014
  4. Thanks - The rods and cones comments are very interesting along with the Bad Astronomy article. I also checked out the other links. Regarding the color of the sun, the wiki site states the following:

    The Sun is a G-type main-sequence star (G2V) based on spectral class and it is informally designated as a yellow dwarf because its visible radiation is most intense in the yellow-green portion of the spectrum, and although it is actually white in color, from the surface of the Earth it may appear yellow because of atmospheric scattering of blue light.

    This states that the yellow dwarf star is actually white. Can you clarify?
     
  5. Drakkith

    Staff: Mentor

  6. Vanadium 50

    Vanadium 50 18,137
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    As pointed out, the perceived color of a star depends on its intensity and thus distance. That means it doesn't have an "actual" color. The color it appears to have depends on how close you are.
     
  7. Simon Bridge

    Simon Bridge 15,274
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    Difference between everyday "color" and "wavelength of light emitted".

    The color is subjective to the specifics of the detectors (i.e. eyes, intervening dust etc) as well as the specific properties of the surface being viewed.

    In the case of stars - they emit light in a spectrum from far infra-red and radio to gamma rays. Our eyes are not sensitive enough to detect the variation that would give a specific peak wavelength in the narrow range of light that we can detect.

    The spectrum depends on the star, what we see depends on how our eyes work.
     
  8. D H

    Staff: Mentor

    Here's a photograph of the noon sun taken from the Kitt Peak Observatory (elevation 6,875 ft) on a clear day:

    [​IMG]
    Source: http://solar-center.stanford.edu/suns-color-story/page22.html

    It's pretty white, isn't it?

    You see the Sun as reddish at sunrise because that and sunset are when the greatest amount of scattering occurs. As the Sun climbs, it turns from reddish to orange and then yellow. After that you can't look at the Sun, even obliquely. You can't see that the Sun is white at noon because looking at the high Sun for even an instant is blinding. Do not try; it will burn your retina.

    You can do this with a camera. Here's a time lapse series of photographs of the Sun at winter solstice.

    [​IMG]
    Source: http://scienceblogs.com/startswithabang/2013/12/18/top-6-facts-about-the-years-final-solstice/
     
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