Here is the situation. A planet fairly similar to Earth is orbiting binary stars, one O or B type and one F type. Sometimes the stars are close together in the sky, sometimes far apart, sometimes only one is visible, and sometimes one eclipses the other. The atmosphere is 56% nitrogen, 40% oxygen, and 3% sulfur. It is lethal to non-native life and the rain is acid.

The two stars orbit a central point, with the planet orbiting both of them in the Goldilocks zone.

It has three moons, of various sizes.

It does have life on it, but is closer to fantasy then sci-fi.

What color is the sky during daylight?

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    $\begingroup$ This might be on topic, but details on your configuration would help. What's the configuration? What orbits what? What are distances?.. $\endgroup$
    – Mołot
    Commented Apr 30, 2017 at 17:47
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    $\begingroup$ A lot of questions on here rely on physics knowledge but wouldn't necessarily be well received on physics SE. I think this belongs here but we need more information on how the planet orbits these stars to say what it is like at different times of day. $\endgroup$ Commented Apr 30, 2017 at 18:12
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    $\begingroup$ Arget, would you be willing to define these stars more closely? You say "blue" and "white", but those classifications are actually awfully broad. For example, the Sun is a yellow G-type (spectral class) main-sequence (type V) star, or a G2V type star. If you can tell us more about the spectral class of your two stars, that will help us give you good answers. Wikipedia has a table of sample star parameters with real-world examples which you may find useful. $\endgroup$
    – user
    Commented Apr 30, 2017 at 18:46
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    $\begingroup$ I'm voting to put this on hold for the moment as "too broad", but if you add some extra information about your solar system, it should be possible to take this question from one that is difficult to answer succintly to one that can be without too much trouble, given subject matter expertise. Don't be disappointed at the "on hold" status; that's merely to give us a chance to work out the quirks without risking invalidating answers. If you edit the question during the on hold grace period, it will automatically be entered into the review queues and hopefully be reopened quickly. $\endgroup$
    – user
    Commented Apr 30, 2017 at 18:50
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    $\begingroup$ Also, more details about your planet's atmosphere would probably be helpful. What is its actual chemical composition? Again, Wikipedia has a summary of Earth's atmosphere that may be helpful. $\endgroup$
    – user
    Commented Apr 30, 2017 at 18:56

1 Answer 1


First off, the system is young. O and B type stars don't last long.

To be in the goldilocks zone of this binary star, you need to be a long way out, O and B type stars are big and violent beasts. That makes for long 'years'.

The Rayleigh scattering is the physics behind the blue colour of air. Any otherwise colourless gas will be blue as a result of Rayleigh scattering. This will be the dominant factor in the colour of the sky. You will have a blue sky.

The O type star will then be a brilliant spot. You will need to be so far out that its disc is much smaller than our sun. The second sun is another dot near to it. The O type star is a lot larger than the F type, so their common barycentre is close to the O type. The white F star would be seen as orbiting the Blue one, but you would be a lot further out, so the two stars would be quite close in the sky, Perhaps as close as Mercury appears to the sun.

You mention Oxygen in the atmosphere. That implies something living (or weird) Oxygen is to reactive to remain in element form for long, unless there are something like plant to regenerate it.


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