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I posed a question on here a couple months or so ago about the orbital patterns of each of the seven stars in the two septuple star systems Nu Scorpii and AR Cassiopeiae. It went unanswered for more than 3 years, but it was finally answered. So, now I want to know which multiple star system has the highest multiplicity for which we actually do know the orbits of all the stars in the system.

To be clear, I am NOT talking about a star with multiple planetary or other smaller bodies. I am talking about multiple STAR systems, with a high multiplicity, or number of stars. Again, I'm not talking about every rock larger than a school bus. I mean STARS. Stars that have nuclear fusion occurring.

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As of writing this, 11 October 2023, I am reasonably certain that the star systems with the highest multiplicity, whose orbital periods of each of the stars are known, are those of the two septuple systems Nu Scorpii and AR Cassiopeiae. For a breakdown of their orbits, see this answer to the question How do the orbits of Nu Scorpii and AR Cassiopeiae work?

Further, according to Wikipedia, there are three new star systems of equal or higher multiplicity (and my Google searches do not yield me any results). They are a septuple named V871 Centauri, an octuple named Gamma Cassiopeiae, and a nonuple named QZ Carinae. And, it would appear that not all of the orbits are well understood an all three of these systems.

The only source I could find describing V0871 Centauri's orbits, Multiply eclipsing candidates from the TESS satellite, published 8 May 2022, says "V0871 Cen is probably a septuple-star system of architecture (Aa-Ab)-B-C-D." It continues:

The most inner two pairs are the 2.8 and 2.09 days binaries (we name it Aa-Ab), accompanied with a more distant component B with its only poorly-constrained orbit by Zasche et al. (2009). And much more distant C and D components are probably bound (due to their similar proper motion), but only very weakly.

So, while orbits of the inner binaries are understood, the more distant stars' orbits are not.

Then for Gamma Cassiopeiae, I couldn't find any source to describe its orbits, and the only source I could find confirming it's an octuple star system was Surveying the Bright Stars by Optical Interferometry III: A Magnitude-Limited Multiplicity Survey of Classical Be-Stars, published 15 September 2021. Now this one is really technically dense but, so far as I can tell, it does not give any orbital periods.

Finally, for Qz Carinae, the only source I could find describing its orbits was Towards a consistent model of the hot quadruple system HD 93206 = QZ Carinæ, published 15 April 2022, which explicitly states that it only has orbital information from the four inner, dual binary star systems.

Given that none of the other star systems known to have a multiplicity equal to or higher than Nu Scorpii and AR Cassiopeiae have all of their orbits known, as shown here, I am confident that these two systems currently have the highest known multiplicity, whose orbits are known. And, I can't thank HDE226868 enough for their answer.

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What is the most populated/numerous stellar system in which the orbits of all objects are known?

The answer is none. Other than our own solar system, astronomers don't know if they know all of the large bodies (aka planets) orbiting any given star system. Presumably other star systems have asteroids, comets, and other stuff. The orbits of those small bodies in star systems is unknown, and will remain unknowable for a long time.

We don't even know that for our own solar system. If we did, we wouldn't have to worry about killer asteroids and comets.

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As far as I know, the system with the most stellar members is the black hole at the center of the Milky Way with 8 stars in close orbit. Of course, David Hammen is correct, and if there are other orbiting stars with much longer periods we'd have trouble determining their orbits until they started their close approaches.

At the other extreme, is a galaxy a star system? I assume no, but just thought I'd ask, since you have not defined the term. We know the orbits of quite a few stars in the Milky Way, but that's probably not what you mean.

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    $\begingroup$ This would be answer, except that the supermassive black hole isn't and wasn't a star. $\endgroup$
    – ProfRob
    Mar 30, 2016 at 18:04
  • $\begingroup$ @RobJeffries - No, but the others are, so doesn't that make it a star system? $\endgroup$
    – WhatRoughBeast
    Mar 30, 2016 at 19:22
  • $\begingroup$ if it does, then your second answer is the correct one and needs to be developed. I think it is clear that the question is about multiple stars though. $\endgroup$
    – ProfRob
    Mar 30, 2016 at 19:26
  • $\begingroup$ @ProfRob not that I advocate devils frequently, but suppose there was a system with a couple of identified stars and an additional unseen member of comparable mass; would it cease to be a "star system" once the member was identified as a black hole? $\endgroup$
    – uhoh
    Oct 13, 2023 at 0:26

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