Why Does Our Sun Appear Yellow Instead of White?

[sophiecentaur]

Of course the sun is white.

But is it Persil White?

 

[davenn]

OK ... regardless of how it looks visually or photographically via the Earth's atmosphere,
The Sun is deemed a yellow class star...

cheers
Dave

 

[chemisttree]

But is it Persil White?

Up close it is.

 

[Solon]

@Drakkith.

It's not contentious. The Sun's COLOR is white. But since color is purely perceptive, we don't use it for accurate measurements. Instead, we measure the spectrum and plot the output per wavelength using a spectrograph.

I've never seen a colour calibration card used out in orbit, or a white balance reference card. Maybe it's because colour being perceptive they don't bother?. SOLSPEC has 1 nm sampling, but calibration is needed regularly, and I don't see any charts showing how the spectrum changes, if it does, over time. Is ours a variable star?

Also, we don't base distances off of color. We have a cosmic distance ladder that involves several different techniques, none of which are based on color.

I though they used light and colour curves when examining the Cephid variables? Maybe not.

 

[russ_watters]

I've never seen a colour calibration card used out in orbit, or a white balance reference card.

Not in orbit, but...
http://marsrover.nasa.gov/mission/ spacecraft _instru_calibr.html

 

[Drakkith]

OK ... regardless of how it looks visually or photographically via the Earth's atmosphere,
The Sun is deemed a yellow class star...

Yes, which is a misnomer.

The term yellow dwarf is a misnomer, as G stars actually range in color from white, for more luminous types like the Sun, to only slightly yellow for the less luminous GV stars.[8] The Sun is in fact white, but appears yellow through the Earth's atmosphere due to Rayleigh scattering. In addition, although the term "dwarf" is used to contrast yellow main-sequence stars from giant stars, yellow dwarfs like the Sun outshine 90% of the stars in the Galaxy (which are largely orange dwarfs, red dwarfs, and white dwarfs, the latter being a post-main-sequence star).

http://en.wikipedia.org/wiki/Yellow_dwarf

@Drakkith.I've never seen a colour calibration card used out in orbit, or a white balance reference card. Maybe it's because colour being perceptive they don't bother?. SOLSPEC has 1 nm sampling, but calibration is needed regularly, and I don't see any charts showing how the spectrum changes, if it does, over time. Is ours a variable star?

No, the Sun is not a variable star.
While the Sun does go through changes, none of these have any effect on the average temperature of the Sun's photosphere, which means that it continually outputs light with a spectrum close to a perfect blackbody at about 5,800 kelvin.

I though they used light and colour curves when examining the Cephid variables? Maybe not.

To my knowledge they only measure the magnitude changes.

 

[cjl]

OK ... regardless of how it looks visually or photographically via the Earth's atmosphere,
The Sun is deemed a yellow class star...

cheers
Dave

That picture is clearly misleading though - it shows a 6000K blackbody as being on the orangey side of yellow, which it clearly isn't (as anyone who has seen a 6000K blackbody will attest). Even 3000K, which that picture shows as being distinctly red, will look fairly white to most people (a lot of incandescent light bulbs operate at around that temperature), though it will probably look somewhat yellowish (especially if you have a whiter reference light to compare it to).

 

[sophiecentaur]

Have you never come across false colour before? It's used all over the place to 'model' variables with different colours. Why do you take such a picture so literally? You would never make the same mistake with a thermograph.

This thread has really got out of hand. Stars have a range of surface temperatures and that produces different 'whites', which can be described in terms of yellow, blue, white etc. Those names are only used 'colloquially'. Anyone who wants to be precise will talk in terms of temperature.

The atmosphere changes the colour of anything viewed through it because it is not a neutral density filter. Is there any doubt about that? Any 'experiment' to demonstrate this needs to be quantitative and properly carried out. And it has been done many times. Random photographs do not constitute evidence of anything.

 

[CubicFlunky77]

I have never thought the sun was yellow. It is too bright for me to observe for more than 5 seconds (even with very dark sunglasses). Color doesn't seem to be an objective measure since I cannot tell if my perspective of a certain color (in this case, yellow) has the same shade/physical characteristics as your 'yellow'.

 

[davenn]

Have you never come across false colour before? It's used all over the place to 'model' variables with different colours. Why do you take such a picture so literally? You would never make the same mistake with a thermograph.

This thread has really got out of hand. Stars have a range of surface temperatures and that produces different 'whites', which can be described in terms of yellow, blue, white etc. Those names are only used 'colloquially'. Anyone who wants to be precise will talk in terms of temperature.

The atmosphere changes the colour of anything viewed through it because it is not a neutral density filter. Is there any doubt about that? Any 'experiment' to demonstrate this needs to be quantitative and properly carried out. And it has been done many times. Random photographs do not constitute evidence of anything.

Totally agree

that diagram, which is the standard model for stars
of course there's going to variations within the ranges displayed. It just gives a good indication of where our sun/star is positioned in the great scheme of things

Dave

 

[cjl]

Have you never come across false colour before? It's used all over the place to 'model' variables with different colours. Why do you take such a picture so literally? You would never make the same mistake with a thermograph.

Of course I have, but it seems like a lot of people (at least among those who haven't studied physics/astronomy very much) do take those charts fairly literally as representing the apparent color. In some ways, I'd prefer if it didn't bother with the color at all, and rather just plotted luminosity and photospheric temperature. The color serves no real additional purpose on that chart, and as I said, it's not really even remotely representative of what blackbodies at those temperatures actually look like (since that would require a slightly yellowish white in the lower right, moving to a slightly bluish white in the upper left, none of which would be terribly saturated colors).

This thread has really got out of hand. Stars have a range of surface temperatures and that produces different 'whites', which can be described in terms of yellow, blue, white etc. Those names are only used 'colloquially'. Anyone who wants to be precise will talk in terms of temperature.

The atmosphere changes the colour of anything viewed through it because it is not a neutral density filter. Is there any doubt about that? Any 'experiment' to demonstrate this needs to be quantitative and properly carried out. And it has been done many times. Random photographs do not constitute evidence of anything.

I think you might be taking this a little too seriously...

 

[sophiecentaur]

I think you might be taking this a little too seriously...

'Appropriately' seriously, I think.

 

[D H]

When the Sun is far above the horizon it is indeed "white" as our eyes would perceive it. We only have three color sensors, and how sunlight from a high Sun excites those three kinds of sensors is pretty close to white.

We can't see that white Sun because we can't take anything more than a fleeting glance at the Sun when it is far above the horizon. Even that fleeting glance hurts, and anything longer would cause permanent eye damage. We can only look at the Sun long enough to deduce it's color when it's close to the horizon and the sunlight has passed through a long stretch of the atmosphere. The scattering that results from this long stretch of atmosphere is what makes us perceive the Sun as yellow (or orange or even red). The perceived color changes as the Sun gets closer to the horizon due to this scattering. When the Sun is high overhead, it's white. Just about as perfect a white as can be.

 

[sophiecentaur]

Which 'white' did you have in mind? Look at the controls on your computer monitor and you'll find a lot of options even on that. Are you certain that, if someone altered the 'white' on your screen whilst you were away, you'd necessarily notice?

 

[Drakkith]

Which 'white' did you have in mind? Look at the controls on your computer monitor and you'll find a lot of options even on that. Are you certain that, if someone altered the 'white' on your screen whilst you were away, you'd necessarily notice?

The kind of white the Sun looks like near zenith?

 

[sophiecentaur]

The kind of white the Sun looks like near zenith?

What sort of a day and where in the World? This is just not accurate enough - even for colour printing and certainly not for Science.

 

[Drakkith]

What sort of a day and where in the World? This is just not accurate enough - even for colour printing and certainly not for Science.

I don't know what you're getting at Sophie.

 

[D H]

Which 'white' did you have in mind?

CIE D65. That is as white as white can get, and it's the color of the Sun at noon. Note that correlated color temperature is not quite the same blackbody temperature. CIE D65 is *the* standard definition of white. http://en.wikipedia.org/wiki/Illuminant_D65

 

[sophiecentaur]

I'd go along with that - as long as you acknowledge that D65 is an Illuminant and not a 'colour'. It's defined in terms of Daylight / Sunlight, which is not only direct light from the Sun. The rest of the sky is there too.

 

[sophiecentaur]

I don't know what you're getting at Sophie.

I'm getting at the fact that the Sun is not something that we ever look at, in fact. So, on its own, the Sun is no use as a reference, and the state of the sky and the surrounding scene is going to contribute to what we see.
When Sunlight (as a luminant) is said to be 'white', the light comes from all over the sky and not down a narrow tube. The colour of a sheet of white paper, at midday, in the shade, is not the same as out in the open. All that indirect light that gets into the shade is also getting to the paper when it's not in the shade. If we're trying to be scientific about this, we have to be precise and have a possible measurement system - not just to say what it 'looks like'.

 

[Drakkith]

If we're trying to be scientific about this, we have to be precise and have a possible measurement system - not just to say what it 'looks like'.

I thought we were being pretty precise when we said the Sun radiates like a blackbody at about 5800 kelvin and looking at the Sun through a small portion of the atmosphere overwhelms your color receptors, making the Sun look about as white as you can get even though some of the light is scattered out.

 

[sophiecentaur]

I thought we were being pretty precise when we said the Sun radiates like a blackbody at about 5800 kelvin and looking at the Sun through a small portion of the atmosphere overwhelms your color receptors, making the Sun look about as white as you can get even though some of the light is scattered out.

But that doesn't correspond to D65 (there are a number of other standards), which is the 'white' for an illuminant. That white hot disc up in the sky is not the only thing that illuminates things on the ground. If you took an equivalent filament lamp, producing D65 illumination, it would not 'look' the same colour as the Sun up in the sky (assuming you used a perfect ND filter to view them through).
I think you would agree that overloaded retinal receptors are not appropriate instruments for any assessment of colour.
So:
Sun + Blue sky = D65 standard filament lamp (when reflected by a totally reflecting matt surface)
Therefore
Sun = D65 lamp - blue sky
Can they both be your 'perfect white' then?.

 

[voko]

So:
Sun + Blue sky = D65 standard filament lamp (when reflected by a totally reflecting matt surface)
Therefore
Sun = D65 lamp - blue sky

This is unobservable from the surface of the Earth, while I think the subject question was in the terrestrial setup.

 

[Drakkith]

Can they both be your 'perfect white' then?.

I don't know and I don't care. Per the OP's original question, the Sun is white, it looks about as close to white as you can get when high in the sky, turning to yellow-orange as it approaches the horizon thanks to scattering.

 

[sophiecentaur]

This is unobservable from the surface of the Earth, while I think the subject question was in the terrestrial setup.

?
I am describing exactly what you see from Earth here. You 'see' the familiar old Sun ( through the atmosphere of course) and you see the blue sky up there. Are you not following this?

 

[sophiecentaur]

I don't know and I don't care. Per the OP's original question, the Sun is white, it looks about as close to white as you can get when high in the sky, turning to yellow-orange as it approaches the horizon thanks to scattering.

And when was the last time you observed the Sun in the sky, well enough to assess its colour? In most PF discussions, it would be expected that conclusions would be based on measurement or firm observations - not just assertions. What's different about this one?

 

[voko]

?
I am describing exactly what you see from Earth here. You 'see' the familiar old Sun ( through the atmosphere of course) and you see the blue sky up there. Are you not following this?

This: "Sun = D65 lamp - blue sky" is what you do not see. Because you see the Sun through the blue sky.

 

[Drakkith]

Sophie, I still have no idea what you're trying to get at here. If you could boil down your argument to the most basic thing your trying to get across, please do. Because I'm half lost here. We've gone from Sunlight filtering through the atmosphere to measurement systems to referencing something with something else. I have a nagging feeling that we're arguing over nothing here.

 

[sophiecentaur]

This: "Sun = D65 lamp - blue sky" is what you do not see. Because you see the Sun through the blue sky.

You see the Sun - yes. but illuminant D65 is what falls on a matt surface and what you see from that surface is direct Sunlight plus what comes from a hemisphere of blue sky. l'm afraid I can't see what there is not to understand here. There are two separate issues. There is the colour seen when you look at a neutral matt reflector (which is the result of an illuminant) and the colour you see when you look directly at the Sun through a ND filter. D65 in no way refers to observing the Sun directly (we can't do it conveniently). It refers to what matches for a white surface outside in sunlight (including blue sky contribution) and a black body at 6504K (a filament). We don't look at the sun to tell us what's a 'standard' white and neither do we shine sunlight through a narrow tube; we see sunlight, reflected off a white card.
Of course our Sun (when viewed directly) is Whiteish and so are most other stars we can see. But, as the diagrams show, there is a range of temperatures - hence a range of 'whites'. In order to compare stars, viewed from above and below the atmosphere, you need to compensate for the atmospheric effect, when appropriate.

Afaiaa, standard white is only defined in terms of an illuminant because that is the majority application of the notion of 'whiteness' (i.e. in printing, TV and photography). I wish I hadn't started on this now but it really is relevant. Stars are just assigned a Temperature.

 

[voko]

You see the Sun - yes. but illuminant D65 is what falls on a matt surface and what you see from that surface is direct Sunlight plus what comes from a hemisphere of blue sky. l'm afraid I can't see what there is not to understand here.

And I cannot see the relevance of this argument with respect to the question "why is our Sun yellow". This question is about our visual perception of the Sun, not of its light that we observe indirectly.

The Sun ranges from reddish when close to the horizon, to blazing white when at the zenith, especially at places where humidity is low, and so must be yellow somewhere in between. There is nothing to debate about these facts, I would presume.

The question is "why", and I think the answer is "it is the atmosphere".