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Suppose you're observing a red dwarf star at Noon from the surface of an Earth-like planet orbiting in the red dwarf's habitable zone, so that the red dwarf's luminosity is equal to our own Sun.

Aside from the red dwarf's greater perceived size in the sky, would its color and brightness closely resemble our own Sun at sunset in Earth's sky on a clear, dust and smoke-free evening?

Is it fair to say that since the Sun's color and brightness changes as it dips closer to the horizon due to scattering of blue light, there could conceivably be a close color/brightness match to any G, K, or M class star at some point along the Sun's trajectory in the sky?

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    $\begingroup$ The following video might be interesting. While it doesn't show the stars at habitable distance (but always at 1 au) you may like it. youtube.com/watch?v=7WHi398VG0I&ab_channel=GALAXIANHD $\endgroup$
    – Greenhorn
    Commented Dec 1, 2020 at 10:47
  • $\begingroup$ @Greenhorn Thanks, very interesting! $\endgroup$
    – RobertF
    Commented Dec 1, 2020 at 16:05

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Your question may ulitmately be about the physiology of the eye, which is off-topic here.

The spectrum of the Sun seen low on the horizon is quite different to the spectrum of an M-type red dwarf. The reason that a red dwarf is red, is not just that it is cool, but that there are great chunks of the spectrum that are absorbed by molecules in the photosphere of the star. These molecules start to form when the temperature of the photosphere falls below 4000 K.

In contrast, the Sun has an almost continuous spectrum all the way into the ultraviolet, with relatively narrow absorption lines caused by atomic species in the solar photosphere. This is then modified, mainly, by Rayleigh scattering, which smoothly scatters blue light out of the line of sight towards the Sun.

Whilst any sort of spectrograph or prism could distinguish these differences, whether your eyes could is another matter. So whilst it is conceivable that the colours would appear similar for some particular angle above the horizon, the detailed spectrum would be quite different.

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  • $\begingroup$ Thanks Rob. I've also found light bulb temperature color charts (google "light bulb temperature color") that show 3,000 K black body objects glowing yellowish-white, more yellow on the edge of the object. If that's more accurate, then "red" dwarfs would actually appear a deep yellow color to the naked eye. $\endgroup$
    – RobertF
    Commented Jul 6, 2020 at 19:44
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    $\begingroup$ @RobertF The spectrum of an M-dwarf is quite different to a blackbody. $\endgroup$
    – ProfRob
    Commented Jul 6, 2020 at 20:02
  • $\begingroup$ @RobertF: "more yellow on the edge of the object" isn't that just limb-darkening though, so it's the same spectrum at lower intensity? Basically whether it looks yellow or yellowish-white is going to depend as much on pupil contraction as anything :-) $\endgroup$
    – Steve Jessop
    Commented Jul 7, 2020 at 0:39
  • $\begingroup$ @stevejessop a limb darkened spectrum isn't the same spectrum at lower intensity, it is the spectrum from a higher, somewhat cooler, part of the photosphere. $\endgroup$
    – ProfRob
    Commented Jul 7, 2020 at 6:48

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