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.
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.
> 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.
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?
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.
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.
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.
> 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.
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?
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.
"For many readers, intuition from astrophysics
will suggest that a star that captures a PBH will
be short lived and look nothing like a star dur-
ing that life. However, we will show that stars
with very low mass PBHs could be very long lived
with many surviving their entire main sequence
phase. Ultimately the evolution is highly sensitive
to the accretion physics, which is the subject of
the following sections."
I hadn't known this until recently, but it's theorized that in the early universe there were super huge stars that had black holes in them - https://en.wikipedia.org/wiki/Quasi-star
It sounds like you’re conflating time dilation with the concept of light being unable to escape once it crosses an event horizon.
Nothing is actually “frozen” around a black hole, but if you accept that light cannot escape once it passes an event horizon then it follows that there must have been one final moment when light still could escape. The light that was able to escape in that final moment would reach your eyes as a “frozen” image of the object where it previously was the exact moment before gravity became too much to overcome.
What's mind bending to me is that a second object just short of that point _also_ sees the first object frozen. It doesn't matter how close you get to the horizon, the image of an object that got there first is still infinitely far in your future.
Yeah, that really is so neat to visualize. That there’s a hard line when the escape velocity reaches exactly the speed of light, and poof. Frozen image of the past right in front of your eyes.
And think… once we crossed the event horizon as observers ourselves (leaving a frozen image for observers behind to see), wouldn’t we see the “tracers” of images the person before us left behind every moment we move closer to the singularity? Edit: no… we never would see any light (in front of us) again by definition when crossing the horizon, duh lol.
When referring to black holes, "falling in" means going past the event horizon. For all practical purposes for us on the outside of the singularity, this is "having fallen into the black hole" as any object is gone to us forever once having done so. We don't use falling in to mean touching the singularity, which as you noted, does indeed take infinite time. Using the definition this way isn't particularly useful.
This is my understanding: It takes an infinite time to cross the event horizon from the perspective of a distant, stationary observer, but a finite time from the perspective of the object that is actually falling towards the black hole. Once past the event horizon, reaching the singularity takes a finite amount of time from the perspective of the falling object. From the point of view of a distant external observer, time from event horizon to singularity is a meaningless question because the events inside the event horizon are causaully disconnected from the events outside of the event horizon.
Your explanation is wrong. For an outside observer, time dilation approaches infinity at the event horizon. Singularities don't even exist for an outside observer, they have yet to happen in the infinite future.
Is that kind of the same as the idea that if you are approaching a wall but each step is only 50% of the rest of the way to the wall, so you would never reach the wall no matter how close you got?
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.