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In the story I'm making, it is important that there are two suns in the sky, going at different rythms (maybe the planet orbits one of them each year, and the other each four years, for instance), and the planet rotates, making it so that in regular patterns, let's say in the beginning of the cycle, both suns seem "together", from the planet, like one is eclipsing the other, but as times goes on, in the mid-day (considering the planet which is rotated yearly as reference), the other sun seems to lag behind, until at exactly half the cycle, just as one of the suns is exactly at mid-day position in one side of the planet, the other sun is also at mid-day position, but in the opposite side of the planet.

One thing I thought, which I don't know how possible it is, is that one of the suns is incredibly more massive than the other, and it's very far, so it's like the smaller sun orbits the big one, and the planet orbits the small one, just like earth orbits the sun, and the moon does the same with the earth. The planet I thought of is similar to earth in size (that can change, is not that important) and climate (that is important, I'd like seasons to feel similar and wildlife in general to be similar).

So that the sun that is close to the planet irradiates a similar ammount of light and heat to our sun, I think it may need to have a similar size, but, would the other sun need to be impossibly massive at that point, so that the small sun orbits it? I know technically both suns orbit each other and affect each other, but in order for the visible cycle I'm describing to happen, I think one would have to be more massive than the other, but I don't know if it would be so massive it would just not be a sun anymore, but would just collapse. If I make that sun smaller, would mean the small sun is even smaller, and the planet would have to be either smaller as well (but gravity would than be very different to earth, which I wouldn't like), or way closer to the small sun.

This is just one way I've thought about it, and I'm not married to that idea, my goal is to look for a way so that the cycle I describe is possible. Just to clarify, there is a magic system intended, but I want it to be completely independent from how this sun system works, if possible. I want it to be possible with the science we know, even if it's a very unlikely (but possible) scenario, so I'd like to avoid magic and science fiction altogether.

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  • $\begingroup$ The suns will rotate around common barycenter, and the planet can rotate around either of those suns, or the barycenter, if it's distant. Why do you think one sun should be much more massive than the other? $\endgroup$
    – Alexander
    Dec 9, 2019 at 22:17
  • $\begingroup$ A way of introducing other bright points of interest to the sky that could be described as a sun (if it’s identified as a Sun by characters who aren’t part of a science based story where they’re bobbing about in space physically looking at the star but are part of a ground based magic story) then it doesn’t necessarily have to be a star. An inward drifting gas giant with an orbit of perhaps 0.2 AU and an orbit of just a few days would still be seen an one of the brightest objects in the sky. $\endgroup$
    – Darius Arcturus
    Dec 9, 2019 at 22:39
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    $\begingroup$ Also, diameter doesn’t necessarily dictate gravitational strength. $\endgroup$
    – Darius Arcturus
    Dec 9, 2019 at 22:45
  • $\begingroup$ Not my field, so I am not confident enough to write an answer, but Wikipedia actually has a page on the potential habitability of binary star systems which might address much of your question. en.wikipedia.org/wiki/Habitability_of_binary_star_systems $\endgroup$
    – GeoffAtkins
    Dec 10, 2019 at 7:27
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    $\begingroup$ Maybe worthwhile to have a look at "Universe Sandbox" - AFAIK it lets you simulate a star system of your liking and see how that plays out. $\endgroup$
    – Daniel
    Dec 11, 2019 at 22:11

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I assume that you want the two suns to provide roughly equal amounts of heat and light to your planet.

Since there are two stars, your planet should receive about half the heat and light from each star as the Earth gets from our sun (Sol), or it would be far hotter.

Stars near solar mass have luminosities proportional to their mass to the fourth power. Hence, a star of 0.9 solar mass (spectra class G7) would have roughly two-thirds the luminosity of Sol, and in order to receive half the light and heat, the planet has to be at a distance of 1.15 AU. If we then assume that the larger star has two solar masses (spectral type A4), its luminosity is 16 times that of Sol, and to receive half the light and heat as we get from Sol, your planet needs to be at an average distance of 5.7 AU from this star, or a bit more than the distance between Sol and Jupiter. This may seem close, but your planet would only get about 1/16 the tidal force that the Earth gets from Sol, though a bit more when it is closest.

You can increase the distance by making the larger star bigger and farther away, if you like. Above two solar masses, luminosity is roughly 1.4 x (M/Msol)^3.5. A star of three solar masses (spectral type B8) hence has 65.5 times Sol's luminosity, and it would have to be 11.5 AU distant; a bit farther than the distance from Sol to Saturn.

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  • $\begingroup$ Actually, I was envisioning it as one sun being more bright than the other, so it would be pretty much just as our sun in most ways, and the other sun would be less bright (I've been picturing it in another color, probably red, but I don't know the feasibility of that). If I'm picturing this correctly, if the two suns go around each other one time each 4 years, from the planet's POV, would that mean that the sun not being orbited by the planet would have a "solar year" of four years? I'm having trouble picturing the perspective from the planet during a whole cycle around both suns. $\endgroup$
    – Iván Flores Vázquez
    Dec 10, 2019 at 21:49
  • $\begingroup$ I'd like to keep one sun fulfilling pretty much the same function as ours (around 1 year traslation), and the other one being more of a visual element in the sky; for example, after half of the time it takes for both suns to go around each other, both suns would appear to be in opposite directions from the planet's POV, and on the beggining of the cycle, they would both be in the same direction, one 'eclipsing' the other from the planet's POV. That's why I've been thinking of a sun being much further than the other. I'll add a picture of a model I imagined when I'm home in some hours. $\endgroup$
    – Iván Flores Vázquez
    Dec 10, 2019 at 21:54
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    $\begingroup$ If you want to have the suns go around each other in 4 years, thay will have to be pretty close to each other. roughly like the distance between the sun and the asteroid belt (or around 3 AU). This could be feasible if the smaller sun is a red dwarf, but it would not be very bright - probably less than the full moon. When the planet is closesf to this sun, it will be 2 AU away; when it is farthest (on the opposite side of the main sun), it will be 4 AU away. This results in a factor 4 difference in apparent brightness, making the small sun weakest when it is in the sky close to the main sun. $\endgroup$
    – Klaus Æ. Mogensen
    Dec 12, 2019 at 10:00
  • $\begingroup$ Thank you, this is pretty similar to what I had in mind, and you helped me clarify it and picture it better. So, the planet would orbit a sun similar to ours, and the other sun would be a red dwarf, right? I can investigate the average proportions they would have, and the red dwarf being the weakest when they appear close is actually a desired outcome design-wise. $\endgroup$
    – Iván Flores Vázquez
    Dec 12, 2019 at 16:38

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