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Could a Star orbit a rotating black hole inside the ergosphere? If so, how big should the black hole be? I imagine it should be absolutely massive so that tidal forces are minimal. And if all this is possible, are there any special effects on the Star apart from usual time dilation?

Stellar_Enginner
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    I saw a paper published several years ago claiming to find a stable orbit inside a black hole (I believe a Kerr black hole). They even speculated a planet could survive there in theory. I assume the region was between horizons where geodesics do not have to be radially inward – RC_23 Jul 24 '23 at 04:17

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Stellar_Enginner aked: "Could a star orbit a rotating black hole inside the ergosphere?"

The innermost stable orbit in the prograde direction of a black hole with spin a=1 is at the horizon, which is well below the ergosphere. The higher the spin, the closer to the horizon it gets.

Stellar_Enginner aked: "If so, how big should the black hole be?"

A supermassive black hole like those in the centers of galaxies has neglible tidal forces; the larger, the better. A stellar black hole won't do the job, since those themselves are much smaller than a planet and therefore have too strong tidal forces, let alone the fact that the whole planet or star wouldn't even fit inside its ergosphere. Then the star would get torn apart and end up as an accretion disk.

Yukterez
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  • There is a kind of trivial condition that the outer part of the star needs to be inside the ergosphere, so (ignoring tidal distortion) for an extremal black hole it needs to have a diameter less than half of the non-rotating Schwarzschild radius. For a 1 AU size supermassive black hole this is not too cumbersome. – Anders Sandberg Jul 24 '23 at 11:02