Starship Asterisk*

Posted: **Sun May 30, 2010 3:30 pm**

It's a truism that our Sun is yellow. So why is daylight not yellow if daylight is sunlight?

Ann

Posted: **Sun May 30, 2010 3:42 pm**

Ann wrote:It's a truism that our Sun is yellow. So why is daylight not yellow if daylight is sunlight?

The solar spectrum is flat enough in the narrow visible band to simply call it white.
(Our eyes may contribute to this by being a little less sensitive to green light.)

Posted: **Sun May 30, 2010 4:22 pm**

Ann wrote:It's a truism that our Sun is yellow. So why is daylight not yellow if daylight is sunlight?

"Color" is a physiological phenomenon, not a physical one. The Sun is essentially white.

When sunlight passes through the atmosphere, shorter wavelengths are preferentially scattered. That shifts the color of the Sun itself (as seen from the bottom of the atmosphere) toward the longer end of the spectrum. This is why (with adequate attenuation) we tend to see the Sun as yellowish when it is high in the sky, and increasingly red as it sinks lower (and more of the short wavelengths are scattered by the increased atmospheric thickness). It is also why the sky is blue, since we are seeing the scattered short wavelength component of sunlight.

Daylight illumination is usually a combination of the direct sunlight and the scattered sky light, which sum up to the actual white color of the Sun itself. There are exceptions, however. Shade is shifted to the blue, since it is mostly lit by the scattered light and not the direct sunlight. Light shining in a window appears particularly warm (yellowish) because there is little inside illumination from the blue sky. Your eye compensates for these effects very effectively, but they are often visible on photographs (which is why most cameras have color temperature settings to adjust for different natural lighting conditions).

Posted: **Sun May 30, 2010 4:37 pm**

Thank you for your answer, and I agree. But do you think that the Sun ought to be defined as a yellow star? On what grounds should it be defined as yellow?

Vega is defined as the ultimate white star. But when I have looked at it through a telescope, it has looked definitely bluish to me. Mirfak, an F-type giant, is defined as yellow-white, but it looks white to me. Capella, a "typical yellow star", looks egg-white to me through a telescope. Pollux, an "orange star", looks very pale yellow to me. Arcturus, a "red giant", looks brilliantly yellow to me. Betelgeuse, an M-type supergiant, looks golden-colored to me, and Mu Cephei, the supposedly blood red "Garnet star", displays a rather pale copper color to me.

Are so called "red stars" really red? I don't think they are. Are brown dwarfs brown? I don't think they are. I think brown dwarfs are red, red dwarfs are orange-yellow, "yellow dwarfs" like the Sun are white, and "white dwarfs" are usually blue.

So can you expand a little on why stars are defined as having certain colors, according to the general definition of stars of various spectral classes?

Posted: **Sun May 30, 2010 4:56 pm**

http://en.wikipedia.org/wiki/Yellow wrote:
Pure primary rainbow colors lie along the outside curve of the following color chart.

Stars and other "blackbody" radiators lie along the thermal "Planckian locus."

The Sun's 6,000K comes pretty close to white.:

Complements of yellow have a dominant wavelength in the range 380 to 480 nm.
The green lines show several possible pairs of complementary colors with respect
to different blackbody color temperature neutrals, illustrated by the "Planckian locus".

Posted: **Sun May 30, 2010 11:59 pm**

Ann wrote:Thank you for your answer, and I agree. But do you think that the Sun ought to be defined as a yellow star? On what grounds should it be defined as yellow?

The stellar classification called "color" is only loosely related to the apparent color of a star to the eye. The Sun is a "yellow" star because that is the name given to the G spectral class. Stars in that temperature range have a peak wavelength around the part of the spectrum we call yellow to green. That does not mean they will appear yellow to us. So as the term is used, "yellow" is the correct terminology for the Sun, even though it is visually white.

Vega is defined as the ultimate white star. But when I have looked at it through a telescope, it has looked definitely bluish to me.

Vega is a class A star, which is described as "white" based on it spectrum. Class A stars are perceived on a scale from white to bluish white.

Mirfak, an F-type giant, is defined as yellow-white, but it looks white to me. Capella, a "typical yellow star", looks egg-white to me through a telescope. Pollux, an "orange star", looks very pale yellow to me. Arcturus, a "red giant", looks brilliantly yellow to me. Betelgeuse, an M-type supergiant, looks golden-colored to me, and Mu Cephei, the supposedly blood red "Garnet star", displays a rather pale copper color to me.

All normal. Mirfak is only a little hotter than the Sun- enough to put it into the next spectral class, but not enough to significantly shift its blackbody peak, and therefore not enough to make it look anything other than white. Similarly for the others- their spectral classification colors are generally different from their apparent colors.

Are so called "red stars" really red?

Yes, they are pretty red, or at least orange red. But most are so dim they don't trigger color vision, or do so only weakly. So they may not be perceived as particularly red. Most extreme are carbon stars, which are a deep ruby red. These stars are very impressive through a telescope; if you haven't tried observing one that way, do so. This is probably the richest, most saturated color you'll see in any astronomical object.

So can you expand a little on why stars are defined as having certain colors, according to the general definition of stars of various spectral classes?

Try this Wikipedia article as a starting point.

Posted: **Mon May 31, 2010 12:49 am**

Thank you for the graph, Art. I have seen it before, but I haven't really studied it, and it is very interesting. Stars emit light of many colors, so their light is never a pure primary color, except, I believe, the light of really cool "brown" dwarfs. They are really purely red.

My point is that the spectral classes of stars are ascribed the wrong colors. A-type stars, at least hotter A-type stars like Vega and Sirius, are not white but bluish. F-type stars are not yellow-white but white, and the hottest F type stars may even be described as slightly blue-white. G-type stars, at least hotter G-type stars like the Sun, are not yellow but white. K-type stars are not orange but a rather pale yellow, and M-type stars are not red but yellow-orange. The only truly red stars, as I said, are the brown dwarfs.

I can think of a couple of reasons for the confusion. First, human color vision works in such a way that our eyes are insensitive to color when illumination is faint. The rods in our eyes are color-blind, but they are, or so I think, particularly sensitive to short-wavelength (blue) light. So they are good at picking up faint blue light, but they will see this light as white. The cones are sensitive to color, but they need quite a lot of light to respond to color, and they are particularly bad at spotting faint blue and faint red light. They are a lot more sensitive to faint yellow and green light. (This, by they way, explains why our eyes are incapable of seeing emission nebulae as red. We detect, faintly, the geenish-blue emission lines of hydrogen beta or triply ionised oxygen in these nebulae instead, so that emission nebulae may look faintly green to us.)

Most bright-looking stars in the sky belong to spectral classes A or B, so that they emit most strongly in the blue part of the spectrum. But when we look at the stars with our naked eyes, the A- and B-type stars will look white to us, because of how our eyes work. Bright F-type stars like Canopus and Procyon will also look white, and even a bright G-type star like Capella will look relatively whitish. It is the K-type stars like Arcturus and the M-type stars like Betelgeuse and Antares that will really look non-white to our eyes. They will really look yellow to us, but since they are seen against a backdrop of white-looking stars, our eyes tend to exaggerate their yellow color. They are described as "red" and we think of them that way.

The second reason for the color confusion when it comes to stars is that Vega is defined as the ultimate white star. But if Vega is the definition of stellar whiteness, then only stars that are hotter than Vega can be described as blue. Indeed, only bright-looking stars that are hotter than Vega can be described as blue. Most white dwarfs are hotter than Vega and should be described as blue, but since they are faint they will look white even through a telescope, and so they are described as white even if they are every bit as hot as an O star.

A third reason for the confusion might be the claim that Vega must be white because its spectrum is "flat". Vega supposedly radiates the same amount of light in the "B" as in the "V" part of the spectrum, so that its B-V index is 0.0. Supposedly even its U-B index is also 0.0. Then every star whose B-V index is higher than 0.0 must be considered either yellow or red, and only stars whose B-V index is lower than 0.0 can be considered blue.

I wonder if Vega's spectrum really is so "flat", or if the B-V index that we used has simply been "calibrated" so that it defines the ligt of Vega as white.

Chris, thank you for your explanation. I'm not altogether happy with your reasoning, though. You said:

The Sun is a "yellow" star because that is the name given to the G spectral class.

Similarly, you said:

Vega is a class A star, which is described as "white" based on it spectrum.

To me this means that the Sun is yellow because it is defined as yellow. But the point I have been trying to make is that the Sun shouldn't be defined as yellow because it isn't yellow. And if Vega is described as white based on its spectrum, what exactly is it about is spectrum that makes it white?

Ann

Posted: **Mon May 31, 2010 1:27 am**

One more thing about the color of Vega. I think that people very often "see what they expect to see". If they are told by other people that stars are either white, yellow, orange or red, then those are the colors they are going to see when they look at the stars. That's why they may "see" Antares as red, for example, even if there is no way that Antares can truly be described as red.

But if you believe that stars are either, white, yellow, orange or red, then it might be interesting to try to find the whitest star in the sky. The whitest-looking star is then going to be the most non-yellow and non-red star in the sky. For people in the Northern Hemisphere, I think that the most non-yellow star is going to be Vega. Vega looks clearly bluer than Sirius, for example, because Sirius is often twinkling and giving off little sparks of different colors, and Vega definitely looks bluer than Rigel because it looks brighter than Rigel. But if you believe that stars can't be blue, and if you are looking for a star which can be defined as white because its light doesn't seem to contain even a trace of yellow or red light, the star you are most likely going to pick as brilliantly white is Vega.

So Vega may have become defined as the ultimate white star because it is the star that most definitely isn't yellow or red, and because astronomy before the advent of telescopes never defined stars as blue.

Ann

Posted: **Mon May 31, 2010 1:59 am**

Ann wrote:Chris, thank you for your explanation. I'm not altogether happy with your reasoning, though. You said:
The Sun is a "yellow" star because that is the name given to the G spectral class.
Similarly, you said:
Vega is a class A star, which is described as "white" based on it spectrum.
To me this means that the Sun is yellow because it is defined as yellow. But the point I have been trying to make is that the Sun shouldn't be defined as yellow because it isn't yellow. And if Vega is described as white based on its spectrum, what exactly is it about is spectrum that makes it white?

No, it is less arbitrary than your interpretation makes it seem. The problem is that "yellow" doesn't mean just one thing. It can refer to the human perception of a color, but the term is also used with an entirely different meaning, the color produced by a blackbody with a peak at a wavelength we would call yellow if we saw it isolated from the rest of the spectrum. Neither usage is right or wrong, they are simply different. Stellar "color" is largely based on temperature, and therefore references the blackbody characteristics, not the human perceptual characteristics.

And if Vega is described as white based on its spectrum, what exactly is it about is spectrum that makes it white?

Basically, it is hot enough that it has a monotonic portion of its blackbody output across the visual part of the spectrum; the peak is in the UV. Since "color" still has some reference to human visual response, "white" is a fair description. The highest intensity in the visual part of the spectrum is blue/violet, but the human response is low at those wavelengths, so the combination with the longer wavelengths makes the apparent color quite close to white, possibly with a bluish cast depending on individual color sensitivity.

Posted: **Mon May 31, 2010 2:22 am**

I found a spectrum of Vega which clearly suggests that the peak output of light from Vega is square in the middle of the blue part of the spectrum, near 480 nm:

http://zimmer.csufresno.edu/~fringwal/s ... s-plot.jpg

The spectrum of the Sun peaks in the yellow-green part of the spectrum, not in the blue part of it, and yet we see daylight (and therefore the light of the Sun) as white. So the fact that our eyes are insensitive to blue light doesn't mean that Vega should be regarded as white. If we could see it up close, or at least much closer than we do from our vantage point, we wouldn't see it as white at all, but instead it would be strikingly bluish.

Besides, I read once that an astronaut described what the Sun looked like when it was seen from above the atmosphere of the Earth. The astronaut described the Sun as "brilliantly white".

http://zebu.uoregon.edu/~imamura/122/im ... ectrum.png

The solar spectrum is much "yellower" than the spectrum of Vega, and yet sunlight is white.

Ann

Posted: **Mon May 31, 2010 2:24 am**

Ann wrote:Chris, thank you for your explanation. I'm not altogether happy with your reasoning, though. You said:
The Sun is a "yellow" star because that is the name given to the G spectral class.
Similarly, you said:
Vega is a class A star, which is described as "white" based on it spectrum.
To me this means that the Sun is yellow because it is defined as yellow. But the point I have been trying to make is that the Sun shouldn't be defined as yellow because it isn't yellow. And if Vega is described as white based on its spectrum, what exactly is it about is spectrum that makes it white?

Whether you are happy with Chris's statements or not, the fact remains the Sun is called yellow because that is the color assigned the G spectral class in the Harvard Classification system. It is not reasoning on Chris's part, it's a traditional color reference.

Chris referred you to the Wikipedia article Stellar classification. In that article, there is a table showing the Harvard spectral classification. In that classification, the conventional color is the color traditionally assigned to the spectral classes. The conventional colors are relative with Class A (Vega) being the White reference. Class G (Sun) is then referred to as Yellow.

Conventional and apparent colors

The conventional color descriptions are traditional in astronomy, and represent colors relative to the mean color of an A class star which is considered to be white. The apparent color descriptions is what the observer would see if trying to describe the stars under a dark sky without aid to the eye, or with binoculars.

Posted: **Mon May 31, 2010 2:43 am**

Yes, Bystander, I understand that. The point I have been trying to make the whole time is that the traditional Harvard color classification of spectral classes are really misclassifications. I was trying to see if I could get either of you to admit that.

The way I see it, there is no way that Vega can be "truly" classified as white, apart from the fact that that is its Harvard classification. But Vega looks blue to the eye, at least through a telescope, its spectrum peaks in the blue part of the spectrum, and its blackbody temperature is also "blue".

So Vega is a blue star, but we are required to refer to it as white because Harvard classified it as such in the late 19th or early 20th century. It's a little bit like saying that the Earth was created in sex days because the Bible says so, isn't it?

Ann

Posted: **Mon May 31, 2010 3:56 am**

Ann wrote:
Yes, Bystander, I understand that. The point I have been trying to make the whole time is that the traditional Harvard color classification of spectral classes are really misclassifications. I was trying to see if I could get either of you to admit that.

As an old MIT grad I can assure you that nothing that comes out of Harvard can be trusted.

Ann wrote:
The way I see it, there is no way that Vega can be "truly" classified as white, apart from the fact that that is its Harvard classification. But Vega looks blue to the eye, at least through a telescope, its spectrum peaks in the blue part of the spectrum, and its blackbody temperature is also "blue".

Code: Select all

Class 	  Temperature 	   Harvard "color"       Actual color
-------------------------------------------------------------------------
O 	     ≥ 33,000 K           blue 	               blue
B 	10,000–30,000 K       blue to blue white       blue white
A 	 7,500–10,000 K 	      white      	      white to blue white
F 	 6,000–7,500 K 	      yellowish white 	       white
G 	 5,200–6,000 K 	      yellow                yellowish white
K 	 3,700–5,200 K 	      orange 	             yellow orange
M 	≤ 3,700 K 	              red 	               orange red

Women just have no sense of color:

http://en.wikipedia.org/wiki/Williamina_Fleming
http://en.wikipedia.org/wiki/Antonia_Maury
http://en.wikipedia.org/wiki/Annie_Jump_Cannon

Ann wrote:
So Vega is a blue star, but we are required to refer to it as white because Harvard classified it as such in the late 19th or early 20th century.

It's a little bit like saying that the Earth was created in sex days because the Bible says so, isn't it?

Sex days?

Oh Be A Fine Girl Kiss Me (Right Now [Smack])

Posted: **Mon May 31, 2010 4:42 am**

Ann wrote:
Thank you for the graph, Art. I have seen it before, but I haven't really studied it, and it is very interesting. Stars emit light of many colors, so their light is never a pure primary color, except, I believe, the light of really cool "brown" dwarfs. They are really purely red.

Yes....
brown (color=#770000) actually is really pretty much
purely red (color=#FF0000) with the intensity turned down.

The "Planckian locus" in the above graph should actually progress
through a third vertical color dimension (not shown) of intensity.

Posted: **Mon May 31, 2010 4:48 am**

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

Posted: **Mon May 31, 2010 5:03 am**

brown (color=#770000) actually is really pretty much
purely red (color=#FF0000) with the intensity turned down.

But brown color can also be a purely yellow color with the intensity turned down.

http://upload.wikimedia.org/wikipedia/c ... wn.svg.png

Anyway, I have an old stove, and once I decided to find out if I could make my hottest hot plate glow red by turning it on at the highest setting. When I tried it in the daytime I saw nothing whatsoever. But when it was dark outside, and I turned off all the lamps in my home and closed the Venetian blinds to banish any light from outside from seeping in, my hot plate actually glowed a faint, dull red color when I had left it on the highest setting for about ten minutes. I can say this much, my hot plate didn't seem to glow brown to me.

Ann

Posted: **Mon May 31, 2010 5:16 am**

And I still can't accept the claim that the color of the Sun is "yellowish white". I think it's a pure white, and there is a very simple explanation for its color. It's because our eyes are naturally adapted to seeing the brightest source of light as white, and the brightest source of light on the Earth (apart from the occasional very nearby flash of lightning) must be the Sun. So we see it as white. Therefore, the Sun is white. Otherwise what do we mean by the color "white"?

The "Planckian locus" in the above graph should actually progress
through a third vertical color dimension (not shown) of intensity.

This sounds like a bit of mumbo jumbo to me. Is the Planckian locus a sort of magic wand that magically transforms our white Sun into a star that is really yellowish white, even though the daylight it shines on us is as "neutral-colored" as ever?

Ann

Posted: **Mon May 31, 2010 5:36 am**

Ann wrote:I found a spectrum of Vega which clearly suggests that the peak output of light from Vega is square in the middle of the blue part of the spectrum, near 480 nm:
http://zimmer.csufresno.edu/~fringwal/s ... s-plot.jpg

I don't know what that is, but it isn't the complete spectrum of Vega. I'd guess it is the spectrum generated by one particular instrument, and probably a ground-based one. Vega has a temperature of 9600 K, which means its blackbody peak is about 300 nm, well into the UV. Now Vega is an interesting case, since it has a dust disc that radiates IR as well as an atmosphere that absorbs some of the UV. I'm not sure how close its actual spectral peak is to the theoretical 300 nm; it is certainly very close to the shortest wavelengths seen by the eye, or shorter. But it doesn't really matter- the stellar classification is primarily based on temperature, and therefore on the theoretical blackbody peak.

The spectrum of the Sun peaks in the yellow-green part of the spectrum, not in the blue part of it, and yet we see daylight (and therefore the light of the Sun) as white. So the fact that our eyes are insensitive to blue light doesn't mean that Vega should be regarded as white. If we could see it up close, or at least much closer than we do from our vantage point, we wouldn't see it as white at all, but instead it would be strikingly bluish.

It would look no different up close than it does to us from here. It is described as spectrally white because its blackbody peak is shorter than the shortest visible light. It happens to appear quite close to white (which is not why it is called white, however) because its integrated spectrum produces a visual response we describe as white. There are many spectral curves that do so- such as that of the Sun. If you compare the visual appearance of the two, Vega will look slightly bluer than the Sun to most people, although the difference may be small.

Again, the important point is that stellar colors are a measure of the emission peak, and not of the apparent color to the eye. Visually the Sun is white; spectrally it is yellow. There is no contradiction here.

Posted: **Mon May 31, 2010 5:57 am**

Again, the important point is that stellar colors are a measure of the emission peak, and not of the apparent color to the eye. Visually the Sun is white; spectrally it is yellow. There is no contradiction here.

Are you sure that the Sun isn't green as defined from its spectral peak? I, at least, have heard that the Sun radiates most strongly in the green part of the spectrum.

Clearly the Sun can't be white if we have to ascribe to it a color from the spectrum of colors. Because "white" isn't part of the spectrum.

And then, of course, Vega most certainly can't be white, either.

But if the color of the Sun isn't the very definition of the color white, then I really don't know what "white" is.

Please note, by the way, that stars will look yellower when they are very far away and therefore faint than when they are nearby and much brighter. Because, as I said, our human color vision responds most strongly to yellow and green when illumination is faint. So if we could see the Sun as a distant and faint star in the sky, it might indeed look yellow-white. But that doesn't make it yellow-white in itself, at least not in my book. Mountains tend to look blue when they are far away here on the Earth, but that doesn't make them blue in themselves.

Finally, when I had access to a moderately large telescope, I spent almost all my time at that telescope checking out the colors of the brighter stars. Vega looked strikingly blue to me, while Capella, which is very slightly yellower than the Sun, looked egg-white. The difference in color between these stars was very pronounced.

Ann

Posted: **Mon May 31, 2010 1:47 pm**

Ann wrote:And I still can't accept the claim that the color of the Sun is "yellowish white". I think it's a pure white, and there is a very simple explanation for its color. It's because our eyes are naturally adapted to seeing the brightest source of light as white, and the brightest source of light on the Earth (apart from the occasional very nearby flash of lightning) must be the Sun. So we see it as white. Therefore, the Sun is white. Otherwise what do we mean by the color "white"?

Which is sort of what I was suggesting in my original post:

The solar spectrum is flat enough in the narrow visible band to simply call it white.
(Our eyes may contribute to this by being a little less sensitive to green light.)

However, (as Chris states) most of our illumination is from a combination of
a somewhat reddened 6000K "Planckian locus" sun PLUS the blue sky ,
so our eyes should really be naturally adapted to seeing this whole combination as white.

Ann wrote:
The "Planckian locus" in the above graph should actually progress
through a third vertical color dimension (not shown) of intensity.
This sounds like a bit of mumbo jumbo to me. Is the Planckian locus a sort of magic wand that magically transforms our white Sun into a star that is really yellowish white, even though the daylight it shines on us is as "neutral-colored" as ever?

Grey is a color which doesn't exist on the chart.

The RGB color model is really three dimensional and cannot be reduced to any two dimensional graph.

Posted: **Mon May 31, 2010 1:53 pm**

Ann wrote:
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.

Which all may be true...

On the other hand: you are a Swede.

Posted: **Mon May 31, 2010 2:23 pm**

Ann wrote:
brown (color=#770000) actually is really pretty much
purely red (color=#FF0000) with the intensity turned down.
But brown color can also be a purely yellow color with the intensity turned down.

Nevertheless, brown dwarfs are a Planckian mix of red, orange & yellow
"in combination with low luminance or saturation."

http://en.wikipedia.org/wiki/Brown wrote:
<<Brown is a color term, denoting a range of composite colors produced by a mixture of orange, red, rose, or yellow with black or gray. The adjective is applied to naturally occurring colors, referring to animal fur, human hair, human skin pigmentation (tans), partially charred or carbonized fiber as in toasted bread and other foods, peat, withered leaves, etc. In terms of the visible spectrum, "brown" refers to high wavelength (low frequency) hues, yellow, orange, or red, in combination with low luminance or saturation. Since brown may cover a wide range of the visible spectrum, composite adjectives are used such as red brown, yellowish brown, dark brown or light brown.>>

Ann wrote:
Anyway, I have an old stove, and once I decided to find out if I could make my hottest hot plate glow red by turning it on at the highest setting. When I tried it in the daytime I saw nothing whatsoever. But when it was dark outside, and I turned off all the lamps in my home and closed the Venetian blinds to banish any light from outside from seeping in, my hot plate actually glowed a faint, dull red color when I had left it on the highest setting for about ten minutes. I can say this much, my hot plate didn't seem to glow brown to me.

"Perceived color categories depend on what white they are compared to."
(And for goodness sake, Ann, buy yourself a new stove!)

http://en.wikipedia.org/wiki/Brown wrote:
<<The brown and orange disks of color are objectively identical,
in identical gray surrounds, in this image; their perceived color
categories depend on what white they are compared to.

Brown exists as a color perception only in the presence of a brighter color contrast:
yellow, orange, red, or rose objects are still perceived as such if the general
illumination level is low, despite reflecting the same amount of red or orange
light as a brown object would in normal lighting conditions.>>

Posted: **Mon May 31, 2010 2:29 pm**

Ann wrote:Vega is defined as the ultimate white star.
But when I have looked at it through a telescope, it has looked definitely bluish to me.

Note, however, that Vegans can only see GREEN.

Posted: **Mon May 31, 2010 2:49 pm**

Ann wrote:Are you sure that the Sun isn't green as defined from its spectral peak? I, at least, have heard that the Sun radiates most strongly in the green part of the spectrum.

The Sun's peak is in the green-yellow part of the spectrum assigned to G-class stars. Some G stars are more towards the yellow, some more towards the green. All are simply called "yellow" stars; there is no attempt made to assign some more precise color. The color name used for stellar classification is nothing more than a broad category that tells somebody generally what sort of temperature characteristics a star has. It isn't intended to do more than that.

Clearly the Sun can't be white if we have to ascribe to it a color from the spectrum of colors. Because "white" isn't part of the spectrum.

I don't understand. The Sun is visually white because the mix of wavelengths that make up its spectrum produces a visual response we call "white". Nothing more. Spectrally, it is a G-class star, which means it has a blackbody peak in the green-yellow part of the spectrum. Sometimes G stars are called yellow stars. In terms of visual appearance, the Sun is white. In terms of stellar classification, it is yellow. Again, I don't see the problem.

And then, of course, Vega most certainly can't be white, either.

Same situation. Visually, the range of wavelengths making up Vega's spectrum appear white or bluish white to the human eye. Spectrally, it has its blackbody peak outside the visible range, in the UV, and that is described as "white" by convention related to the fact that a very hot blackbody is perceived as completely unsaturated. Again, "white" isn't being used as any sort of rigorous definition of star color, but as a temperature.

Please note, by the way, that stars will look yellower when they are very far away and therefore faint than when they are nearby and much brighter.

Over a wide brightness range, the hue of a star will not appear to change. Eventually it will get so dim that the blue, and then the red cones will stop responding. So the last bit of color you can see as the star gets dimmer will be green, although it will be very unsaturated. Different people see different things at the transition between photopic and scotopic vision. In general, this isn't a factor in determining star colors. You can always view dim stars telescopically to get more light and see their true apparent color.

Finally, when I had access to a moderately large telescope, I spent almost all my time at that telescope checking out the colors of the brighter stars. Vega looked strikingly blue to me, while Capella, which is very slightly yellower than the Sun, looked egg-white. The difference in color between these stars was very pronounced.

Certainly, there is a wide variation in star color. Equally certain is that there is a wide range in human visual response and in perceptual effects. I generally see Vega as white and Capella as warm white. But the apparent color of a star also depends heavily on the colors of reference stars around it. Normal human vision will make almost any star appear white after a few minutes observation in an otherwise empty field. You can't trust your own vision to assign an accurate perceptual color to stars.

Posted: **Mon May 31, 2010 2:51 pm**

Well, hmmm. That "sex days" thing was a little funny, I'll grant you that.

On the other hand:

http://www.wikihow.com/Count-in-Swedish

As for the colorful graph you posted, it could be that I'm all wrong, but it sure looks to me that even the graph admits that a star whose temperature is 6,000 degrees has its peak light output in the green part of the spectrum. The same even appears to be true of stars whose temperature is more like 5,500 degrees Kelvin.

Any way, I have read an article perhaps in Sky & Telescope which tried to explain why we see no green stars in the sky. The magazine explained that there are green stars, that is, stars whose energy output peaks in the green part of the spectrum, but these stars are just like the Sun! So, ergo, the Sun's energy output also peaks in the green part of the spectrum, so the Sun is green!

I googled "green stars" and found the following article:

http://www.astronomycafe.net/qadir/q72.html

The article points out, correctly, that no stars look green because if they shine most strongly in the green part of the spectrum they look white to us.

However, the guy who wrote the article then goes on by saying that in order to peak in the green part of the spectrum the star ought to have a temperature of about 10,000 degrees. Is he crazy? Doesn't he realize that a star with that temperature will peak in the blue or even the near-ultraviolet part of the spectrum? He also said that 4000 Angstrom is in the green part of the spectrum. Doesn't he know that 4000 Angstrom is the same thing as 400 nm, and that that is in the blue part of the spectrum, near the ultraviolet?

Madness! But at least he is right about the fact that there are indeed green stars, stars which peak in the green part of the spectrum, but they look white to us - like the Sun!

Are there no green stars? Yes, actually, the Sun is one!

Finally, a few words about the color brown:

"Perceived color categories depend on what white they are compared to."

Yeah? You mean that the "brown-ness" of brown dwarfs depends on what shade of white makes up the blackness of space that these stars are seen against?

And why should I buy myself a new stove when I can make astronomical experiments with the one I've got?

This is admittedly a dollhouse stove. Its hotplates may not glow red.

Ann

Starship Asterisk*

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