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Let's say we have a binary star system, with a high mass star and a low mass, red dwarf star. High mass stars emit their light mainly in the ultraviolet while low mass stars emit mainly in the infrared. If a planet were orbiting the two stars and some happy aliens were living there, could the alien life evolve to see both ultraviolet and infrared light? Or are those wavelengths of light just too far apart on the light spectrum for eyes to evolve to see both?

It doesn't have to necessarily be a P type star system, it could maybe also be an S type star system with the planet orbiting just the red dwarf, but with the high mass star close by and shining on it. Something like this: enter image description here

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  • $\begingroup$ Our own Sun emits strongly in both UV and IR. The answers on Biology Se to the question How Can some animals see UV or IR light may help. $\endgroup$ – StephenG Jan 13 '18 at 5:56
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    $\begingroup$ All stars other than brown dwarfs emit lots of light in the visible spectrum, regardless of where their peak emissions are. A red dwarf, for example, has a similar spectrum to an incandescent light bulb, and isn’t very red at all. $\endgroup$ – Mike Scott Jan 13 '18 at 8:20
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    $\begingroup$ There is no way for a star to emit only ir or only uv. That said, if you have totally alien biology in totally physically impossible environment, everything is possible, or impossible, as you please. $\endgroup$ – Mołot Jan 13 '18 at 10:08
  • $\begingroup$ @Mołot That's not strictly true, though depending on where you draw the line between small stars and big planets. The Y-class brown dwarf WISE 1828+2650 emits essentially no visible light, only infra-red. And of course Jupiter, and to a lesser extent the other gas giants in our own solar system, emit infra-red but not visible light. $\endgroup$ – Mike Scott Jan 13 '18 at 15:09
  • $\begingroup$ You've got a big problem here: Figuring the star has a surface temperature of 10,000K it appears to be about an A0--with a lifespan of about 300My. Oops--your planet gets fried long before anything can evolve vision. $\endgroup$ – Loren Pechtel Jan 14 '18 at 4:51
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Stars don't emit a narrow band frequency of light

enter image description here

Stars are not lasers. Stars emit energy based on their surface temperature, in rough accordance with expected blackbody emissions from an object of that temperature. You can see on the graph above the emission magnitude (in log scale on the y-axis) for different wavelengths (x-axis). These wavelengths are a small pieces of the overall spectrum, but you can see how all stars emit at all frequencies.

Also note that the white and blue giant stars (in the 10000 K temperature range) emit more light at all wavelengths, often by orders of magnitude, than cooler F and G stars like our sun (with its surface temp of 5777 K).

In your example, a 'UV' star would in the 10000 K range, with lots of emitted energy in the 'UV' range as you can see on the graph. An 'IR' star would be cool M-type red dwarf like the 3000 K line, where peak radiance is in the IR range.

The important thing to note, is that if a planet received radiation from both stars, there would still be a great deal of visible light that the planet received. Both visible spectrum 'eyes' and photosynthesis would be viable on such a planet.

Its also important to note that the graph is on a log scale. There is no IR peak in combined magnitude; the hotter 10000 K star will be so much more powerful as to overwhelm the lesser star. There is no reason to specifically develop sensors for IR, since there will be more energy in the visible spectrum, and even more still in UV.

Overall, despite having two stars with different spectral peaks outside the visible range, you can still use the standard visible light range on your planet.

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  • $\begingroup$ Another factor to consider is your planet's atmosphere, and what wavelengths of light it allows through. For instance, Earth's atmosphere blocks a lot of IR wavelengths. $\endgroup$ – jamesqf Jan 14 '18 at 3:07
  • $\begingroup$ @jamesqf I think any atmosphere is going to block a lot of IR. $\endgroup$ – kingledion Jan 14 '18 at 3:33

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