# Distance between star systems

I'm writing a story that will eventually escalate from being focused on one planet to the entire star system that planet belongs to, and finally to a big story involving the two nearest star systems to the one the Original planet is located in.

So what I would like to know is what is the minimal distance these different star systems will have to be apart from each other? (I don't want them so close as to be binary star systems but the 3 different systems should not influence each other with gravitational pull either)

P.S. If it matters what type of stars we are talking about, if we call the Original star system A and the others B and C then A has a Main Sequence Star (basically our sun), system B has a Blue Supergiant, and system C has a Red Dwarf.

Any advice will be greatly appreciated.

• I won't venture a general answer, but as far as our own Solar System is concerned a star at less than 1 or 2 light years (or half a parsec) would significantly perturb the Oort cloud; so says Wikipedia. Jan 21 '19 at 13:25
• The real question is how long you want the time required to travel between the systems, or communicate between them. With today's technology, we'd require decades or centuries to reach Proxima Centuri. Jan 21 '19 at 15:50
• Well travel or communication is not really a problem here, that part of the story does not need to be hard science in any way. Lets just say that radio signals between the systems travels fast enough to have an actual conversation (with a little waiting time but not anything like weeks) and that the main planet has a certain tech that once in space allows their ships to pretty much accellerate to ''ludicrous speed'' levels of speed. It was more a question of how close can i get these systems without them wreaking havoc on each other through things like gravity or other physics factors. Jan 21 '19 at 15:57
• Could it be Ok if these stars form one loose star system rather than be fully independent from each other? Jan 21 '19 at 18:10
• If you're using actual radio (i.e. electromagnetic radiation, at whatever frequency), then your wait time = distance at the speed of light. 1 day wait is 1 light-day. 1 week is 1 light-week. 1 year delay is 1 light-year. If you want "not anything like weeks", then you'll need some handwavium, as < a couple light-weeks is likely to be significantly too close. Given you want a blue supergiant, you also need to consider luminosity, which can be 6 orders of magnitude more than the Sun, which will make it extremely noticeable (e.g. Rigel, at 863 light-years, is the 7th brightest star at night). Jan 21 '19 at 18:56

What makes a couple stars a binary system is the both of them having a stable orbit around a common barycenter. If they don't share that, then they are not a binary no matter how close they are. They will be doing a flyby on each other and will spend years to millenia close, depending on relative speeds.

As Alex said in the question comments, any star closer than a couple light years to the sun would disturb the Oort cloud. The practical consequence of that is a comet bombardment on every planet. Other than that, check this table of the closest stars to the sun and their distance to us over time:

You can use this for inspiration. Those stars are probably the ones we are going to visit first if humanity doesn't destroy itself before we go interstellar.

Take into account that stars orbit the galactic core just like planets orbit stars; but their orbits are much more varied. Think about the timespan of your story. Over dozens of millions of years, stars have moved so much relative to each other that you don't get to see the same constellations. A star that is close today may be too far in the future and vice-versa. But for a timespan of millenia they should stay put, figuratively.

• Do all star systems have Oort clouds? If not, I'm not sure using that as a constraint on an answer is a good idea. Jan 21 '19 at 19:07
• @TracyCramer This is not currently known. See here. Nevertheless, the size of the Oort cloud does give us the order of magnitude of the Sun's sphere of influence, suggesting roughly sun-sized stars not part of a binary shouldn't be much closer than that. Jan 21 '19 at 19:30
• The linked source states that a star entering the Oort cloud would cause comet bombardment of the planets, but only after 2 million years or so. So it's possible that a neighboring star system could've gotten well within the Oort cloud and, in the 10's or 100's of thousands of years before it left, a sentient species could become space-faring, colonize this system, and have quite a few millenia left before the system continues on its way and long before either system would get the comets resulting from such an interaction. That could put them at a distance of 1 light year or maybe even less. Jan 21 '19 at 19:35
• @TracyCramer We haven't even proved our star has an Oort cloud. Jan 22 '19 at 0:45
• @tracyCrammer, as curiousdannii posted, the Oort Cloud is still theoretical, but highly accepted. That being said, that if we do have an Oort cloud it would be foolish to assume other stars do not have one them selves. They may very well be intermixing in interstellar space between different star Oort Clouds. Jan 23 '19 at 23:24

If you give a look at the list of the closest stars to Earth (from where the image below is taken), you see that the closest is at a few more than 4 light years.

Since our planet has been not significantly disturbed by neighboring stars to the point of being ejected from its solar system, and considering ours is the only system we have been able to investigate in such fine detail, you might take that as a reasonably upper limit for a safe distance.

• -1: no image sourcing Jan 21 '19 at 14:03
• @dot_Sp0T, the image is contained in the page that I linked
– L.Dutch
Jan 21 '19 at 14:04
• How does the fact that Earth has not been disturbed by neighbouring stars indicate that the distance is a bottom limit? Hypothetically there could be other, nearer stars that still leave Earth's orbit intact. Jan 21 '19 at 14:19
• @zovits hypothetically there could be a lower limit, but the data this answer is based on does not display any lower limit - thus the mentioned distance is the lowest limit that can be assumed... Jan 21 '19 at 14:22
• Technically what this answer gives is an upper limit on the minimum safe distance. That is, we don't know what the minimum safe distance is, but we know it is less than 5 light years, because there are stars closer than that. The minimum safe distance seems to be 4.24 ly or less based on the data. Jan 21 '19 at 14:54

## It Depends on Your Stars

Escape velocity, $$V_{escape}$$, = $$\sqrt{{2 G M}\over{r}}$$.

G is the gravitational constant ($$6.67 \cdot 10^{-11} {{m^3}\over{kg * s}}$$),

M is the mass in kilograms of the primary body ($$1.989 \cdot 10^{30} kg$$ for the sun) and

r is the distance.

Typically (for stars near our sun) stars are moving in our part of the galaxy at speeds of about 10 to 20 km/s. Which is 10,000 to 20,000 m/s. However, only the relative velocity counts, since we are looking at everything from the perspective of one of the two stars. That being said, I think you are looking for the distance at which an escape velocity is under a kilometer per second.

Plugging in different values for r, I get these values for $$V_{escape}$$ from a star as massive as the sun :

• 1 light-second : 665 km/s
• 1 light-hour : 11 km/s
• 1 light-day : 2 km/s
• 1 light-month (30 light days) : 413 m/s (about the outer edge of Sol's outermost comet cloud)

A blue supergiant, like Rigel, has 23 times the mass of the sun. The escape velocity at different distances would be -

• 1 light-month (30 light days) : 1.9 km/s
• 9 light-months : 660 m/s
• 1.5 light-years : 463 m/s (would not be surprised to find the edge of a comet cloud here)

And a dwarf, like Proxima Centauri, has a smaller mass of 0.1221 Suns. The escape velocity at different distances would be -

• 1 light-hour : 3.8 km/s
• 1 light-day : 790 m/s
• 3 light-days : 456 m/s (expect some sort of comet cloud edge here)

This is additive, so your blue supergiant could be no closer than 1.5 light-years + 3 light-days to the red dwarf

• +1, this appears to be the answer that directly answers the question rather than pivoting to Oort cloud issues. Jan 21 '19 at 18:07
• It looks like the OP edited to include the types of stars. Can you update your answer (if necessary)? Jan 21 '19 at 19:05
• I've added the escape velocities for all three stars in the OP. Jan 23 '19 at 11:07
• By this, a blue supergiant may not have gravitationally captured the main sequence star, so they remain independent, but how would it effect other habitability aspects? 1) I would assume, it would deposit extra energy on the planets, potentially skewing the habitable zone for the time its in close proximity. 2) it would definitely start perturbing the orbits of the outer solar system. 3) the life cycle of a blue supergiant is ~10 million years. This star coming within ~5 light years may be bad for the system because it will definitely have an effect once it goes through its end of life cycle Jan 23 '19 at 23:39
• (1) Virtually no impact. Dropping the Stefan-Boltzmann and blackbody constants from the Stefan-Boltzman radiant power equation (P = A e sigma T^4), radiant power is a function only of A (which is itself a function of r^2) and T^4. Rigel is only about twice the temperature of the sun. At the edge of the Oort (1 light-month), the Sun emits about 1E-15 (whatever the units are if I drop the constants). At 6 light-month's distance, Rigel's power output is 1E-16 (already colder). And that is far inside the 1.5 light year gravity boundary. Jan 24 '19 at 11:24

The Traveller RPG assumes star systems are approximately 3 light-years apart (one parsec to be accurate, about 3.2 light years). The wiggle-room is roughly plus or minus a light year. This allows sufficient buffer space to (1) fit the interstellar "jump" drive scheme for the game itself, and tangentially (2) not interact with others' gravitational fields.

Here is a link to another question and answers with a similar topic:

You could get other stars quite close, within a tiny fraction of the distances of the nearest stars to our solar system, without disturbing the orbits of planets.

But a nearby star at the distance of the Oort Cloud of a solar system would disrupt the orbits of millions of comets and send some of them into the inner solar system to bombard planets causing multiple extinction events. Intelligent life on a planet would have to have a very active defense against incoming comets. So they would have to develop advanced interplanetary travel before the comets started arriving.

And if the stars were packed as close as in globular star clusters or the galactic core, any habitable planet would have been irradiated and sterilized by a number of nearby nova and supernova explosions during the billions of years it would take for intelligent life to evolve on that planet.

Perhaps super powerful aliens created a force field generator to stop the dangerous radiation from nearby novas and supernovas and then seeded the planet with life that eventually evolved intelligence. And perhaps the natives find and accidentally turn off the generator and then their astronomers say a nearby star is about to go supernova.