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# Close multiple

edited December 2012
Hello, AstroSynthesis friends.
I've a question about binary systems.

When you look at the preview picture of a binary system made of two close stars, you can notice that both stars are surrounded by the orbit of one planet that turns around the system centre of mass. This is perfect: it means that the distance between the two stars is smaller than the distance between that planet (the one orbiting around the centre of mass) and the centre of mass.

Well, when you open the preview window, the picture changes, and you can see a completely different situation: while the planet seems to still orbit around the centre of mass, at the centre of the system you can see just one star, because the other is located at a very incredible distance (usually a thousand AUs). This means that, in the new diagram, the distance between the two stars is bigger than the distance between the "famous" planet and the centre of mass, a very strange fact if I consider what I've studied about astronomy.

Since I am perfectly sure that the programmers are very great and astronomy experts, I'd like someone to explain me the reason of what I've noticed. Is there something about astronomy that I ignore, or are there other things to consider?

Thank you!

The smaller preview window & multiple star system view shows orbits at an exagerated scale because otherwise the orbits wouldnt be visible at that scale. Might that be what you are seeing?

The system view only shows one star (or parent body) at a time.
• Hmmm... I admit I haven't looked up in the help file to see how the program defines the difference, but I would have assumed the difference between "multiple star system" and "close multiple" would be:

Multiple Star System: Each star in the system has its own debris disk/planetary disk.
Close multiple: The system has a single debris disk/planetary disk, orbiting system barycenter *outside of all stars in the close multiple*. No?

The other possible definition I would have expected is to say a multiple is "close" when stars have noticeable tidal effects on one another's shape, to the point of sharing a single outer atmosphere. Beta Lyrae pairings and such.

So much for what happens when you assume.
The system view only shows one star (or parent body) at a time.

That may be what he considers the problem. In a "close" multiple by either of the above definitions, the "parent body" (system barycenter) isn't visible at all, but there are 2 stars orbiting the same parent *both within the orbit of any planetary disk* (and you'd expect within any likely camera POV.)

I don't know enough astronomy to know if you'd run into the same problems with moons of planets in planetary systems. I'd think if they were close enough to swap orbits now and then, they'd be close enough to steal moons off each other.
• Close multiples are essentially stars that are close enough that they interfere with the orbits of the planets orbiting the other star. Its not a hard & fast definition, though.
• Ed_NBOS wrote:
Close multiples are essentially stars that are close enough that they interfere with the orbits of the planets orbiting the other star. Its not a hard & fast definition, though.

Hmmm, I think I'm beginning to see the problem. Looking through the help file...
Close Multiples are two or more stars that are very close to each other, and do interfere with each other's planetary systems. Planet systems in these types of star systems are not very stable, so there are usually fewer planets present. Those that are present will either revolve around one of the stars at a very close distance, or revolve around the center mass of the close star system at a very far distance. Those planets that may have been in between these two ranges would be in unstable orbits, and get flung into space or, made into planetary toast.

I wasn't considering the possibility of stars within a close multiple still having 'single parent' stars in their skirts. If it wasn't for that, I'd say the solution is to come up with a formula for "system children" of a close multiple, that orbit system barycenter instead of any given star. You could still do that, but it would need a sub-formula to check for single parent children in the skirts of each star in the multiple... unless the stars are so close they're stealing atmosphere off each other, in which case there won't be anything in the skirts....

And even aside from that, if you have a semi-detached or contact multiple where the masses are starkly different, even that outer planetary disk might come up with some wild orbits, no? Stable perhaps, but... "different."

Too many exceptions, not enough rule that the exceptions are *to*.
• I'm still not exactly sure what is being discussed here. If its why, in a close multiple, only one star is shown in the orbital view, thats because the software only supports one 'parent' body at a time in the orbital view. It has nothing to do with astronomy, its simply a limit of the program's graphical display.