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I've been imagining an RPG universe consisting of eight solar systems orbiting around a black hole in a finite universe with an edge, and I'm wondering what the night sky would look like to beings inhabiting a world in one of those solar systems.

My thought process is that for half the year they would see the actual stars, and for the other half they might see, 'stars' in the form of other planets in their system. However, when it comes to the half of the year where they do see actual stars, I'm not really sure how many they would see.

How many phantom stars could gravitational lensing around the black hole create? Would the other stars, assuming they're roughly all identical to our sun, create any phantom stars? Thanks.

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  • $\begingroup$ This setup would not be stable at all. How big are the stellar systems, by the way? $\endgroup$
    – HDE 226868 ♦
    Sep 13, 2015 at 17:33
  • $\begingroup$ What size of black hole? Stellar? Galactic? $\endgroup$
    – celtschk
    Sep 13, 2015 at 18:10
  • $\begingroup$ Do the systems orbit the black hole in the same plane, or different planes? You also need to specify the distances, masses, and number of bodies (and their masses, initial positions, and velocities) involved. $\endgroup$
    – HDE 226868 ♦
    Sep 13, 2015 at 20:02
  • $\begingroup$ Stellar systems roughly equivalent to our own, orbiting on the same plane at 3x the event horizon of a black hole with 2 billion solar masses. $\endgroup$
    – OnionDruid
    Sep 13, 2015 at 21:03

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Assuming solar systems similar to ours orbiting a massive black hole (similar to the one in the movie Interstellar) then you would have some effects, a lot which would depend on how close you are to the black hole itself. For the solar systems to remain stable over eons of time, they will need to be orbiting at a great distance (with a period measured in thousands of years).

The night sky would actually be rather similar to what we see here on Earth, until you look in the direction of the black hole itself. The black hole will be noticeable because it will bend starlight around itself forming an "Einstein ring", which will be pretty mind bending for even primitive priests and shamen doing unassisted night sky astronomy.

Einstein ring

If the black hole is active, then things will be even more exciting from a visual perspective. Infalling matter will tend to orbit around the black hole at ever increasing speed. As the matter accelerates it also heats up, eventually becoming a visible disc of matter (and even energetically emitting high energy radiation.) If the infall is great enough there might even be beams of radiation coming from the poles of the black hole.

Accretion disc

The last effect will be gravitational lensing, which will occasionally bring different distant objects into focus. The various solar systems might actually become briefly visible as they pass in the focus of the black hole (this is related to the Einstein ring, and you will not be getting a "naked eye" view of the other solar system, but a telescope will be able to make out more details as the other solar system or object passes into the focus of the gravitational lens).

So there will be some visible effects, but not as spectacular as you might see in a Hollywood movie.

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  • $\begingroup$ Maybe the planets cannot revolve too far from the black hole as the gravitational pull of a black hole, though being severe, is quite limited in scope of it's range. That's one reason why they don't gulp down everything around them upto far distances. $\endgroup$
    – Youstay Igo
    Sep 14, 2015 at 9:56
  • $\begingroup$ @YoustayIgo Err, the gravity isn't limited in any way. It simply decreases with distance. $\endgroup$
    – HDE 226868 ♦
    Sep 14, 2015 at 16:22
  • $\begingroup$ * shrug * maybe. I am a big astronomy fan and I have never read about any solar system orbiting directly around a black hole, let alone 8 of them. Plus, since all of them are revolving around the same black-hole, so all of the planets would be regarded as ONE solar system, no matter what their number, inter-planetary distance or size. $\endgroup$
    – Youstay Igo
    Sep 14, 2015 at 16:54
  • $\begingroup$ @YoustayIgo Since "solar" refers to a sun-centric system, there could be eight solar systems orbiting a black hole. There's a black hole at the center of the Milky Way, and there's more than one solar system in this galaxy. $\endgroup$
    – Frostfyre
    Sep 14, 2015 at 17:22
  • $\begingroup$ Hmmf. This makes some sense now. So the solar systems each have their own sun and they are (as a whole) revolving around the black-hole. It would help if OP specifies whether all solar systems are coplanar or the system is three dimensional. $\endgroup$
    – Youstay Igo
    Sep 14, 2015 at 17:26

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