Is the Sun yellow?

[owlice]

After my previous post, I mused more on this, and what came to mind were string quartets. Generally, I'm not crazy about string quartets and rarely listen to them, though I have recordings of a few, including those of Brahms. I also have these quartets arranged for piano, four-hands; these were transcribed by Brahms himself. I love these! I would MUCH rather listen to the piano arrangements of these works than to the original works. Is the music any less music because it is played on piano than on strings? No. Both versions illuminate different things about the music, but music it remains.

"true color"

:: now hears Cyndi Lauper on the radio in her head ::

[Ann]

It isn't the intent of most scientific astronomical imagers to produce results that show some sort of actual color. The intent is to ascertain what makes up objects, and to see structure.

Yes, it is obvious that that is the kind of astronomy that the Hubble telescope usually does. And I don't consider it an unworthy enterprise, very far from it. Believe it or not, I'm a true astronomy nerd, and I'm really fascinated by all the amazing astronomical discoveries that have been made during the last few decades. I admire astronomers who labor to reveal more of the secrets of the universe. Don't ever get me wrong about that!

This is a "wrong-color picture" that is of great scientific interest:



Dwarf planet Pluto is shown as blue??? Even though Pluto is about as red as Mars?

But, okay. I understand why NASA chose to photograph Pluto in blue light. It's because Pluto itself, although it is tiny, is nevertheless so much larger and brighter than its moons Nix and Hydra that its light would have overwhelmed these tiny bodies if NASA had photograped the Pluto and its moons in red light. Since the moons are more neutral-colored than Pluto, it was much easier to detect and bring out the moons if they (and Pluto) were photographed in blue light. I understand it, I support it, and I also think it's a great picture and an important astronomical discovery. Nevertheless, the image of the blue Pluto still looks totally weird to me. I very much admire the image, but I don't love it.

So let me show you some images that I truly love because I find them so beautiful:



This is one of my favorite images hands down. I love the Pleiades, which are beautiful to the naked eye, beautiful in a pair of binoculars, and beautiful when you scrutinize the stars up close in a telescope to really detect their color. In this image, it's amazing to see the color difference between the greenish coma of Comet Machholz and the blue reflection nebula of the Pleiades. I also love the blue ion tail of the comet versus the neutral color of the comet's anti-tail. Obviously the composition of the picture is stunning. I do have one complaint, however, and that is that all the stars in the picture seem to share an identical blue color.

Another of my all-time favorites is this image, which is so large that I'll just post it as a link:

http://apod.nasa.gov/apod/image/0709/sc ... uisard.jpg

Milky Way in Scorpius. Wow. The picture is amazingly detailed, fantastically resolved, stunningly colored and just so darn beautiful. Wow. What's not to love about it? It may or may not be fantastic new science, but it sure is astronomy as true-color, lovely color art.

Ann

[Ann]

But hah! Art! You have almost convinced me! Who would be the greatest ever authority on the color of the Sun, if not Shakespeare? I bow to your erudition and insight, if not to your sense of color, oh grand master of wisdom and blank verse! I think I do believe in you and Will, so the Sun must be golden indeed!



And hey, there are yellow turds, yellow bellies, yellow pie-slingers, and gold and sons mixed up in the whole thing, at least in your post! What more proof do I need that the Sun is a ball-shaped gold ingot in the sky that got so hot and bothered for some reason that it started giving off light and heat out of sheer embarrassment?



Ann

[Chris Peterson]

This is a "wrong-color picture" that is of great scientific interest:

http://www.nasa.gov/images/content/1508 ... _moons.jpg

Dwarf planet Pluto is shown as blue??? Even though Pluto is about as red as Mars?

But, okay. I understand why NASA chose to photograph Pluto in blue light.

Pluto was not imaged in blue light in this shot. Because the moons are so dim, the camera needed lots of light. So they used the F606W wideband filter of the ACS, which covers 480-700 nm.



The resulting B&W image was then displayed using a false color scheme that maps different intensities to different shades of blue- a standard mapping that allows the human eye to see a greater dynamic range than a simple gray scale would.

[Ann]

They didn't image Pluto in blue light? And yet they showed it as blue, even though Pluto is about as non-blue as a planetary body can get???

I don't get it. This is the kind of false color usage that I have no respect for, sorry.

It's like when they image the Sun in ultraviolet and show it as orange and red. Orange and red are the visible colors that are farthest away from the ultraviolet part of the spectrum, so why "translate" ultraviolet into red?

http://www.xe-media.ch/demoV3/administr ... escope.jpg

Ann

[Chris Peterson]

They didn't image Pluto in blue light? And yet they showed it as blue, even though Pluto is about as non-blue as a planetary body can get??? :facepalm:

I don't get it. This is the kind of false color usage that I have no respect for, sorry.

It was a black and white image, using a filter with a bandpass similar to ordinary B&W film. How would you have them present the image? Simply using a grayscale? That would be stupid, since the human eye can't resolve very many different intensity levels. By using a false color palette that maps intensity to color, they greatly increase the amount of detail the eye can resolve.

The choice of display palette was not based on aesthetics!

[Ann]

I think they should have shown the Pluto picture in orange, since Pluto is an orange-brown world. And while I don't know anything about the color of Nix and Hydra, chances are that the Earth and Neptune are the only blue planetary bodies of this solar system. Why do astronomers have to use the false color hue that is most unlike the true color of the object, or, in the case where the object is colorless, why do they have to choose the visual color that is furthest from the part of the spectrum that was isolated by the passband filter that was used when they made the picture?

Ann

[Chris Peterson]

I think they should have shown the Pluto picture in orange, since Pluto is an orange-brown world. And while I don't know anything about the color of Nix and Hydra, chances are that the Earth and Neptune are the only blue planetary bodies of this solar system. Why do astronomers have to use the false color hue that is most unlike the true color of the object, or, in the case where the object is colorless, why do they have to choose the visual color that is furthest from the part of the spectrum that was isolated by the passband filter that was used when they made the picture?

Because the blue false color palette has greater dynamic range to the eye than other palettes, such as red or heat. In this case, the blue palette used for display is fully within the passband of the filter, just as a red or orange one would have been.

[Ann]

Because the blue false color palette has greater dynamic range to the eye than other palettes, such as red or heat.

Let me offer an alternative explanation.

As a false color, orange is almost routinely used to show stars and other "hot objects". Therefore, astronomers may expect the general public to subconsciously associate orange objects with stars.

Now suppose that the Pluto image had been colored orange. Wouldn't the orange color make a lot of people assume that the picture showed a quadruple star system, or possibly a binary star system with two huge planets?

Blue isn't generally used to show stars. By coloring the Pluto image blue, NASA was delivering a subtle message to the people watching the image: Look, we have something to show you but the bright objects you can see in the picture are not stars!

As for dynamic range, I think orange will do as well as blue. Look at this image of the planet orbiting bright bluish star Fomalhaut:



This picture has been made using a blue and an infrared filter. The central star has been blotted out, but is shown here as a white dot. The rest of the image is shown as red, which, all things considered, must be false color. The planet, a red dot, doesn't show up too clearly, but NASA didn't want to show the disk around the star as blue to make the planet show up better. Personally I rather doubt that the planet would have been that much more obvious as a blue dot in a blue-colored disk.

But anyway, Fomalhaut is hot, and it is easy to think of the disk swirling around it as "kind of hot", so therefore NASA made the picture red, since red is a "hot" color. Well, it is hot to us humans, that is. But the realm of Pluto is cold, and blue is a "cold" color, or at least many people think of it like that. Since Pluto is far removed from the warmth of the Sun, NASA showed it as blue to signal to the general public that the picture didn't show any stars or any other "hot" objects. At least that is my interpretation.

Ann

[Chris Peterson]

As a false color, orange is almost routinely used to show stars and other "hot objects".

I don't think that is true. Astronomers select all sorts of different palettes for false color images, depending on which brings out detail the best.

As for dynamic range, I think orange will do as well as blue. Look at this image of the planet orbiting bright bluish star Fomalhaut...

That's a very different situation, since the image was made through two filters. So this isn't a simple intensity-to-color map, but a channel mapped image.

For the Pluto image, the processors picked a reasonable false color palette. Picking an orange one would have been no nearer to "true" color. This particular palette is a very commonly used one. The text released with the image is absolutely clear that the image was produced as B&W and then mapped to a color palette, and that it bears no relationship to what the objects would look like to the human eye.

[Ann]

One more post her, and then I'm done, I hope.

Chris, you said this about the Hubble images of NGC 3310 and M104:

The images you link, especially NGC 3310 and M104 are, in fact, carefully color balanced to reflect their true colors if our vision were more sensitive. I think you are mistaken about these being unrealistically blue or red.

Well, I can't believe that. In particular, there is no way I can believe that M104 would look like that to us if our color vision was more sensitive. The overall color of M104 is comparable to the color of K0 star Pollux. In fact, M104 is, if anything, a bit more yellow than Pollux, or at least it has a slightly yellower B-V index. Most of the light from this galaxy comes from stars that are yellower than the Sun. There is no way you can make me believe that a galaxy that is yellower than the Sun would look as bluish as M104 does in that Hubble picture.

Let me tell you about the very first time that I made an astronomy observation myself. I was fifteen, and I had read somewhere that you could actually observe the Andromeda Galaxy in a pair of binoculars, and the Andromeda Galaxy was a huge collection of millions or billions of stars. That sounded fantastic to me, and since my parents had a pair of binoculars I decided to use them, so that I could see this "city of stars". It took me a frustratingly long time to locate my target, and it cost me a very sore neck, but in the end I found it. And can you imagine what my first impression of the galaxy was? It was two things. It was that the galaxy was fuzzy-looking, absolutely not like the stars that were brilliant points of light. Also the galaxy was yellow. Yellow! Wow! You can't imagine how impressed I was. To me color is actually linked to a sense of reality: if something is colored it exists. And here was this fuzzy patch of light in the sky which was yellow. You can't imagine how real it became to me. Up until then I had regarded the sky as moderately boring to look at, even though I had learnt one year before that all stars are suns. That, too, almost knocked my socks off, because that piece of news suddenly expanded the size of the universe into something that I couldn't possibly imagine, but at least I could spend time thinking about this impossible vastness and ask myself this question: How far away would the Sun have to be to look as faint as the stars? That question changed my idea of the sky forever. But I still didn't think it was much fun to look at the stars. Back then I didn't realize that the stars were colored. I thought they were all little points of light of a uniform white color. Why would I want to look at them?

That is probably why I didn't realize that some of the stars in the vicinity of M31 are not only colored but blue, which is my favorite color. But it is my impression that the blue color of stars rarely shows up all that well in a pair of binoculars. You need a telescope to really see their color. Also I never really looked at the stars that I saw that first time I used binoculars to look at the sky, since I was so concentrated on trying to find the galaxy. But it could be that I saw the galaxy as so remarkably yellow because my eyes had "adapted to the blueness" of some of the nearby stars, particularly Nu Andromedae. Anyway, I'll never forget my first look at a galaxy, the amazingly yellow Andromeda Galaxy.



The Andromeda Galaxy. I only saw the center of it. And boy, was it yellow.

Ann

[neufer]

I'll never forget my first look at a galaxy, the amazingly yellow Andromeda Galaxy.

The Andromeda Galaxy. I only saw the center of it. And boy, was it yellow.

The Andromeda Galaxy is the biggest YELLOW-belly in the Local Group!
http://www.colbertnation.com/the-colber ... axy-is-big

[bystander]

Yellow

Look at the stars,
Look how they shine for you,
And everything you do,
Yeah they were all yellow,

-- Coldplay

[Ann]

Guess why I don't like planetariums? Because, at least in the few I have visited, they project the stars on that dome so that all the stars get the same artifical light color. All yellow.



Unsaturated yellow stars, all of them.

Ann

[swainy]

Yellow

Look at the stars,
Look how they shine for you,
And everything you do,
Yeah they were all yellow,

-- Coldplay

Here's a link.

http://www.youtube.com/watch?v=qI8I6qcxWyU

tc

[Beyond]

Sex days?

You got me there, neufer. I'm a Swede, and in Sweden "six" (6) is written and pronounced "sex". No kidding. And I was in a hurry when I wrote that post, so I had no time to check it too closely.

Ann

That being the case Ann, how do Swede's write and pronounce "sex?"

[makc]

how do Swede's write and pronounce "sex?"

I think it's "o ja" in german

[Ann]

How do we Swedes pronounce the number 6 ("sex" in Swedish)?

We pronounce it like you pronounce the activity.

Ann

[Beyond]

So Ann, what then is the "activity" of the number 6 ??

[Ann]

Would you believe it's the same word? It is. Same word, same spelling, same pronunciation. Different meaning. Ouch.

Well, you know, context is everything.

"Honey, how about some 7 tonight?"

"Don't you mean 6?"

"I don't mean the number. I mean the activity."

Ann

[neufer]

Would you believe it's the same word? It is.
Same word, same spelling, same pronunciation.
Different meaning. Ouch.

It looks pretty much
like 6 to me, Annita:

[Beyond]

What an amazing Alphabet and Numbering system the Swede's have. No wonder we do not hear anything about them and no wonder they take so many hot saunas in -20' or -30' weather and then go run around naked through the snow for a while. Verrrryyyyy interesting people those Swede's !! I almost forgot, i think they drink a lot of during the sauna to help sweat a lot.

[swainy]

What an amazing Alphabet and Numbering system the Swede's have. No wonder we do not hear anything about them and no wonder they take so many hot saunas in -20' or -30' weather and then go run around naked through the snow for a while. Verrrryyyyy interesting people those Swede's !! I almost forgot, i think they drink a lot of during the sauna to help sweat a lot.

Long live Stockholm. Blonds all the way, (for me). beautiful country. I could find a link, but I better not.

tc

[makc]

"Honey, how about some 7 tonight?"

Well, on a scale of one to ten... ten being the most depraved act of sexual theatre know to man... one being your average Friday night run-through at the Lomaxes' household... I'd say, not to be immodest, Mary Ann and I got it on at about... [counts on his fingers] ... seven.

[Helio George]

Ooh, fun topic... the Sun's color that is.

There are a number of things stated in this thread that may deserve a little more attention. I did not read every post carefully, so I am probably restating some things.

The Sun has a number of colors, so one must clarify whether its true color (as seen in space and at a proper attenuation level) or associate color, if placed next to another star, or its color seen when sunlight passes through a fixed amount of atmosphere, including dust and other particles.

The true color of the Sun is.... white.

For some fun with this and a fair amount of information....[the last is the best]
http://www.scientificblogging.com/solar ... of_the_sun
http://www.scientificblogging.com/solar ... un_part_ii
http://www.scientificblogging.com/solar ... revelation

A solid demonstration of this can be seen by simply doing a solar projection of a mid-day sun. You don't even need a telsecope, just a pinhole in a plate or paper allowing sunlight upon a white colored sheet of paper or whatever. But a bigger job of this was done a few years ago...

The last link above has an image taken by Dr. Potter of the Solar projection as it is seen at the McMath-Pierce telescope at Kitt Peak. [It's the world's largest.] He and a technician, Roy Lorenz, provided this for me and they were kind enough to add red, green and blue reference objects to help color calibrate the image, not that it needed calibration, but it is clearly not a b&w image. The telescope does not use any filters, so color alteration is not an issue.

Their image reveals no hint of yellow or any other single color found in the sunlight. Yet we do have to add whatever colors that were taken out (extinctions)by our atmosphere back into this white projection. But, as has been stated before, these colors are predominatly blue, but only a little bit is lost to the atmosphere. So what does our little math viewpoint give us? If we add these "blue" photons (mainly) to a white object what color, and only a little bit of blue, will we get a yellow star? Nope! Can we get a blue star? Well, that's highly unlikely.

It's really that simple, though other methods, if accurate, will give you the same result.

As for the Sun's peak energy wavelength, it is commonly misstated. It is not yellow or green or yellow-green, at least as seen from space (AM0). The peak wavelength can be found in the blue portion of the spectrum. I have seen the peak vary from about 450nm (almost violet) to 480nm (almost cyan). These peaks are more like a plateau so it is not surprising to see a tiny change from time to time, I think. [You can find numerous spectral irradiance graphs on the web, but look for AM0 ones. At least one has already be given in this thread.]

Our eyes, however, are better modelled using photon flux rather than watts/unit area per unit wavelength. But photons require more energy for shorter and shorter wavelengths (E = hf), so we end-up with an entirely different energy (i.e. photon flux) distribution. Here the peak is.... brace yourself... yellow! Yikes!

Well, if a yellow photon flux was all there was from the sun, it would always look yellow (ignoring evolutionary issues), but all the other wavelengths must be taken into consideration. Also, the spectral sensitivity of our eyes must alos be considered. It gets very complicated, but we already know the answer for the simpler work, so why bother.

Some simply argue that the Sun is white because it consists of all the colors, and white has all the colors. But this approach does not work since red giants and blue stars also have all the colors, too. Stars are close to being blackbody objects so, except for the chilly T-class stars, you will always find all the visible colors emitted.

One other issue, that applies to which star is the whitest, is the problem or advantage we have with color constancy. Our eyes & brains (retinex) take the brightest and whitest object seen as a color reference and converts everything else to help us maintain objects truer colors in the absence of sunlight. Look at your car's headlights in the daytime (preferably not the halogen ones) and then look at them at night. In the daytime, the Sun's direct sunlight color temperature is around 5800K, whereas your headlights have a color temperature closer to a wimpy 2000K. So the Sun is your retinex white color reference. But at night, your headlights are king and they will appear whiter than they do in the daytime.

So, if we see a star all by itself that is anywhere close to the temperature of the Sun, probably by several thousand degrees, we will see it as white due to color constancy. Red stars will appear orange partly for this reason, but more for the fact, as mentioned above, that the other colors are also present (blue to orange), which shifts a strong red end toward the blue. Some observers have told me that certain "red" stars do appear red to them, so now we can talk about the subjective issues of color vision. Or perhaps not...