APOD: Conjunction Colours (2012 Aug 23)

[Helio George]

I lost a number of comments I made, somehow, because I failed to login.

The picture that Philip Hart posted in the discussion thread, where he had applied no increase in contrast, saturation and vibrance, makes the color of Spica to appear very "normal-blue-sky"-like to me.

Fortunately, Philip noted that he slightly enhanced the image. Perhaps stars can become as blue as he depicts, but even this is still a desaturated blue, but stars simply do not get hot enough to produce a deep blue, as can be found in the deeper blue skies seen, perhaps, from mountain tops or on especially clear and calm days. Ironically, white dwarfs -- not stars by definition --might be hot enough to appear a deeper blue than what is shown in the APOD imaage, though they fade to white and, eventually, become dark.

However, the surface brightness of hot stars is so very much higher than the surface brightness of cool stars. A high surface brightness will burn out the impression of color in the human eye. Personally I believe that the color of hot stars gets "burnt out" to the human eye much more easily than the color of cool stars.

I agree that at some point as we approach a distant star its light flux will max-out all our color cones so that any one color could not be distinguished from the other colors; white is all that would be seen. This is true of the Sun, though some side effects besides atmospheric extinctions can cause some to see the Sun as yellowish-white.

But as Chris pointed out, the human retina contains far fewer blue-sensitive rods than red- and green-sensitive rods.

Our color cones -- rods refer to the non-color receptors -- are only part of the overall color processing equation. Remarkably, this processing compensates for the small percentage of blue color cones. The receptivity charts of the retinex (eye-brain per Dr. Land) reveal that blue is weaker than the other colors, but not significantly so.

This is an important reason why it is hard for us to see blue color in stars. The light that reaches us from the stars is faint and concentrated (the light seems to emanate from a single point in the sky), and the blue-sensitive cones are few and located away from the macula of the retina, again making it harder to spot the blue color of hot stars.

This may be tested by using averted vision on the really hot stars like Rigel, or perhaps Spica,too.

I don't expect, however, that there will be a lot of difference. [Shill time] Last march, our astronomy club flashed on and off two 800 watt search lights, along with my 1 watt blue laser (probably about 1/2 watt contrary to its rating) at the ISS where Don Pettit and Dan Burbank observed us from the cupola. They saw the tiny blue light as blue, especially when the search lights were blocked. [This was historic since, supposedly, no one had ever accomplished a light signal to any space venturer in the history of mankind.]

P.S. By the way, speaking of the color of the Sun, I am very well aware that it is intrinsically white. However, seen through the Earth's atmosphere, it gets reddened to a slightly yellowish color.

Yep, and I enjoyed those discussions in the yellow sun thread here.

[Chris Peterson]

But as Chris pointed out, the human retina contains far fewer blue-sensitive rods than red- and green-sensitive rods.

Our color cones -- rods refer to the non-color receptors -- are only part of the overall color processing equation. Remarkably, this processing compensates for the small percentage of blue color cones. The receptivity charts of the retinex (eye-brain per Dr. Land) reveal that blue is weaker than the other colors, but not significantly so.

When viewing large areas of color, our eyes and brain compensate. But the system is easily fooled, and one way to do this is with point sources. When viewing stars, it is possible to change the S/M/L ratio just by moving the eye a little... with the S sensors being the major contributor to the effect, because of their low density. This results in significant color shifts when the point source contains short wavelengths.

[Ann]

However, when you discuss the color of the sky, I see a great problem here - the exact color of the blue sky isn't fixed. It changes. Due to the presence or absence of moisture in the sky, it changes. Due to the height of the Sun above the horizon, it changes. Due to the height above the horizon of the sky itself, the color changes. If we are going to have a serious discussion about the difference or similarity between the color of the sky and the color of Spica, we first have to define what exactly we mean by "blue sky".

Right. I think when we want to be more techinical about the sky, we look for regions that have the greatest purity, which is typically overhead, except when the sun is near the horizon. The spectrum for the blue sky is the information needed to make comparisons.

Should it be defined as something that can only be seen at 60 degrees or more above the horizon? Should it be defined as something that can only be seen at a certain elevation above sea level? Should it be defined in such a way that people living near the coast, with permanent moisture in their ambient air, can never see this color? Should it be defined as something you can best see in, say, the Nevada desert when the air is perfectly still and there is no wind to stir up tiny grains of sand?

You are certainly correct to note the variations of blue in the sky; they are too varaible to merit much definition. Nevertheless, overhead spectral irradiance measurements, where the blue has its greatest saturation, may be useful, somewhat. The sky is blue because it is the only color of the spectrum that matches the sky, and we don't seem to like to say cyan (for the more bluish-white regions, which can be the whole sky at times).

I think you and I look at this differently. When I say that a star is as blue as the sky, I don't mean a maximum-purity blue sky. I agree that stars don't get as blue as a maximum-purity blue sky. (Even though I once observed Lambda Orionis, an O8 star, through a 14-inch telescope, and got the most incredible impression of blue color.)

Rather, when I say that a star is as blue as the blue sky, I mean a color that people would identify as "normal" for the blue sky, whatever that means. I posted a picture earlier in this thread, where the blue sky is very desaturated. Nevertheless, practically everyone would identify it as "blue sky".

I rarely use the word cyan myself. To me, it is not clear where the color blue ends and cyan begins. I do know that very obvious cyan is not a very beautiful color to me.

I wouldn't describe the sky in the picture I posted as cyan. And while I have spent a lot of time observing hot stars through a telescope to assess their colors, I have never come across one that looked cyan to me.

This is a color that wikimedia describes as cyan. I have never seen the sky or a star look like this. (I may add, completely unscientifically, that I find this color rather astoundingly ugly.)

However, this color is also described as cyan. The sky can look like this. However, the color is too saturated and greenish to look like the color of a hot star.

Ann

[Ann]

Helio George wrote:
Last march, our astronomy club flashed on and off two 800 watt search lights, along with my 1 watt blue laser (probably about 1/2 watt contrary to its rating) at the ISS where Don Pettit and Dan Burbank observed us from the cupola. They saw the tiny blue light as blue, especially when the search lights were blocked. [This was historic since, supposedly, no one had ever accomplished a light signal to any space venturer in the history of mankind.]

That's really fascinating. Congratulations for being part of this historic experiment.

I totally agree with you that hot stars are not like blue lasers. A blue laser should, ideally, emit blue light only. It should not be mixed with any other photons. A hot star can never be like that.

The point I'm trying to make is not that our eyes can't detect faint blue light. Rather, I've tried to say that faint point sources of desaturated blue light, mixed with other colors, will probably not look particularly blue to us. The red- and green-sensitive cones in our eyes will react sufficiently strongly to the green and red photons that are mixed with the blue light that their response will mostly drown out the response of the blue-sensitive cones to the predominantly blue but faint light. Also, our color-blind rods will respond more strongly to blue than red light, triggering a moderately strong sense of "white light" when we observe predominantly blue light from stars.

Ann

[Case]

I rarely use the word cyan myself. To me, it is not clear where the color blue ends and cyan begins. I do know that very obvious cyan is not a very beautiful color to me.
I wouldn't describe the sky in the picture I posted as cyan. And while I have spent a lot of time observing hot stars through a telescope to assess their colors, I have never come across one that looked cyan to me.
This is a color that wikimedia describes as cyan. I have never seen the sky or a star look like this. (I may add, completely unscientifically, that I find this color rather astoundingly ugly.)
However, this color is also described as cyan. The sky can look like this. However, the color is too saturated and greenish to look like the color of a hot star.

With a background in graphic design, my main reference for a color named ‘cyan’ is the printing color cyan (as used in professional offset printing (which is very similar to what your home inkjet printer or your office color laser copier uses)). That cyan should look something like the color below on the left, which is quite close to the color depicted in Ann’s last link.

[Ann]

Thanks, Case!

To me, that color is 100% blue. I wouldn't dream of calling it anything else.

http://en.wikipedia.org/wiki/Cyan wrote that there is a so-called "subtractive primary cyan color" which looks just like the color you showed in your post. But basically all the other colors shown by that wikipedia page are much greener than that.

There is a shade of cyan that wikipedia called an "additive secondary" color. This cyan color is defined, in RGB, as 0, 255, 255. So it is just as green as it is blue.

The subtractive primary shade of cyan is defined, in RGB, as 0, 183 and 235. So it is considerably more blue than green.

Ann

[Chris Peterson]

To me, that color is 100% blue. I wouldn't dream of calling it anything else.

http://en.wikipedia.org/wiki/Cyan wrote that there is a so-called "subtractive primary cyan color" which looks just like the color you showed in your post. But basically all the other colors shown by that wikipedia page are much greener than that.

There is a shade of cyan that wikipedia called an "additive secondary" color. This cyan color is defined, in RGB, as 0, 255, 255. So it is just as green as it is blue.

The subtractive primary shade of cyan is defined, in RGB, as 0, 183 and 235. So it is considerably more blue than green.

In the most common usage these days, RGB levels define colors as seen by cameras and displayed on monitors (which use RGB primaries), and CMY define colors that are printed with inks. These two schemes are related: cyan is what you get when you remove red from white, magenta is what you get when you remove green from white, and yellow is what you get when you remove blue from white. "White" in this case is equal levels of red, green, and blue. This show nicely how it works:

colors.jpg (12.81 KiB) Viewed 2230 times

Yellow is the sum of red and green (i.e. no blue), magenta is the sum of red and blue (no green), and cyan is the sum of green and blue (no red). The RGB system is additive (we add together primaries to make new colors) and the CMY system is subtractive (inks act as filters, blocking certain colors).