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The most common concepts of multi-star systems, it seems to me, are either unrealistic / impossible OR do not have the "feeling" of a multi-star system commonly depicted in films etc., i.e. multiple sunrises...

Just for clarification, the concepts I'm talking about are:

A) Two stars in the center: unstable unless star-planet distance is large compared to star-star distance (the latter removing the "feeling" because the two stars appear as one)

B) One star orbiting another: The planet having (approx.) the same distance to the outer planet at all times is not realistic, everything else will cause the smaller star to be on the opposite side of the system at some point, which is very far away.

My concept would be a large star obited by a small star which in turn is orbited by the planet. If my smaller star is about at the lower mass limit for hydrogen fusion and the larger star is about at the upper limit for being stable, then is it possible that the star star distance has a size so that:

a) the system overall is stable (like the milky way but a much smaller scale)?

b) the distance is so large that the temperature difference is not extreme (i.e. not a lot more than summer/winter on earth) between times when the planet is inbetween the two stars and times where the large star is ecplipsed by the small one?

c) the distance is so small, that the larger star is easily visible?

And if the small star is at the low limit of size and the planet is at the inner limit of the habitable zone: is it possible to have a very short orbiting period (20, 40 days or something) so that the respective days (respective to the different stars) are not so different (i.e. < one order of magnitude)?

Thanks in advance!

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closed as unclear what you're asking by John Dallman, sphennings, rek, Mołot, Bellerophon Feb 23 '18 at 13:22

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ if you would post an actual question we could also answer. $\endgroup$ – L.Dutch Feb 17 '18 at 9:23
  • $\begingroup$ The part starting at "is it possible" is the question. I should put question marks. $\endgroup$ – Anagkai Feb 17 '18 at 9:26
  • $\begingroup$ do you explicitly need a small star to be very small? Alpha Centauri could be a good example, just needs to increase one of the stars. you will need something maybe +1 absolute magnitude star to be big star, to add about +10% energy flow to a planet around the second star. $\endgroup$ – MolbOrg Feb 17 '18 at 11:03
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    $\begingroup$ Your proposal has already been made on this site. See my answer to the question here: worldbuilding.stackexchange.com/a/80627/23519. Bottom line, you can have an Earth-sized planet orbiting 1.7 AU out from two stars, one if which is a bit brighter than our sun. This orbital configuration is stable. I marked this question as a duplicate, since it is already answered elsewhere. $\endgroup$ – kingledion Feb 17 '18 at 13:33
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    $\begingroup$ Well then you need to make it clear in your post what you are asking. Like that other post, make a drawing, put a set of proposed orbital coordinates. Then I can go in with my orbital simulator and solve it for you. $\endgroup$ – kingledion Feb 17 '18 at 14:04
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If you want both of the stars to appear as sun like discs in the sky of a habitable planet, it is best to have the planet orbit both stars.

If you want both of the stars to appear with almost exactly the same apparent diameter in the sky of the planet, true twin suns, then the planet almost certainly has to orbit around both of the stars.

In a few cases of planets that orbit one of the stars and not the other in double star system, it may be possible for the two stars to have the right diameters and distances to sometimes line up so that they look about the same size in the sky of the planet for a short period of time, maybe a few percent of the total time it takes for the two stars to orbit each other, which can be many Earth years.

In your System A), a circumbinary or P-type system, where the planet orbits around the two stars, the planet will have to orbit at least 2 to 4 times the distance between the two stars. Both of the stars should appear as discs in the sky of the planet. As the two stars orbit around each other first the bright one would eclipse the dimmer one and then the dimmer one would eclipse the brighter one. And most of the time the two stars would be between eclipses and appear side by side in the sky. It is quite possible for the two stars to be separated by ten or twenty times their diameters, for example, and the planet to orbit at a distance of about 100 times the diameters of the two stars. Thus the two stars would appear clearly separated most of the time.

Your saying that the two stars would appear as one bright point in the sky in system A) is incorrect. A planet would have to orbit a type O supergiant or something for it's star to appear as a point of light in the sky at a habitable distance. And it is impossible for a type O supergiant star to have a habitable planet anyway, so that doesn't count.

As for your system B), which is called a S-type orbit, the farther star will not be much father away when it is on the opposite site of the nearer star than when it is on the closer side of the nearer star.

Suppose that the two stars have almost circular orbits around each other that are ten times the distance of the planet's orbit around the nearer star (Which is a bit farther than the minimum separation needed for stable planetary orbits. Thus the distance from the planet will vary between 9 times and 11 times the distances of the nearer star from the planet.

If the two stars have exactly the same diameter and brightness (and of course they do not have to), the farther star will seem to have between 0.0909 and 0.1111 the diameter of the closer star, and will be be between 0.0082 and 0.0123 times the brightness of the closer star.

Suppose that the two stars have almost circular orbits around each other that are 100 times the distance of the planet's orbit around the nearer star. Thus the distance from the planet will vary between 99 times and 101 times the distances of the nearer star from the planet.

If the two stars have exactly the same diameter and brightness (and of course they do not have to), the farther star will seem to have between 0.0099 and 0.0101 the diameter of the closer star, and will be be between 0.000102 and 0.000098 times the brightness of the closer star. Note that on Earth the full moon is only 0.00000025 times as bright as the Sun, so if the farther star appeared only 0.000098 times as bright as the nearer star it would still be many times as bright as the full moon on Earth and easily visible in daylight on the planet

Thus if the two stars have almost circular orbits around their common center of mass, instead of highly elliptical ones, the apparent diameter and apparent brightness of the farther star will not vary by very much. Your worry about the planet's orbit taking it much farther away from the farther star is not scientifically valid.

The big problem with your system B) with a habitable planet in a S-type orbit around only one of the stars, would be getting the two different stars to appear to have the same angular diameter and the same apparent brightness as seen from the habitable planet.

In your system type A), with the habitable planet in a P-type orbit around both the stars, both of the stars will be about the same distance from the habitable planet all the time. Thus the apparent diameter and the apparent brightness of both of the stars will have the same ratio as seen from the habitable planet as they actually have. So if the two stars happen to have almost identical actual size and luminosity they will appear to have almost identical apparent size and luminosity as seen from the habitable planet.

But in your system type B) with the habitable planet orbiting in a S-type orbit around only one of the stars, the farther star will have to be at least several times farther from the planet than the star the planet orbits. Thus if you don't need the two stars to have the same apparent diameter and the same apparent brightness as seen from the habitable planet, there is no problem. There are plenty of possible configurations that will have the farther star have a smaller apparent diameter as seen from the habitable planet, and also a much less apparent brightness, while still shining brighter than any star in Earth's sky or even the full moon.

But if you want the two stars in your system type B), with the habitable planet orbiting in a S-type orbit around only one of the stars, to appear to have the same diameter and the same luminosity as seen from the habitable planet, you will have a problem because one of the stars will have to be at least several times as far away as the other star.

In that situation it will be difficult to have the farther star appear to have the same apparent size as the nearer star as seen from the habitable planet, and it will be difficult to have the farther star appear to have the same brightness as the nearer star as seen from the habitable planet, and it will be just about impossible to have both.

You might be able to make the apparent size match, and you might be able to make the apparent brightness match, but it would be almost impossible to make both match.

If there is a habitable planet in your solar system, you will want both of the stars in your system to be main sequence stars. The size and brightness of main sequence stars depend on their masses. And if you change the mass of a main sequence star the size and the luminosity will change according to different formulas. You can never get the size and the luminosity to change in the same amount, so that the farther star will have both the necessary actual size and the necessary actual brightness to have the same apparent size and the same apparent brightness as seen from the habitable planet.

So in your system type B) with the habitable planet orbiting in a S-type orbit around only one of the stars, you will have to settle for noticeable differences in the apparent diameter and apparent brightness of the two stars as seen from the habitable planet.

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I envision this set up as similar to Sol, Jupiter and Ganymede except the role of Jupiter is played by a brown dwarf star

brown dwarf and friends

The neat thing about this is that close parallels exist in our own system so less handwaving or speculation required. Brown dwarfs are very close to gas giants - maybe they are a kind of ambitious gas giant. You could tweak the luminosity of your brown dwarf as you see fit; I am intrigued to read that these objects do not do normal star-type hydrogen fusion but might fuse other elements and so still be radiant.

One could definitely still see the sun from the vantagepoint of Ganymede. You could swap in a brighter star if you wanted things brighter out in gas giant land.

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    $\begingroup$ "You could swap in a brighter star if you wanted things brighter out in gas giant land." Careful, though, or the brighter star might push the rocky-planet belt out into your "gas giant land", requiring you to push the gas giant further out, in the end accomplishing nothing... (Yes, I know we've discovered extrasolar gas giants orbiting very close to their stars, but last I looked, we hadn't yet figured out what makes them form and remain there. Come up with a plausible enough for the audience explanation for that, and you can do just about whatever you want...) $\endgroup$ – a CVn Feb 17 '18 at 19:11

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