If the Sun was red, rather than yellow, how would the color of the sky be affected?
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If the Sun was red, rather than yellow, how would the color of the sky be affected?
Order of Kilopi
My educated guess is something resembling the sky shortly after sunset or before sunrise, when the illumination of the sky is strongly reddened by atmospheric extinction. The blue tint becomes washed out compared with midday. That is assuming that by "red", you mean the color of an M star, which is a pastel orange.
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Yes, M, specifically red dwarf but I think any M would do.Originally Posted by Hornblower
The effect you are describing... specifically the darkening of the sky, is it because of blue/green light not being scattered by the atmosphere (like with our star) or is it more just the dimness of an M type?
If it is dimness, then if you were close enough to get near earth like climates wouldn't you be close enough for near sol levels of illumination?
Order of Kilopi
I intended for my previous post to be about the tint, not the brightness. My apology for not being clear about that.
We see blue in a clear sky because the blue end of the Sun's spectrum is scattered more strongly than the red end. If we substitute an M star with its greatly reduced blue component, I would expect the sky to look more nearly white, perhaps with a tinge of green. That is consistent with what I actually saw around the setting Sun tonight, when its strongly reddened light was the source of what was being scattered in the sky.
As for brightness, we must remember that an M star has a higher percentage of its radiant energy in the infrared. I would expect less visible luminance at a distance that warms us just as much as the Sun does. Considering how adaptable our eyes are to different light levels, we may not be very aware of the difference.
Order of Kilopi
The quirk that may need considering is whether it makes sense to assume that the colors we see will be the same colors seen by a "red" host star's planetary sky observers. The peak photon flux will be in the infrared, which might be a yellow, orange or red to the indigenous species to the planets orbiting the star, but that assumes I have a clue about evolutionary visual tracts, but I don't.
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This is perfect. Thank you.
Order of Kilopi
The more I think about it, the more I expect our sensation to be similar to that of our familiar blue sky. The key point is that our color vision has an enormous ability to adapt to the ambient light tint over a wide range of blackbody color temperature. When I take a color swatch that matches the blue sky when seen in daylight and then look at it under a 3000oK tungsten lamp at night, it still looks blue.
The appearance of the sky at sunset may be a poor model because of the difference in the geometry of the incoming light that is being scattered. It is coming in through a vastly larger amount of air than when the Sun is overhead.
Edited to add: The tint of the tungsten lamp is similar to that of an M star.
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Even better...
I'm working on a short story that takes place on a chilly little world around a small M star.
One of the most important parts of describing a scene (in my opinion) is the lighting. Now, I'm not afraid of taking a bit of creative license, but I like my sci-fi to be at least a little credible/possible if not probable/expected.
Now here is a bit of a curveball... how would the light from an M star tint a moon (without atmosphere) around this world?
Order of Kilopi
An M class star is even hotter than a tungsten filament in an incandescent light bulb, and these light bulbs, unlike many other types, are close to being blackbody radiators. So if a light bulb is yellowish-white, then the moon would be even less yellow under a higher temperature illumination. This ignores atmospheric effects that would make it look more yellow or even orange.
Last edited by George; 2012-Apr-06 at 04:04 PM.
Order of Kilopi
My guess is that our sensation of it would be similar to that of our own Moon, that is, more or less white. Once again we are dealing with pastel incandescent blackbody sources, for which our color vision is enormously adaptable. Our Moon has approximately the same color index as Arcturus, which looks pale orange to the unaided eye in a dark sky, but that tint washes out at the far brighter level of the Moon. Only when the color saturation is much higher do we see conspicuous tints in ambient light.
Order of Kilopi
One estimate of sky colour on other worlds I've seen showed that sky colour wasn't really affected on planets orbiting M-Class stars until you start to consider the very coolest and reddest stars. These tiny stars would be so dim that the planet would also be tidally locked; so you would have an (apparently) vast, cool, red star, motionless in a whitish sky.
Order of Kilopi
And no moons, since you can't have a moon around a tidally locked planet.
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I read a story in Astronomy some years back that speculated that the skies on M-dwarf planets would be black, because they wouldn't emit enough blue light for the atmosphere to scatter. I, however--along with some in this thread--am of the opinion that an M-dwarf planet's sky would look pretty much like ours; sure, the "sun" would be bigger, and I think the light might be noticeably ruddier, but on the whole it would be the same old blinding brightness and blue sky. I can only echo the reasons already stated:
1.) Bright light overwhelms our cones' color perception. As aforementioned, a tungsten filament is about as hot as an early M dwarf, and to the eye it looks white.
2.) Our eyes are more sensitive to blue light, so even if there's less to scatter we'll definitely see more of it. The caveat on this point, though, is how sensitive the eyes of creatures that evolved there would be to blue light; our sensitivity makes sense for this atmosphere and this Sun, but we can't assume that will hold for M-dwarf planets.
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You must take account that we adjust our idea of white to the general lighting around us and then judge colour accordingly. The yellow light of for example candle light allows full colour vision but also has a psychological effect seeming "warmer" while paradoxically blue tinged lighting from a higher temperature source psycholgically seems cooler while still allowing full colour vision. It also affects perception of relative brightness. If you light a picture with a task light and adjust the colour of the light towards yellow , such as by dimming a tungten lamp, the overall lighting effect improves in that more of the picture appears lit evenly. These psychological effects are presumed to be useful is the primitive environment for finding food and avoiding predators, thus expected in any evolution. So I think moons will seem white if they have general reflective properties whatever the main star light colour is.
Order of Kilopi
There are 2 factors here. First is if we went to a Earth like planet orbiting around a M class star. The second is if we evolved on a Earth like planet orbiting a M class star. The 2 outcomes are very different because our perception of light is different in those 2 situations. More then likely if we evolved around such a star we would probably see much more in the infra red.
The first question is easier to answer because there are less variables and has been already answered pretty well I see.
Order of Kilopi
My bold. Whoever wrote that must have been unaware that the atmosphere scatters light over the entire visible spectrum, not just in the blue end. It is enhanced in the blue end, giving us the familiar pastel blue. In addition, an M star still has lots of blue in comparison to the dull reddish glow of a stovetop burner.
Order of Kilopi
Yes, the approximate difference may seem like a lot, but it's not. Using a Planck distribution for, say, 3000K, reveals that the blue end is about 200 times weaker than the Sun's. But that is still only a loss of about 6 magnitudes from a -26.7 Sun.
For a low altitude sun, another blue boost comes from what is known as the Chappuis Effect, which shows that the very thin ozone layer greatly attenuates the mid range colors, so blue gets favor from both Rayleigh scattering and ozone passage. [Grant and I, however, have argued on this effect, but it is addressed nicely in Gotz Hoeppe's book on "Why the Sky is Blue".]
[Added: The net effect might be more of a bluish-white or possibly a light purple color sky for human eyes since the gain due to Rayleigh scattering is lost to the conversion to photon flux values and the red end is about 5.5x greater in energy (before conversion). The Chappuis Effect might be the dominant factor in what is seen during the sunset hour.]
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Ozone production would be much lower due to the cooler spectrum of light from the star, wouldn't it? So you might find this effect to be reduced as well. Is there an atmospheric physicist in the house? We need a UV flux -> Ozone level converter, stat!
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