dungeon blaster said:
Let's say you had a binary star system with a yellow sun like ours revolving with a black hole. Then you had a planet revolving around the both of them. First, is this physically possible? Would the planet be forced into a highly elliptical orbit? What would it look like from the planet inhabitants point of view if the the sun and black hole rotated? Would the sun and black hole need to be of equal mass (and therefore hole would need to be quite small)?
Physically, yes, it's possible, but it has a number of problems.
- To maintain a reasonably normal orbit around the common center of gravity, it would have to maintain a pretty good distance from the star; probably enough so that it would be frozen and lifeless.
- Stars that are partnered with black holes have short life-spans. The black hole usually sucks material from its stellar partner like a vacuum cleaner. A side effect of this is also a disk of supercharged material that is slowly spiralling into the black hole which end up being some of the strongest sources of x-rays we know of. Again, the radiation from this system would probably render any planets lifeless.
- Black holes are the cores left over from massive stars that go supernova. If the black hole was always part of the system, it would already have blasted any planets lifeless, if indeed, any planets could maintain an orbit following a supernova. Of course, the planet coul have been captured after the fact, which is probably the only option. But that beggars the question; where did the planet come from and how did life appear on it?
- If the black hole is a migrant into the system, it would have perturbed the orbits of any planetary bodies on it's entry into becoming the star's companion.
There's some potentially interesting answers to the questions raised by the last few points there, but that first few points are difficult. You're better off having it not be a yellow, sun-like star, but rather a more massive A or F class star, which will burn hotter, and thus potentially heat a planet at a farther distance. Have the planet be the latest migrant into the system, to explain how it has a stable orbit at a safe distance, give it one hell of a magnetic field to protect against radiation, and an intriguing backstory of alien (or godly, depending on if you want to lean more towards sci-fi or fantasy) intervention on the development of life, and you're more or less good to go.
They don't have to be the same mass, and in fact, I'd find it odd that they would be. A black hole has a minimum mass to even form, and a star like the sun can't do it.
Both would rotate around a common center of gravity. You wouldn't actually see the black hole, but you'd see the big disk of gas, which would probably obscure, although be illuminated by, the stellar companion. The old movie
The Black Hole is oddly enough fairly accurate, but make it brighter, have it be out during the day. Heck, given the mass that the planet has to orbit, it's also possible that the planet would be tidally locked; i.e., only one side would ever face the black hole and sun duo, so you'd have a permanent dayside and nightside. That's potentially interesting too, IMO.
The disk of gas and dust can actually be bigger than the orbit of the planet, so the night sky might well be lit by a kind of ambient glow from the lit gas particles all over the system. In fact, and I'm getting myself interesting in this idea now, this could be pretty cool. The gas and dust would probably also obscure the sun during the day; maybe "day" and "night" would be relatively light and dark variations on twilight, without really truly deep darkness or bright day either one due to the ambient glow of the gas in the system.