Would a circumbinary planet have a differently colored sky depending on which star was visible?

Question

One of the things that affects the color of the sky is the wavelengths of light that the star emits, like the image in this question. Here is the image again:

If you are standing on a circumbinary planet with an earth-like atmosphere orbiting a binary star system containing one star from the left side of that image and one star from the right side, would the sky appear different colors depending on which star was currently visible? If not, what sort of atmospheric conditions would make it possible?

For the purposes of this question, I am counting different shades of the same hue as the same color, i.e. light blue and dark blue don't count as "different colors".

Answer 1

Yes, because:

Here on Earth you see a wide range of colours in the sky depending on time of day, cloud cover, dust etc. as the atmosphere absorbs and scatters different frequencies of the generally yellow light from our Sun.

If your planet orbits a very blue star you won't expect to see the reds you get here at sunrise and sunset.

If it orbits a very red star, you will never see a blue sky as you just won't get light at those wavelengths.

So:

Assuming a similar atmosphere to our own, when your red star is in the sky you will have an overall red sky, with the blue star in the sky you will have a blue sky. With both you will have variation.

The interesting times will be at or around sunset and sunrise as the light from one appears or disappears and is attenuated through atmosphere.

Answer 2

I agree with Rory's answer, particularly about the exaggerated variations at sunrise and sunset. I just want to add that if both suns set or rise at the same time, you won't see those variations. Both suns will set/rise at the same time daily only if the planet is orbiting close to 90 degrees off of the plane of the ecliptic, or the binary stars are orbiting each other 90 degrees off the plane of the ecliptic.

For a normal planet orbiting within the plane of the ecliptic, this will sort of happen at the poles: the suns will appear to drop below the horizon at the same time with the passage of seasons.

Answer 3

If you want to build a calendar to determine what the sky will look like throughout the year you'll need to know a few things.

1. The orbital periods of the stars to each other. This will determine the relative positions of each star in the sky.
2. The distance of your stars from each other as well as there size. If the distances between your stars are great enough than as they move around their size relative to each other will vary in relation to your planet.
3. The Incline of the orbit of your planet. what maters is the relative position to the orbital plane of your stars. The more out of sync it is with that plane the more vertical variation will be observed over the year.

One thing to keep in mind is the possibility of eclipses. If your orbital period lines up with the stellar plane this can happen all year round. If the planet orbits at an incline then it will have a two windows every year during which an eclipse can happen.

Depending on the relative size of each star partial eclipses will allow for the light of the further star to pass over the other and according to Einstein the light of the further star will be refracted inward.

Answer 4

Probably not much.

The colour of stars isn't that variable. A red giant is reddish white. A blue giant is bluish white. Even a red dwarf is a dark orange. Your eye has maximum sensitivity in the green.

In addition your eye adjusts to changes in illumination. That's why in a room lit by tungsten lighting you don't notice that the world looks like a photograph shot through a Holsum bread wrapper.

Tungsten light corresponds to about 2700 K which is cooler than a red dwarf Most people don't notice it, except that things that in daylight are a vibrant blue or green in tungsten lighting look a lot darker.

Take a look at a house plant. Your eye adjusts because your brain remembers what the houseplant is supposed to look like. You can have interesting discussions about fabric colours. If you haven't seen a clothes article in daylight, it's sometimes hard to say what colour it is lit by tungsten.

The blue of our present sky is mostly raleigh scattering. This is roughly linear with frequency, so higher frequncies get scattered more. The average is the blue we see.

A blue sky at 90 degrees from the sun has a colour temperature of 12,000. This corresponds to a stellar class B or A Yet if you look at something lit only by the northern sky, it looks more or less normal. (Artists like northern light because it's fairly constant over the day.)

A red star would mean less blue in the light so it would be shifted further away from the blue. However I suspect that it would mostly appear just to be darker.

If you get dust in the air you get a more pronounced difference. Dust being larger than air molecules scatter more, and also absorb more. See the copper coloured sun when you have forest fire upwind. But with dust the scattering is more uniform with frequency. The sky will be a smeared out version of the colour of the star.

You will also get interesting shadow effects. If both stars are in the sky, you will have two shadows. The shadow of the red star is illuminated by the blue one, and vice versa.

You can get a bit of the feel of this by going out on a sunny winter day. Compare where the sun hits the snow to a place where the snow is lit only by blue sky. The snow shadows are quite blue compared to the sunlit snow. This can be very pronounced in photographs.