I know next to nothing on black holes/star formation and astronomy in general, but isn't this a big deal? I thought a pretty big defining factor of black holes was that nothing (not even light!) ever escaped it?
> I thought a pretty big defining factor of black holes was that nothing (not even light!) ever escaped it?
Nothing ever escapes from inside the hole's horizon. But there can still be a lot of interesting things happening outside the hole's horizon as matter either falls in, or orbits the hole, or some combination of the two; and we can certainly observe things happening outside the hole. That has been known for decades.
I wasn't including Hawking radiation because, however likely it is considered, it's still not a confirmed prediction, and the theoretical basis for it is not rigorous (since we don't have a consistent theory of quantum gravity to back it up, just various heuristic arguments). (Also, even if it does exist, it is negligible by many, many orders of magnitude in the scenarios under discussion here.)
Also, if Hawking radiation does turn out to be confirmed, it might end up that the radiation does carry information from inside the hole. The quantum effects that are involved in Hawking radiation, at least as we currently understand them, violate the conditions that ground the "no information can escape from a black hole" theorems in classical GR.
You can orbit a black hole like any other object, and most things do, for a long, long time. Hell, the whole milky way is basically orbiting a black hole (and a lot of other mass near the center).
I recently learned about delta-V as a concept. Flying directly into the sun, for example, is basically not possible for a spacecraft with today’s technology, without dozens of gravity boosts. You need to basically undo the speed of your initial orbit, which for something in orbit around a star is huge!
A black hole would be even more difficult and require more Delta-V to fly into, if you’re in any sort of orbit. So you should definitely expect tons of stuff in orbit around them!
No.
For the same reason all matter in the solar system is not doomed to get sucked into the sun.
A black hole is just a very dense objet things can fall into if they have the right trajectory. But more often than not, the trajectory will be an hyperbola or a parabola around it, and the matter don't reach the event horizon.
Might help to think of it as more of a reprieve. In that the star itself or its remains will still likely end up inside the black holes Schwarzschild radius never to be seen again except as Hawking radiation till the BH evaporates.
That the environment directly outside a BH is energetic enough fling some stuff "up" does not mean the stuff can't/won't fall back "down".
In this case the stuff flung "up" happens to have the necessary properties to trigger star formation further away, which is mostly an incoming shock wave and an preexisting cloud of "cold" stuff.
The stuff the BH is throwing in its shock wave is not going to be "cold".
So to facilitate star formation the shock wave has to be less hot / less dense than and maybe slower than a larger BH hole would produce (which would more typically shred the cold cloud to tatters instead of causing it to collapse in on itself precipitating a star)
In General Relativity (which is the theory we have to use if we're talking about black holes), gravity is not a force, it's spacetime curvature. The spacetime curvature at any point in spacetime depends only on the matter in the past light cone of that point. Heuristically, "gravity" travels at a finite speed in GR. But there is matter and energy everywhere in the universe, so there is spacetime curvature everywhere; there is no place you can go to "escape" from its effects.
So thinking of gravity as a Newtonian "attraction over infinite distance" doesn't really work in GR, but neither does thinking of it as a "force" with a finite range. It works differently from either of those.
Thinking in continuous terms, it would be infinite and inversely proportional to distance squared. But thinking in terms of a distortion in space-time and also considering that it may be quantized, perhaps there is some limit. Also for points that are separating faster than the speed of light due to inflation gravity couldn't alter that space.
> for points that are separating faster than the speed of light due to inflation gravity couldn't alter that space.
This is not correct. The effects of inflation on spacetime geometry are a form of "gravity" as far as GR is concerned. They're just not a form of "gravity" that you could ever get out of the Newtonian approximation.
I don't see how this could work. Say there's a significant increase in mass at a point in space A. And there's another point B that's where the distance A-B is separating faster than the speed of light. How could any changes in mass at point A alter the spacetime/gravity at point B? Are we presuming a means of propagating changes faster than light via graviton?
Of course, nothing can have infinite range in an infinite Universe, because the infinite sum of gravitation fields of an infinite number of objects can create infinite force at any point of the Universe.
At some range, any field will decrease to below thermal noise of medium.
This has to be a question someone here might be able to answer.
Is there some mass-distance(-velocity?) function
that defines the alternative fates?
Where below some threshold, things are fated to merge and beyond which ...not so much.
I am pretty confident all the mass on the earth would be expected to end up together, and would be comfortable imagining that theoretical planets on opposite sides of the observable universe may never merge but what is the transition point? the solar system? the milky way? the local group?
Systems that are gravitationally bound will stay that way, yes. (At least as long as a "big rip" scenario does not happen; our best current observations say it won't, but the error bars don't completely rule it out.) Generally speaking, the boundary in our universe seems to be around the galaxy cluster level: galaxy clusters are the largest systems that are generally gravitationally bound (although often quite loosely, so that objects near the outer edge of the cluster might fairly easily acquire escape velocity through random encounters with other objects). That corresponds roughly to a distance scale of a few tens of millions of light-years.
I don't think this finding challenges any of those assumptions. I think the explanation was that the relativistic jet for smaller blackholes located at galaxy centers is moving slow enough that the compressive effects of the jet are not overcome by the speed. The compression therefore helps with star formation since the jet isn't moving fast enough to disrupt that process.
It is definitely an unusual observation, but I don't think it's really that big of a deal. A strong gravitational field would pull matter closer together, possibly kick-starting the fusion process if the energy density gets high enough in a certain region. If you asked an astrophysicist what would happen to a massive dust and gas cloud near a black hole, star formation would be 2 or 3 on the list that they rattle off.
> But the gentler outflow of gas from the black hole in Henize 2-10 is compressed just enough to facilitate star formation.
It's a Goldilocks thing. Black hole is strong enough to have a significant gravity, but weak enough that the shell of hot dense matter it harbors doesn't completely obliterate everything that falls toward it.
No it’s not a big deal, they just mean that far away from the black hole its effect stirs the pot a bit in a way that triggers star formation. Interesting to observe but also a pretty obvious consequence of a black hole being around.
