Sun's Color

[George]

I would like to know what the true color of our sun really is.

Here, below the atmosphere and from noon to dusk, it goes from white to yellow to orange to red. This is due to the fact that the higher frequency light gets scattered more and more (Rayleigh Scattering). It is also why the sky is blue. It is white when high in the sky because the intensity of the colors are all about the same to our eyes rendering a white image even when the light intesity is highly attenuated.

But what is it really like in space? I am wondering if it is bluish. Our atmosphere takes a heavy toll on the blue portion of it's light. An irradiance curve will show that about 1/2 of the blue light is lost due to our atmosphere (some of it is seen as blue sky). The blackbody spectrum peak for the sun is on the edge of blue. Astronauts see it as blinding white but at 1,370 watts/m^2, who wouldn't?

Space scopes don't seem to be much help on this issue. Usually they avoid the sun "at all costs". If Spitzer gets closer than 80 deg. toward the sun, instruments get damaged. Hopefully I'm wrong here, but I suspect solar observation scopes aren't great in getting data necessary for "true color" rendering. These scopes are designed for other areas of the spectrum which are much more critical to science.

Our eye favors green light. Firetrucks are getting a lime color look for this reason.

We assume it's yellow. We grew up with it being yellow. We used our green crayons for grass and trees (and aliens), red for fire and barns, blue for sky and water, brown for land, so, we had to use yellow for the Sun. :rolleyes: Well, it does emphasize the psychological aspect of it all.

A computer model based on spectrum and eye/brain rendering produced a sun which is peachy pink! What if he's right.

If I make a strobe, do ya'll have any astronaut friends who will take it up? B)

[Josh]

Having not left Earth's atmosphere recently (sadly) I can only go by what the physics says and what I've heard so ...

The long answer is, yes. The sun is all those colours. But I'm not going to explain that. The shorter answer is, white. If you went up there and looked at the sun (which I'd advise against) then it'd appear white. The reason it looks white when it's high in the sky is because it is travelling to your eyes through the least amount of atmosphere. The lower down in the sky the sun gets then the more atmosphere it has to travel through. Here is the reason which I posted in another thread somewhere ...

The Sun will look redder at both the morning and the evening - just before sunrise and sunset. The reason for this is something called Rayleigh scattering. At those times when the sun is lowest in the sky, the distance the light has to travel from the sun to reach your eyes is at its greatest and has to travel through more of the atmosphere. A lot of the blue and violet end of the visible spectrum has been scattered and lost meaning that only the longer wavelengths (ie those closer to the red part of the spectrum) reach you. The result is that the sun looks more red at those times.

When outside the atmosphere all the colours of the visible spectrum are reaching your eyes and so the light will appear white.

[George]

What you say is correct. However, "blue" stars also radiate all colors. They look blue because of the greater intensity in the blue relative to the other colors.

The irradiance curve of the sun is flat when seen down here on the surface. A flat curve, of course, will give you equal levels of input to our eyes so it will look white.

Above the atmosphere is a much different story. The blue level is double what it is down here. The peak blackbody wavelength of the sun (around 480 nm) is not quite as much in the blue segment as the "blue" stars but these stars are "blue" even after suffering 50% loss of their "blue" due to our atmosphere.

Here's the graph...Sun's Irradiance (http://hyperphysics.phy-astr.gsu.edu/hbase/vision/solirrad.html)

This describes the input. The eye becomes the next issue. I would bet that if all three of our color cones receive enough photons, it will register an object white. This would be true of a blue or violet or orange local star as the eye would be "blinded" as to the true color. Once the intensity is lowered where the eye can discriminate the proportions of actual color emission, it should render its "true" color. This is just my opinion, however. I have not found supporting evidence for this idea but it seems logical, right?

A simple plate strobe used above the atmosphere would resolve this issue. I made one and, no surprise, we all see a white sun as long as it is not on the horizon.

I appreciate any help on this as it is one of this "can't shake it loose questions" even though it has little scientific value.

I suppose if SETI shouts "Eureka" we will not want to tell a likely visitior that we are the 3rd planet from the "yellow" sun. :D

Would we tell them it is "peachy pink" as per here (http://casa.colorado.edu/~ajsh/colour/Tspectrum.html). :rolleyes: