Proxima Centauri is about 0.21 light years from Alpha Centauri, which is close-by in stellar terms but a long way for a companion star (the jury is still out on whether Proxima Centauri is in orbit around Alpha Centauri AB or just formed with them and is thus on a similar path through space).
From Earth, at a distance of 4.37 light years, Alpha Centauri A has a magnitude of 0.01 and Alpha Centauri B a magnitude of 1.33. The brightness increase from Proxima Centauri will be
2.5 log (4.37 / 0.21)2 = 6.6 magnitudes
So, Alpha Centauri A would be Mag -6.6 and Alpha Centauri B would be Mag -5.3. That's about three times brighter and about half as bright again than Venus at its brightest respectively.
From Proxima Centauri, the maximum separation of Alpha Centauri A and B would be about 0.15 degrees at most. Much of the time they would probably look like one bright star perhaps four times brighter than Venus. In other words, not enough to affect how bright daylight seemed on Proxima b.
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timeasmymeasure
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Date: 2016-08-27 10:32 am (UTC)From Earth, at a distance of 4.37 light years, Alpha Centauri A has a magnitude of 0.01 and Alpha Centauri B a magnitude of 1.33. The brightness increase from Proxima Centauri will be
2.5 log (4.37 / 0.21)2 = 6.6 magnitudes
So, Alpha Centauri A would be Mag -6.6 and Alpha Centauri B would be Mag -5.3. That's about three times brighter and about half as bright again than Venus at its brightest respectively.
From Proxima Centauri, the maximum separation of Alpha Centauri A and B would be about 0.15 degrees at most. Much of the time they would probably look like one bright star perhaps four times brighter than Venus. In other words, not enough to affect how bright daylight seemed on Proxima b.