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PlutonianEmpire
2012-May-21, 04:14 AM
A while back, I read a page on alien sky colors, and at the end of it, it mentioned that sky colors are influenced by what spectral type a sun is, as a result of said sun emitting light and radiation at different wavelengths, and so, around K and M suns, the sky would appear purplish or something while late F through O would appear violet-blue.

Is there any truth to this, or should I disregard it, considering I found the page on a sci-fi oriented website?

Swift
2012-May-21, 04:20 AM
A discussion from early April on this topic. (http://www.bautforum.com/showthread.php/130306-alien-sky)

I also recall an article in Scientific American on this topic within the last 3 or 4 years, IIRC, though I couldn't quickly google a link.

PlutonianEmpire
2012-May-21, 04:34 AM
A discussion from early April on this topic. (http://www.bautforum.com/showthread.php/130306-alien-sky)

I also recall an article in Scientific American on this topic within the last 3 or 4 years, IIRC, though I couldn't quickly google a link.
Interesting read; thanks. :)

m74z00219
2012-May-21, 04:35 AM
A while back, I read a page on alien sky colors, and at the end of it, it mentioned that sky colors are influenced by what spectral type a sun is, as a result of said sun emitting light and radiation at different wavelengths, and so, around K and M suns, the sky would appear purplish or something while late F through O would appear violet-blue.

Is there any truth to this, or should I disregard it, considering I found the page on a sci-fi oriented website?


Hi there, here is the article that Swift was thinking of.

http://pubs.giss.nasa.gov/abs/ki00100w.html


M74

Ufonaut99
2012-May-21, 07:37 AM
I also recall an article in Scientific American on this topic within the last 3 or 4 years, IIRC, though I couldn't quickly google a link.
Was it this one : The shocking colors of alien plants (http://www.scientificamerican.com/article.cfm?id=the-shocking-colors-of-alien-plants)

Swift
2012-May-21, 12:58 PM
Yes (m74z00219) and yes (RobA) and I guess my memory is faulty and those weren't exactly on topic.

eburacum45
2012-May-21, 07:03 PM
As it happens I co-wrote this page, which may be the one you were thinking of
http://www.orionsarm.com/xcms.php?r=oa-page&page=gen_skyonalienworlds

the section you are referring to is probably this one

Stellar Type and Kinds of Sunlight
Changing the illumination changes the sky colour yet again. Though a type G star like the sun has a peak in the green/blue range, a type M star has a peak in red, K peaks in orange, F stars are brightest in the blue, and A and higher are brightest in the ultraviolet range. Because K and M stars provide less blue and green light for scattering, the most likely result for earth-like planets circling them may be a purplish-black sky, given an earth-like gravity and one bar of atmospheric pressure. At higher atmospheric pressures, since the blue component in the light is not so strong, then the other colours of the rainbow (green to yellow to orange or red) show up more, and under these conditions the colour balance might simply make the sky look white. It is likely that brighter G and early type F type stars would not change the sky colour significantly. The rare world (almost certainly terraformed, since garden worlds would not have time to evolve) around spectral types from late F to O and B might have violet-blue skies. The shorter wavelengths would likely be so abundant that the human eye's bias towards blue and green could be overcome.
...and I must admit I'm not sure I currently agree with it. The sky on an Earth-like planet orbiting a K- or M- type star would probably be blue, even at Earth-like atmospheric pressures; the section about a purplish black sky is probably wrong. If anyone has any good data about this I would be grateful; although we found quite a lot of data pertaining to the effect of atmospheric pressure on sky colour, we couldn't find much about the effects of star temperature.

Hornblower
2012-May-21, 10:48 PM
As it happens I co-wrote this page, which may be the one you were thinking of
http://www.orionsarm.com/xcms.php?r=oa-page&page=gen_skyonalienworlds

the section you are referring to is probably this one

...and I must admit I'm not sure I currently agree with it. The sky on an Earth-like planet orbiting a K- or M- type star would probably be blue, even at Earth-like atmospheric pressures; the section about a purplish black sky is probably wrong. If anyone has any good data about this I would be grateful; although we found quite a lot of data pertaining to the effect of atmospheric pressure on sky colour, we couldn't find much about the effects of star temperature.

I strenuously disagree with it. The author appears to grossly misunderstand the characteristics of the source and the scattering in the atmosphere, along with the functioning of our color vision. Let's start with the source. The blue component in an M star's light is greatly reduced in comparison to the Sun, but it is by no means absent. In addition our color vision system has a phenomenal ability to adapt to the tint of this type of light over a wide range of temperature and perceive it as white if it is the ambient light source. The tint of an M star is similar to that of an ordinary incandescent lamp. My best information on this is what I see when looking at a defocused star image in a telescope while looking at a distant lamp with the other eye. They are a pretty good match.

If the composition of the atmosphere is unchanged (a big if, I realize), it will scatter whatever blue light is present from an M star just as effectively as with sunlight. It will scatter the red, orange, yellow and green components likewise, just somewhat less strongly than the blue. Thus the tint of the scattered light should be similar to what we get with reflected light off a sky-blue piece of paper. I have such a piece of paper, a color print of the sky, hanging on the wall as I write this. It looks just as blue to my indoors-adapted vision under an incandescent lamp as it does to my daylight-adapted vision in sunlight.

The author must have been assuming that the blue light is vanishingly weak in an M star's light, and that the atmosphere does not scatter an appreciable amount of the red to yellow light. Such is not the case either way.

If there is any major difference in the sky tint on our hypothetical planet, as percieved by our color vision, it would have to be due primarily to differences in the composition of the atmosphere.

PlutonianEmpire
2012-May-22, 01:22 AM
As it happens I co-wrote this page, which may be the one you were thinking of
http://www.orionsarm.com/xcms.php?r=oa-page&page=gen_skyonalienworlds

the section you are referring to is probably this one

...and I must admit I'm not sure I currently agree with it. The sky on an Earth-like planet orbiting a K- or M- type star would probably be blue, even at Earth-like atmospheric pressures; the section about a purplish black sky is probably wrong. If anyone has any good data about this I would be grateful; although we found quite a lot of data pertaining to the effect of atmospheric pressure on sky colour, we couldn't find much about the effects of star temperature.
Actually, yes, that was the page I mentioned. I didn't say anything more because I didn't know how people would react. :o


I strenuously disagree with it. The author appears to grossly misunderstand the characteristics of the source and the scattering in the atmosphere, along with the functioning of our color vision. Let's start with the source. The blue component in an M star's light is greatly reduced in comparison to the Sun, but it is by no means absent. In addition our color vision system has a phenomenal ability to adapt to the tint of this type of light over a wide range of temperature and perceive it as white if it is the ambient light source. The tint of an M star is similar to that of an ordinary incandescent lamp. My best information on this is what I see when looking at a defocused star image in a telescope while looking at a distant lamp with the other eye. They are a pretty good match.

If the composition of the atmosphere is unchanged (a big if, I realize), it will scatter whatever blue light is present from an M star just as effectively as with sunlight. It will scatter the red, orange, yellow and green components likewise, just somewhat less strongly than the blue. Thus the tint of the scattered light should be similar to what we get with reflected light off a sky-blue piece of paper. I have such a piece of paper, a color print of the sky, hanging on the wall as I write this. It looks just as blue to my indoors-adapted vision under an incandescent lamp as it does to my daylight-adapted vision in sunlight.

The author must have been assuming that the blue light is vanishingly weak in an M star's light, and that the atmosphere does not scatter an appreciable amount of the red to yellow light. Such is not the case either way.

If there is any major difference in the sky tint on our hypothetical planet, as percieved by our color vision, it would have to be due primarily to differences in the composition of the atmosphere.
Does that include sunsets? (I know the other thread touched on this...)

John Xenir
2012-May-22, 01:22 AM
I wonder if there exist an application, which would roughly based on several parameters (star, distance, air pressure, gravity, maybe some particles too), display a color scheme of the sky of a planet.

eburacum45
2012-May-22, 04:00 AM
Thanks! I'll amend the page, after contacting the other author. The only exception to the 'blue sky' perception might be planets orbiting very cool stars or brown dwarfs, where the light is very red indeed; a number of people on this forum and elsewhere have suggested that the sky on a world orbiting a very red, cool star would have an even spread of scattered light, which would look white; I'm a little unsure about this.

As John Xenir suggests, this is the sort of phenomenon which could probably be simulated quite effectively.

PlutonianEmpire
2012-Jun-03, 05:03 AM
Hey, sorry for bumping, but I was wondering, would the sky remain blue even for significantly hotter stars, from F to O? (Granted, such worlds would be terraformed, assuming it was even possible.)

chornedsnorkack
2012-Jun-03, 11:41 AM
Black body spectrum can never get bluer than the Rayleigh-Jeans distribution, for high temperature limit.

How blue is Rayleigh scattered Rayleigh-Jeans light? Is it still blue, or getting violetish?

Hornblower
2012-Jun-03, 01:26 PM
Black body spectrum can never get bluer than the Rayleigh-Jeans distribution, for high temperature limit.

How blue is Rayleigh scattered Rayleigh-Jeans light? Is it still blue, or getting violetish?

I would expect a somewhat more saturated but otherwise similar pale blue. In other words, a sky somewhat bluer than the source, but still pastel.

George
2012-Jun-04, 02:34 AM
I too think it would be a bit richer (more saturated) in blue.

The ratio of blue photons to red increases with higher temperatures. A 10,000 K star, compared to the Sun, would produce about twice as many "blue" photons relative to "red" photons.

eburacum45
2012-Jun-06, 07:31 PM
Here's part of the reply I got from Stephen Inniss, who co-wrote that page with me a few years ago;
-------------------------------------
Hi Steve,

Thanks for this; I do believe the sky colours article is due for an overhaul. Here's a very thorough theoretical treatment:
http://www.xenology.info/Xeno/5.4.2.htm

-----------------snip---------------
and my reply;

Thanks! I'd forgotten about that page.
This bit is interesting;

Net Sky Color: F0 vivid blue
G0 powdered blue
K0 light blue
M0 pale-whitish blue

I'll add this to the next rewrite.

eburacum45
2012-Jun-06, 07:33 PM
I also have a set of interesting comments by George (of this forum) which we shall also address in the next rewrite.

eburacum45
2012-Jul-01, 06:48 AM
Sorry to resurrect this old thread, but here is an updated version of our page about sky colour;
The Sky on Alien Worlds (http://www.orionsarm.com/page/321)
mostly updated to include Robert Freitas' estimates of the blue fraction of skylight under various common star types.
Bear in mind this is written at least partly from an in-universe viewpoint for the Orion's Arm scenario.

PlutonianEmpire
2012-Jul-01, 06:57 AM
No offense, but.....

Yet another case where reality is FAR less exciting than science fiction. :(

Haters gonna hate.

eburacum45
2012-Jul-01, 09:14 AM
I think that the palette of colours afforded by Rayleigh and Mie scattering is quite exciting enough, especially when augmented by coloured gases and particulates. The only colour I can't imagine ever occuring in the sky of a real exoplanet is that curious tomato-soup red that used to be common on Star Trek TOS.
http://216.172.180.111/~jja/wp-content/uploads/2011/07/kirk_and_landing_party_in_tombstone.jpg
And even that colour might occur at, or near sunset or sunrise on some worlds.

chornedsnorkack
2012-Jul-01, 09:41 AM
What could the colour of direct sunlight be for the said tomato-soup red?

True, it is said that the light of most red dwarfs would actually look relatively whitish yellow for an adapted eye, like the light of an incandescent light, because the black body spectrum does have a Wien tail into yellow.

But the carbon stars are notoriously bright red - they do not have black body spectrum because although they are not so cold, the Swan bands of dicarbon absorb the yellow tail.

Are there any red dwarfs which already on main sequence are rich in carbon?

eburacum45
2012-Jul-01, 09:56 AM
Sunset on a world orbiting a carbon star would probably be the reddest sky one would ever be likely to see. That is an intriguing idea.

George
2012-Jul-02, 01:25 AM
Sorry to resurrect this old thread, but here is an updated version of our page about sky colour;
The Sky on Alien Worlds (http://www.orionsarm.com/page/321)
mostly updated to include Robert Freitas' estimates of the blue fraction of skylight under various common star types.
Bear in mind this is written at least partly from an in-universe viewpoint for the Orion's Arm scenario.Those look nice, though a Mars-like atmosphere is tricky since particle sizes allow selective scattering.

The zenith colors will likely be very dependent on ozone levels. [see Chappuis effect]

eburacum45
2012-Jul-02, 08:20 PM
That is interesting;

on an artificially terraformed planet orbiting an F-type star (or hotter) the atmosphere would hopefully hold a respectable ozone fraction, to protect tender human skin and eye tissue; that would make the sky even bluer, and increase the redness of sunsets.

George
2012-Jul-02, 09:14 PM
That is interesting;

on an artificially terraformed planet orbiting an F-type star (or hotter) the atmosphere would hopefully hold a respectable ozone fraction, to protect tender human skin and eye tissue; that would make the sky even bluer, and increase the redness of sunsets. Or perhaps purple in some cases, since both ends of the spectrum are much less attenuated. It does take a fair amount of atmosphere for the effect to work since the ozone layer is so very thin. Our ozone layer is only 300 Dobson units, which means at one atmosphere pressure it would be only 3mm thick if all the ozone was squeezed into one layer.

Grant and I had a few rounds with each other on this effect because he was dubious of its true net effect. I don't mind stating that he is smarter at than I am. Too bad he's wrong. ;)