Erik Max Francis wrote in message ...
Jonathan Silverlight wrote:

It's interesting to compare that with the depiction in David Hardy's
painting "Proxima's Planet" - I've got the print over my desk. He
shows
them in a dusty red sky, which makes sense as -6 stars would be
visible
in daylight even on Earth with our blue sky, but they are about one
inch
(2.5 cm) apart which corresponds to at least a couple of degrees at
normal viewing distance.

But this only takes into account the brightness of the A and B
components. Proxima itself would be very bright, because for it to be
in the habitable zone (presuming that's where it is, I'm not familiar
with the painting), the planet needs to be very close to Proxima. For
it to be in the habitable zone, in fact, it needs to be at a position
where the intensity of sunlight is the same as at Earth's orbit, which
means that Proxima will have the same brightness as the Sun, about
magnitude -27 -- a little less because Proxima has enough infrared
excess that that will go into heating the planet a little, requiring
less raw illumination in the form of visible light.

Do you have any idea how much less?

A and B will be easily visible, but most of the illumination will come
from Proxima itself. It will be a cool, red light, but it will be
nearly as bright as the Sun on a full day. Almost by definition.

How large bolometric correction towards red would be acceptable for
terraforming?

On Earth, temperate and tropical shadow-tolerant land plants
habitually grow in 1/100 of total sunshine. Indeed, photosynthesis
suffers from "light inhibition". But what light they do get has a lot
of blue (scattered from sky) or at least red (filtered through leaves
of other plants).

How low light temperature can plants evolved on Earth endure? And what
would be the corresponding bolometric correction?