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Context: I am creating a hard science-fiction world set in the Alpha Centauri system.

How large and bright would the binary star Alpha Centauri AB in the sky, when observed from an exoplanet (Proxima C) in Proxima Centauri? Would it be visible during the day? Is it close enough to cast any light onto the planet?

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    $\begingroup$ Welcome to worldbuilding. Can you explain what have you already searched on your own? At the moment your question sounds a bit like "do the math for me". Please read the help center for what we want from a good question $\endgroup$
    – L.Dutch
    Oct 13, 2021 at 19:12
  • $\begingroup$ I know that our sun is 8.3 light-minutes from Earth. After conversion, the Alpha Centauri AB binary system that Proxima Centauri orbits is 105,192 light minutes away. So since the Alpha Centauri A and B stars are sun-like, if we assume they are the same size as the sun, then they would be 12673 smaller in the sky. However this does not account for brightness, and I'm not sure how the two stars would combine to create a new star, and how large that star would be. $\endgroup$ Oct 13, 2021 at 19:19
  • $\begingroup$ While we do allow real-world questions, there is a discussion going on about clarifying their suitability without an actual worldbuilding problem. Do you have an actual worldbuilding problem? If not, this question might be better asked at Astronomy. $\endgroup$ Oct 13, 2021 at 20:05
  • $\begingroup$ Magnitude and Luminosity calculator $\endgroup$
    – Alexander
    Oct 13, 2021 at 21:16

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According to this page, Proxima is about 13,000 AU (or roundly 1/5 light year) from the AB pair that orbit only 23 AU apart (comparable to Neptune's orbit around the Sun). From that distance, the individual stars of AB would be, well, stars. Very bright stars (seemingly of magnitude -1 or brighter, according to this calculator, brighter than Sirius and comparable to Venus), but without any hint of a disk.

Given that Alpha A is a little bigger and 1.5 times brighter than our Sun, while Alpha B is a little smaller and a little less than half as bright (not to mention, at different temperatures, they'll be slightly different colors, one bluer and the other yellower than our Sun), they should be comparable to what our Sun would look like from the outer Kuiper belt or inner Oort cloud.

Now, the more interesting question is whether they'd be a visual binary (we'll assume vision as good as human standard): arcsin (23 / 13000) ~ 0.1 degree or about 6 minutes of arc -- 1/5 the width of the Sun or Moon as seen from Earth makes the AB system an easy visual binary as seen from Proxima B, and it would be obvious over a pretty short time (orbital period just under 80 Earth years) that the two stars orbit each other.

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    $\begingroup$ "less bright than Sirius or Vega" - oh no, according to this calculator both of them should be brighter than Venus. $\endgroup$
    – Alexander
    Oct 13, 2021 at 21:16
  • $\begingroup$ Thanks, @Alexander -- I've edited, and added the link you gave. $\endgroup$
    – Zeiss Ikon
    Oct 14, 2021 at 11:05

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