@HDE226868 Cool :) I literally do not know how to proceed, though. Is there a PMing system on this site? Or a way of setting up a message thread dedicated to the topic? Or shall we just proceed here? :)
@HDE226868 thanks for the answer, and the edit to same. It looks like you're saying the second star is about 15x brighter than the full moon and the other answer is saying about 40x. I suppose we could say "well, it's within the same order of magnitude", but I'm still curious what the difference is. Any idea where its' coming from? (Either way, "bright enough to dominate the night sky" is very useful information.)
If you're right and I wanted the second sun to be a little brighter, I could presumably move it in by an AU or two without damaging much else. (Is this the part where you cringe at my casual treatment of the science? :-) )
@HDE226868 I'm idly curious how close it'd have to get before it produced noticeable effects on the planet, but I don't know if it's worth asking as a separate question.
If a second star were to provide enough light for a "second day", though, that'd be interesting. I'm trying to imagine how that would work; it'd be a seasonal effect, depending on where the planet is in its orbit around the first star.
At its farthest point from the second star there's only one day (everything lines up); at its nearest point there are two equal-length (?) days, and in between (let's call them "fall" and "spring") there's transition, but I'm fuzzy on what it would look like. (Twlights bumping into each other? Overlap?)
@HDE226868 how would other planets be relevant? Their gravity should be negligible compared to stars, right?
@HDE226868 it seems like the sort of thing that I ought to be able to work out with paper and pencil (by drawing, not by math), but I haven't managed it. Maybe I will ask.
@MonicaCellio Oh, I had been thinking along different lines, e.g. how civilizations on each one would react to each other. But yes, gravity should be negligible . . . until you get up close. I'm not sure how close.
(I've tried to research this online but I'm a linguist, not a physicist, and most places that discuss this quickly get too technical for me. Simple answers will get tons of gratitude!)
I imagine two suns locked in a fairly tight orbit (for simplicity's sake let's say Sun 1 is the same size as ou...
Consider a dual-star system, with the distances between S (larger star) and J (smaller star) similar to Sun–Jupiter. A planet P rotates around J on a 90-degree ecliptic and is Earth-like otherwise.
Due to a tidal lock, one hemisphere of the planet permanently faces J. J is hot enough to sustain ...
@HDE226868 I think I saw that, but the tidal locking seems like a significant difference to me. If any given part of the planet never either gets a break (on the day side) or gets sun (on the dark side), that's going to affect lots of stuff there.
@HDE226868 "circumbinary" is a good word that I never would have come up with on my own. Reading.
Actually, maybe you can help me think this through before I ask. So, based on the answers I've got so far, the sun has an apparent magnitude of -27, the distant sun here is -17, the full moon is -13. -27 is clearly enough to "feel like day" (by definition). If I want a "lesser day" that still feels more like day than like night, around what should I be aiming for? This affects distance, which might affect my seasons question.
Ok, thanks. I want the stars to be far-enough apart to not break things on the planet. Your blog post mentioned 100AU as being a safe distance, but you never claimed it was the minimum safe distance. If ti were instead 10AU, would that still work for a habitable planet? If that's not bright enough yet, how much closer could I get it?
I'm fine with ranges for all of this, of course. It's just that you need to have some numbers in mind to ask the question, and asking the question might help you refine the numbers, which you use for the next question, and...
> There is a direct correlation between the period of revolution of a binary star and the eccentricity of its orbit, with systems of short period having smaller eccentricity. Binary stars may be found with any conceivable separation, from pairs orbiting so closely that they are practically in contact with each other, to pairs so distantly separated that their connection is indicate only by their common proper motion through space.
> Among gravitationally bound binary star systems, there exists a so-called log normal distribution of periods, with the majority of these systems orbiting with a period of about 100 years. This is supporting evidence for the theory that binary systems are formed during star formation.
I'm trying to visualize what "40x as bright as the full moon" would look like. I mean, away from obstacles the full moon is actually bright enough to walk around and stuff, but not bright enough to read.
@HDE226868 wow, 1/3 binary? I thought this was much more unusual.
> The secondary star B orbits A at only 9.8 times the semimajor axis of A's planet. Despite how compact the system is, the planet's orbit is stable if it is coplanar with that of the binary companion.
Gamma Cephei does see like a good choice, though. One is a subgiant and one is (probably) a red dwarf, so the analogy is imperfect, but a change to solar analogs is possible.
So Gamma Cephei has a planet orbiting A, with B about 10AU away, and that's stable. What I can't tell from that page is whether that planet would be habitable. Or, rather, if we promote the primary from red dwarf to G-class star to get sun-like properties, does that work for the planet.
Ok, so we have an existence proof of a sun-like star with a planet, and another star ~10AU away. I don't see in the article how far out the planet is, but even at a distance of only 10AU between the stars, it seems like it should be possible for it to be in the goldilocks zone. Thanks!
I think, now that we've gone through this, that I should be able to ask my seasons question without specifying the distance; it's more of a geometry problem, I think. I also want to know how bright I can get that "second day", which depends on getting the second star closer, but that's separable I think. So, two questions. :-)
@HDE226868 oh, thanks. So for purposes of making a new world (since GC is just a reference, not the actual setting), I could put the planet at 1AU from star A and things wouldn't fall apart, it looks like.
I was worrying about the case of the planet being closer than 1AU already.
@MonicaCellio I found a set of notes on stability that can be used to quantify whether or not the system will work (it comes together ~ page 37; I haven't read through the whole thing yet). It apparently comes down to "resonance overlap".
That's going to take a while to get through, though. :-)
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