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From the surface of an earthlike planet orbiting star 1, would start 2 be visible during the day?

Would it have any noticeable effect on the light level at night?

Star 1:
Class: Main sequence type K2V (orange)
Distance from observor: 0.76 AU
Mass: 0.82 suns
Radius: 669,766 km
Luminosity: 0.286 Lsun
Temperature: 4932°C
Star 2:
Class: Main sequence type M2V (red)
Distance from observer: 56.7 AU (orbiting around the larger star)
Mass: 0.48 suns
Radius: 347,750 km
Luminosity: 0.0296 Lsun
Temperature: 2622°C
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    $\begingroup$ In all your questions you keep leaving out a vital parameter: how far are these bodies from the observer? $\endgroup$
    – L.Dutch ♦
    Commented Apr 4 at 14:12
  • $\begingroup$ @Fallenspacerock, check here meta.stackexchange.com/q/356997/644344 $\endgroup$
    – L.Dutch ♦
    Commented Apr 4 at 14:31
  • $\begingroup$ @L.Dutch Thank you I will definitely use similar tables for my own questions in the future. $\endgroup$
    – Fallenspacerock
    Commented Apr 4 at 15:21
  • $\begingroup$ @www I am pretty sure that the red star would be visible in the day as the moon often is also visible in that time. And u mean it is way bigger than the moon and it's own light source. I don't know how it would effect the night this depends on your orbits but it would probably make the sky red in the night. $\endgroup$
    – Fallenspacerock
    Commented Apr 4 at 15:30
  • $\begingroup$ Regardless of what you can see, it sounds cold there. $\endgroup$
    – Escaped dental patient.
    Commented Apr 5 at 19:05

1 Answer 1

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This star's bolometric apparent magnitude (that is, summing over the entire spectrum) is -14.14, which at first glance makes it brighter than the full moon. But we then have to apply a bolometric correction in reverse: we know the total light output, and we essentially multiply by the fraction of that which is in our visual band of choice.

An M2V star is much hotter than 2622 C ~= 2895 K; per Erik Mamajek's models which are sometimes cited on this stack as a quick reference, you would expect closer to 3560 K.

The models suggest that the former star (around M5.5V in the table) has absolute V-band magnitude 15.30 compared to bolometric 11.72, which means that at any distance (because these scale with distance at the same rate), the star will be 3.58 magnitudes dimmer in the V-band. But the latter star, with higher temperature, has 10.21 and 8.59 respectively, a difference of only 1.62. This reflects that a hotter star has more visible emission compared to infrared.

The difference is very significant! With our hotter 3560K M-dwarf, we have an apparent V-band magnitude of -12.52, very close to the full moon's -12.74. The cooler 2895K dwarf will be closer to -10.5 (rounding for uncertainty here), which is about six times fainter. The difference should be something like that between the full moon and a half moon.

But you could definitely see both of those during the day, so the answer to your first question is yes.

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