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I apologize for the way this question is worded but I don't think I know the proper verbiage. Basically, I want to know how the stars orbit one another in the two septuple star systems. For example, are there two massive stars in the middle that have the five remaining stars orbiting similar to the planets of our system; does one of the orbiting stars have one or more smaller star-"moons;" maybe there are stars trapped in one of the Lagrange Points. I hope that clears up the question.

I am aware that the orbits of these systems may not be known/understood. If that is the case, then I would appreciate the orbital relationships between the stars in the highest-multiple star system. If that is confusing, I mean, if we know the orbits for the stars in a sextuple system; if not then a quintuple... etc.

Finally, as an added bonus, I would appreciate being told the correct verbiage. Thank you, much.

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I checked the Multiple Star Catalog for data on these systems. It has hierarchical information on Nu Scorpii and AR Cassiopeiae, which I also used for this answer. I'll try to explain the orbital structure of both systems.

Nu Scorpii

Think of Nu Scorpii as two subsets of stars, Nu Scorpii AB and Nu Scorpii CD, orbiting each other with a period of 105,000 years. AB contains four stars:

  • Nu Scorpii Aa, Ab and Ac: Aa and Ab are a binary system (period: 5.5 days) orbited by Ac (period: 8.3 years).
  • Nu Scorpii B: This star orbits Nu Scorpii A - the three stars above - with a period of 675 years.

Nu Scorpii CD contains three stars:

  • Nu Scorpii C: C is a single star.
  • Nu Scorpii Da and Db: Da and Db are a binary system (period: unknown), which together orbit C over 3,100 years.

AR Cassiopeiae

AR Cassiopeiae is more complicated; it contains three main subsets:

  • AR Cassiopeiae AB: The contains three stars: Aa and Ab, a binary orbiting each other every 6 days, and B, orbiting A every 545 years.
  • AR Cassiopeiae CD: These pair of binary stars orbit each other once every 2,200 years, and orbit component AB in 438,000 years.
  • AR Cassiopeiae FG: This is another binary, with a period of 58,000 years. It orbits AB every 381,000 years.

Multiple star systems are usually nested like this - binaries and triple systems inside more binaries. I would image it's rare to have any other setup, like four stars orbiting together, because such a system would be highly unstable. You really need to nest the stars, with large separations between components (i.e. the distance between Nu Scorpii AB and CD must be much larger than the distance between Nu Scorpii A and Nu Scorpii B).

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  • $\begingroup$ Three years. Thank you so much. You should look at the question I have about a black hole-magnetar binary and information retrieval from the even horizon. Just got the tumbleweed achievement on it and think it'd be cool if I didn't have to go three years on it. You have no idea how excited I am to have this answered. $\endgroup$ – Jimmy G. May 26 '18 at 4:40

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