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I have come across many galaxy maps showing the dominion/borders of one empire or another in different works of fiction. The first that comes to mind is WH40k for which the galaxy map of the different factions are like this. For star wars, it looks like this

Now with the fact that there are many different factions sharing the galaxy in this way and with the fact that different parts of the galaxy orbit the center at different speeds, wouldn't that eventually (in millions of years) mean that all of these factions will chaotically mix with other factions resulting in undefined borders?

If so, then how would different factions configure their worlds to keep them part of their faction after a long time has passed? Would a ring configuration for example do the trick?

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    $\begingroup$ Do they have ftl travel? If so, what sort? $\endgroup$
    – Nepene Nep
    Dec 31, 2022 at 16:04
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    $\begingroup$ There is no empire that is going to exist for millions of years. $\endgroup$
    – Ian Kemp
    Dec 31, 2022 at 16:44

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Over the course of a couple of galactic rotations (250 million years each) any set of neighbouring stars will be dispersed, due to variations in orbital period, eccentricity, and gravitational perturbations. There are two ways to tackle this problem, if you want to maintain a polity of stars over that kind of timescale:

Globular Cluster

Form your polity in a globular cluster, and confine it to the cluster. These clusters are gravitationally bound, and will stay together over long timescales, unless they collide with another cluster, which is extremely rare.

A globular cluster has a limited size - about 100 light years diameter for a typical large one - and thus a limit on the number of stars in it. Typical examples have hundreds of thousands of stars, and the largest one in the Milky Way has about ten million. The stars also tend to be older and poor in elements heavier than helium ("population II stars"), so you may be short of planets.

The stars within a cluster move around, so they won't be at fixed distances from each other, but they stay relatively close together, in terms of the size of a galaxy.

If you want to form a large polity, with a few billion stars, this will be a substantial fraction of the stars in the galaxy with terraformable planets. At this point, you might as well take over the entire galaxy.

Move your stars around

This is quite ambitious, and quite slow. Stellar engines that allow you to use a star's power to propel it have been proposed, although their thrust is quite low, and building them is a huge project. A useful piece of SF, if you want to explore this concept, is Alistair Reynolds' novella Thousandth Night, which deals with a project to make the galaxy more compact, by moving its stars closer together, reducing interstellar travel times. Yes, this is hubris on a vast scale.

On a smaller scale, you might well be able to get a small polity, of a few hundred stars, "flying in formation" this way. They would stay together for perhaps half a galactic rotation, until perturbations started to disrupt your formation.

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  • $\begingroup$ The globular cluster is a great solution. But is there an upper limit to how many stars you can pack into a single globular cluster to keep them gravitationally bound while preventing them from merging with each other? For example, can a globular cluster be made out of 10 billion stars? $\endgroup$ Dec 31, 2022 at 11:51
  • $\begingroup$ @AbanobEbrahim: The entire Local Group is gravitationally bound. $\endgroup$
    – AlexP
    Dec 31, 2022 at 11:56
  • $\begingroup$ @AbanobEbrahim: Added some numbers. $\endgroup$ Dec 31, 2022 at 15:27
  • $\begingroup$ An open star cluster might be better. It wouldn't stick together nearly as long as a globular star cluster, nor have as many stars, but if the planets could be terraformed it might stick together for millions of years. A moving group is a set of stars which escaped from a star cluster but still have similar velocity and direction of travel. So claiming a moving group and all the stars in between the moving group might be a good idea. $\endgroup$ Jan 2, 2023 at 18:05
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The most likely explanation is that millions of years means that empires rise and fall, and the shifting of stars is an infinitesimal factor in their reconfiguration because the empires don't last for longer than an infinitesimal time.

Assuming that your intelligent beings have configured themselves to maintain social structures for millions of years, the borders are less important than they look. The vast majority of space is hard vacuum between Oort clouds. It is very hard to maintain a military presence in the quadrillions of cubed lightyears of hard vacuum. Therefore the borders would look much like those on Earth if you tried to draw borders in the 19th century that encompassed a country and all its possessions about the world. That would look like a chaotic mix, with much overlap, but in reality every imperial country could get to its possessions by the high seas, just as your far future empire could reach its by hard vacuum.

They would relocate a world only if they had vast power, and a powerful reason to do so -- remember, that power could also be used to reach the location.

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The stars all orbit around the center of mass of their galaxy like asteroids orbiting the Sun in our solar system. If two stars had exactly the same orbit they would bein in the same place and be a binary star.

The orbit of each star system must be different from the orbits of its neighbors. Thus as they orbit the galactic center they will move farther and father apart and won't start to move closer to each other again until they have completed more than half of a galactic orbit a hundred million or so years later. But passing close to other stars will change their orbits, and they will pass close to a lot of stars during a single galactic orbit. Thus a group of stars which are currently close to each other will probably never get that close together after a hundred million years.

Here is a link to a list of the stars which have passed or will pass within 5 light years of the Sun within a few million years in the past and future.

https://en.wikipedia.org/wiki/List_of_nearest_stars_and_brown_dwarfs#Distant_future_and_past_encounters

This indicates the rate at which stars may move relative to each other.

So if two interstellar realms have a nice flat border between them, it is quite possible that after a few tens of thousands of Earth years, the border will be very bumpy as stars from Realm A move in between stars of Realm B, and vice versa.

This may be a problem for interstellar realms lucky enough to last for tens of thousands of years.

I don't know the time frame that will be covered by your single short story, or long series of epic multigenerational novels, set in that fictional galaxy. Thus I don't know whether the time covered will be long enough for that to happen, nor do I know how the space realms would deal with the changing borders when and if happens.

Added January 2, 2023.

Another answer suggested claiming a globular star cluster, whose stars are gravitationally bound to each other and travel together.

Here is a variation on that idea.

One possibility would be to claim all the stars in a moving group.

A moving group, in astronomy, is a group of stars that share a common motion through space as well as a common origin.

https://en.wikipedia.org/wiki/Beta_Pictoris_moving_group#:~:text=A%20moving%20group%2C%20in%20astronomy,is%20named%20for%20Beta%20Pictoris.&text=The%20Beta%20Pictoris%20Moving%20Group%20is%20an%20important%20object%20for,of%20stars%20to%20the%20Earth.

The stars in a open cluster would all be young, since it would disperse rapidly by astronomical standards.

Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years.

https://en.wikipedia.org/wiki/Open_cluster

So some advanced society could colonize an open star cluster, terraforming young planets to make them habitable. And as a star cluster gradually disperses, it will form a moving group of stars. The farther apart the stars get, the more ordinary stars will be between the members of the moving group.

I believe that the Sun is within the volumes occupied by the members of at least one nearby moving group - the stars in the moving group are in every direction as seen from the Sun.

As the moving group spreads out, there will be tens, hundreds, thousands, & eventually millions of more or less background stars within its volume for each star that is actually part of the group.

So a space realm could claim all the space within a distance of X light years or parsecs or whatever distance unit is used around each of its colonized stars, and claim as a border a number of flat planes connecting the spheres around the outermost stars it claims. Thus it would claim all of the stars which pass into the volume occupied by its moving group, and colonize their planets.

And that would be fine for the interstellar realm so long as the nearest other interstellar realms were hundreds or thousands of light years away with unclaimed space and stars between them.

The closer various interstellar realms get to each other, the more likely it is that stars will pass from the volume of one realm into the volume of another realm. Especially when the space realms directly border each other.

But as I said above, the distances between stars are so great that the time for a star and its planets to pass out of the volume of space realm to that of another space realm should be extremely long by human standards.

Thus it quite possible to write a long, long, epic space opera series which covers thousands of years, in which stars passing out of one space realm into another doesn't happen because the time scale is too short.

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  • $\begingroup$ I think the link is missing? $\endgroup$
    – user86462
    Jan 1, 2023 at 9:53
  • $\begingroup$ @Sean OConnor Oops!. I have added the link. $\endgroup$ Jan 2, 2023 at 17:45
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The configuration would have to be variable in world membership. There would have to be set (or somewhat set) xyz coordinate bounderies where all worlds within are a part of a faction and all worlds without are not a part of that faction. As a world moves from within to without their membership in a faction changes. The bounderies could be set up via military might and then held with military might. The Bible says that strong fences make good neighbors, thus every faction that can build a strong (military based) border can look forward to a potential of good neighbors as long as that faction is able and willing to destroy any and all other incursions by other factions that militarily cross the border.

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