# Can BY Draconis variable stars sustain any form of complex life?

I wanted to ask this, because while I was working on two fictional planets back on the Worldbuilding SE, one of them being habitable, which orbit the 61 Cygni A star of the 61 Cygni binary star system, I realised something: 61 Cygni A is a BY Draconis variable star.

My problem is: can a BY Draconis variable K-class star like 61 Cygni A permit the development of complex life, unlike its partner, 61 Cygni B, is a flare star that can barely get life past the primordial stage, if any planets are there? Or does it have some problems in that regard? If so, what are those problems that a habitable world in that star system could have, and other star systems of the BY Draconis variable star category?

• I can't see any opinion-based in this question. It is roughly clear, which environmental parameters are needed for complex life, the question is, BY Draconis has them or not. – peterh - Reinstate Monica Mar 6 '18 at 19:33
• @peterh The headline question is totally an unanswerable matter of opinion. What does complex life need? Light, heat, carbon? All present. Will UV be a problem? Who knows? Any scenario for a planet can be hypothesised to remove the problem (e.g. ozone in the atmosphere). – Rob Jeffries Mar 6 '18 at 22:41
• @RobJeffries I think "no one knows for sure, but ..." would be a correct answer here. "No one knows" answers, or the negative answers for questions like "does ... exist", are all valid answers on the SE - only no one likes to write them. The question could be offtopic saying that astrobiology != astronomy, although it might be a too narrow interpretation of the site topic, in my opinion. – peterh - Reinstate Monica Sep 4 '19 at 9:06

Suppose you have a star with luminosity varying between $L_{min}$ and $L_{max}$. Then the inner limit of the life zone will be in the interval $[c_1\sqrt{L_{min}},c_1\sqrt{L_{max}}]$ where $c_1\approx 0.7$ AU (or whatever value you think makes sense for the inner limit). The outer limit will be in $[c_2\sqrt{L_{min}},c_2\sqrt{L_{max}}]$ with $c_2\approx 1.4$ AU. So if $c_1\sqrt{L_{max}}<c_2\sqrt{L_{min}}$ there will be a region that is always in the life zone.
61 Cygni A has magnitude varying between 5.19 and 5.27, $L_{min}=0.666807$ and $L_{max}=0.717794$. That gives a constantly habitable zone between 0.61 and 1.14 AU. Narrower than the Sun's, but still there.