[pdonis]

Not necessarily. A black hole doesn't have any stronger gravity than an ordinary object of the same mass. And matter falling into the hole would radiate strongly (the paper calls this "accretion luminosity"), which would help to maintain an equilibrium with the rest of the star. That's the kind of model the paper is studying.

How long such an equilibrium can last is a different question. The paper only briefly comments on this when it says that the time scale of the numerical simulations they did is of the same order as the hydrodynamic timescale of the Sun. That means, roughly, the time it would take the Sun to collapse to a white dwarf if fusion reactions in its core stopped, which is, I believe, tens of millions of years. So a star with a black hole at the center would not have the same lifetime as an ordinary main sequence star with similar mass, but it would have a long enough lifetime that would could not conclusively rule out that at least some stars we see have black holes at their centers.

[hinkley]

A black hole at the center of a gaseous body would mess with the fusion cycle would it not? There’s no pressurized core at the center. Just a drain that all pressure exits into.

[pdonis]

> A black hole at the center of a gaseous body would mess with the fusion cycle would it not?

Not necessarily. That's the sort of question the paper investigates, and it finds models for which fusion can continue in the star's core for an extended period of time.

> There’s no pressurized core at the center.

Yes, there is, because, as I noted, the matter falling into the hole radiates strongly, and the radiation has pressure.

[hinkley]

How do you capture a PBH unless there’s substantial, fast mass (momentum) transfer from the star to the black hole? Otherwise the ballistic trajectory would carry it out and through. Wouldn’t you be more likely to find a PBH orbiting a main sequence star? Or gone altogether?

[hinkley]

It came to me while doing some housework that this feels like “what if the moon crashed into the earth?” The moon cannot crash into the earth. Any alien that could make that happen would be so terrifyingly powerful they wouldn’t have to crash the moon into the earth. It would be the twentieth most interesting way to doom us. Tidal waves would be easier.

[pdonis]

While your statement about the ballistic trajectory is true in the short term, over longer time scales (my off the cuff guess is thousands of years, much shorter than the time scale covered by the numerical simulations in the paper), there will be momentum transfer due to the PBH perturbing the star's matter as it passes through, and the PBH will settle into the center of the star if there are no other perturbations (i.e., no other massive bodies near enough to affect the process). The paper doesn't discuss the capture process in any detail, but I suspect that something like that is what they have in mind.

[pointlessone]

I thought infalling matter radiates because of friction in accretion disc. Does a similar structure form in a much denser interior of a star?

[pdonis]

Matter falling into a spinning black hole surrounded mostly by vacuum forms an accretion disc. But friction and heating and radiation will occur in infalling matter no matter how it is falling in. A black hole inside a star would probably not just have an accretion disc, it would have accretion happening in all directions. But the accretion would still involve friction and heating and radiation.

[hinkley]

Would there be a disk at the center of a star? Not a lot of angular momentum there.

[Sharlin]

It's very very difficult to "drain" anything through a molecule-sized hole.

[andrewflnr]

FWIW, the sun's schwarzchild radius is more like inches than Angstroms. Still small compared to the sun, but not that small.

[pdonis]

> the sun's schwarzchild radius is more like inches than Angstroms.

No, it's not inches, it's about 3 kilometers.

But the holes at the center of stars that the paper is talking about have tiny masses, much, much smaller than those of the stars they are inside. Their schwarzschild radius could indeed be of the order of Angstroms.

[andrewflnr]

Oops. I didn't remember it being that much bigger than Earth's.

[Sharlin]

The article is not talking about solar-mass BHs but ~asteroid-mass ones, with a Schwarzschild radius on the order of nanometers to micrometers.

[hinkley]

How do you catch a molecule sized black hole? Maybe in stellar nurseries? But then the black hole has been feeding for a very long time. Yes?

[jiggawatts]

It would be like a pinhole leak. A large enough container can maintain pressure even with a few small holes letting the contents slowly stream out.

[joot82]

it would only be a pinhole at first, though start to grow quite rapidly(? no idea in what time scale?) and at some point consume the sun quite violently from within, no?

[pdonis]

> no idea in what time scale?

The numerical simulations in the paper go on for a time on the order of the Sun's hydrodynamic time scale, which is tens of millions of years. After that time has elapsed, yes, the star could be completely consumed by the hole.

[hinkley]

Tens of millions is off by more than two orders of magnitude, isn’t it?

[pdonis]

The hydrodynamic time scale is not the same as the age of the Sun.

[the8472]

https://en.wikipedia.org/wiki/Quasi-star also mentioned in the paper, powered by larger black holes

[LorenPechtel]

It's not stable, it's that the consumption rate is low enough the star can live quite a while.