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Why is our sun yellow?

  1. Aug 14, 2013 #1
    Hi!

    I wonder why our sun, being a star and all, is yellow and not white?

    What is it in our atmosphere or jonosphere that makes it yellow?

    Best regards, Roger
     
  2. jcsd
  3. Aug 14, 2013 #2

    mathman

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    It appears yellow mainly because the blue part of the spectrum gets scattered more than the rest by our atmosphere. That's why the sky is blue.
     
  4. Aug 14, 2013 #3

    Nugatory

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    Google for "Why is the sun yellow?"
    The first two or three hits have pretty decent answers.
     
  5. Aug 14, 2013 #4

    cjl

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    Honestly, it looks pretty white to me.
     
  6. Aug 14, 2013 #5
    Two things:

    1) Scattering, what is that?
    2) Reading that link suggests that the amount of yellow is due to the distance sunlight has to travel through the atmosphere.

    Scattering sounds to me like something being reflected "away" from the observer. My thinking is that if scattering of blue light happens then the sky should be everything but blue. Please correct me while I'm wrong :)

    Then take the only star I know, the Alpha Centauri (AC).

    The light from AC certainly appears white to me.

    Whiter than our sun at noon, even.

    The only difference is that I/we cannot see it unless it is dark.

    Correct me if I'm wrong, but isn't the distance for AC's rays of light through our atmosphere equally long as it is for our sun's rays of light?

    Yet Alpha Centauri looks white while our sun looks yellow...

    Best regards, Roger
     
  7. Aug 14, 2013 #6

    Drakkith

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    The Sun is not yellow, it is white. The only time it looks yellow is near sunrise and sunset.
     
  8. Aug 14, 2013 #7

    sophiecentaur

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    Sunlight can be altered either by partial absorption of some wavelengths or by some of the energy at some wavelengths being scattered from the direct path. Short wavelengths (the blue end of the spectrum) are scattered more than the longer wavelengths. We see the sky as blue(ish) because sunlight that would go direct to points on the Earth's surface remote from where we are standing is scattered in our direction. Meanwhile, the shorter wavelengths that would come directly to us, are scattered to other places - making the sky look blue to them.

    Different stars have different temperatures so, inherently, they appear different colours. Viewing them through our atmosphere affects them all but still the bluish stars (hotter) look less yellow than the (less hot) stars like our Sun. The angle in the sky will also affect the colour - which is why the Sun looks much more red when it's near the horizon (sunset) - the path through the air is much longer, so more scattering. The short wavelengths the we 'lost' are seen by people to the west of us (it's still only afternoon for them).

    If you actually measure the colour of AC when it's low on the horizon, it will also look more yellow than when it's high in the sky. But our subjection appreciation of the colour difference may not be good enough to spot this effect unless we are well trained observers.
     
  9. Aug 14, 2013 #8

    SteamKing

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    Is Alpha Centauri where you are from originally?
     
  10. Aug 14, 2013 #9

    D H

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    Alpha Centauri is fairly close in color to that of the Sun. It's a question of how our eyes work.

    You can't see Alpha Centauri in the daytime. Too much sunlight! You can only see it at night. Of course it looks white. Almost all of the stars look white. Use a telescope and the multitude of colors will jump right out. It's quite beautiful.

    You don't see color at all (scotopic vision) or not very well (mesopic vision) at night. The cones that are responsible for our color vision don't work very well under low lighting conditions. The rods in our eyes take over when illumination decreases. There's only one kind of rod, so all we see under extremely low lighting conditions is black and white: No color at all. With slightly greater illumination we can see some color, but it is rather muted. You are looking at the stars with your cones working very poorly at best. That's why most stars appear to be white.
     
  11. Aug 14, 2013 #10

    Drakkith

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    I just went outside and looked at the Sun for a moment. It looked pretty white to me.
    Now excuse me while I go replace my retina...
     
  12. Aug 14, 2013 #11
    I need to say thanks officially to all you guys but especially sophiecentaur and DH.

    Take care!

    Best regards, Roger
     
  13. Aug 14, 2013 #12

    sophiecentaur

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    You're welcome.
    It's amazing just how counter intuitive a lot of this stuff is. Working it out on one's own can be really hard.
     
  14. Aug 14, 2013 #13

    sophiecentaur

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    Apparently, the navigators on old ships (pre-sextant days) all went blind in one eye at a very early age. At least they had the sense to use the same eye each time for sun sightings.
     
  15. Aug 14, 2013 #14
    It sure is.

    It thus seems like you are aware that you and DH have explained the phenomenon in a completelly different but yet scientific way?

    Ain't that fascinating :)

    Best regards, Roger
     
  16. Aug 15, 2013 #15

    cjl

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    Despite all of this though, I would still say the sun itself is white, not yellow. People think of it as yellow in part because that is how it is depicted in paintings/artwork, and in part because it does actually look yellowish when it is near the horizon, which also attenuates the light enough that it isn't as uncomfortable to look at. Since it is normally painful (and potentially damaging) to look at the sun, we unconsciously assume that it is always yellow (as it is when it is near the horizon).

    However, if you need further proof, look at a piece of white paper sitting in the sunlight. It is reflecting the same spectrum that is incident on it. Does it look like it is sitting in yellow light? If you think that the blue from the sky may be affecting this, do it inside, with the sunlight coming in through a smallish window to prevent much skylight from being able to change your results, and it should still look quite white.
     
  17. Aug 15, 2013 #16

    sophiecentaur

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    Sunlight is certainly taken to be one of the 'white' illuminants. A surface (paint) that reflects all wavelengths equally will look White (that is what we call white) when it's outside. But of course, it is getting blue light from the sky around the Sun, too. Depending on the state of the clouds (red absorbers), elevation of the Sun and the presence of snow on the ground, we can get all sorts of 'whites', from warm white to cold white.

    This is all subjective; it's not really a matter of "proof". You would find that your white paper will look very convincingly white when indoors, under all sorts of illumination (very yellow, dimmed filament lamps, for instance). That's because your colour vision is desperately trying to take out any perceived colouration due to to the illumination. Your brain tries to 'integrate to grey' - just like your digital camera also tries to do, and jiggles its colour channel gains accordingly. It can be very successful and it can also fail abysmally, when trying to match clothes colours in a shop, under artificial light. (Colour film is hopeless because it has only one setting for its colourimetry). That's why we take things out into the street to check.
    The Sun is not particularly blue or particularly red/yellow, compared with all the other stars around there are much more extremes on other side of our 'white' Sun - intense blue stars that are burning up very quickly and are much hotter and some red giants that are cooling down at the end of their lives. Their particular colours of illuminant would be taken as normal (white?) by the local inhabitants.
    Overall, coulorimetry is a fascinating subject, I think.
     
  18. Aug 15, 2013 #17
    Here is an image of the Sun taken with a neutral density filter.
    Sun Spots. 30 Sept 2011. 1203hrs 200mm | ISO 50 | f/13 | 1/8000sec | 10 stop ND filter
    http://mcalisterium.wordpress.com/2011/09/30/sun-spots/
    So surely if we used exactly the same set-up from outside Earths atmosphere, we would know how much effect the atmosphere has on what we see from Earth? A simple experiment, but has never been done AFAIK.
     
  19. Aug 15, 2013 #18

    sophiecentaur

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    What, no happy snaps taken on the Moon with standard film? - Perhaps before your time!
    But photographs tell us nothing about perceived colour. Colour is subjective and even the best camera can only make a stab at it. There is no doubt, though, that there is atmospheric absorption and scattering so the colour would have to be different out in space. Is this really an issue worth discussing?
     
  20. Aug 15, 2013 #19

    cjl

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    The "blue light from the sky around the sun" is the reason I suggested doing it through a smallish window, with the sky mostly blocked. That having been said, you're right that white isn't terribly well defined.

    Sure. I debated going into a more lengthy discussion about various color temperatures, and how an incandescent light looks white despite a significant red/infrared bias in its spectrum, but I didn't feel like going into that much detail. That having been said, I don't think anyone following normal conventions would call a 5800K blackbody "yellow", and looking at solar spectra, I cannot see any evidence of significant blue attenuation (compared to the rest of the spectrum) when the sun is at a high angle in the sky. Sure, blue is scattered more than other colors (and thus the sky is blue), but the vast majority of blue light from the sun is still making it through the atmosphere unimpeded. As a result, I don't see how the sun could be considered anything other than white to most people, at least when it is high in the sky. (Admittedly, this is all somewhat academic, since the intensity of the sun is far too high to look at directly without a filter in any case, and once you add a filter, the apparent color will be strongly affected by the filter's exact properties).
     
  21. Aug 15, 2013 #20

    sophiecentaur

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    There are many sources that are hotter than the Sun - electric arcs, for instance - and many that are not so hot. It is strictly a matter of taste, of course, but it would be reasonable to call the different 'whites' by familiar names - namely blue, yellow and orangy-red. But that's nothing to do with Physics, is it? You'd need to ask your granny or someone else who is strictly non-technical. (Apologies to all the Grannies with PhD's in Physics - there must be quite a few on PF - or perhaps they would have more sense than to get into the testosterone laden maelstrom that is Physics Discussion)
     
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