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Say you have a galaxy, possibly ours, with a central black hole. In an instant the black hole falls through a plot hole and vanishes.

What happens to the rest of the galaxy?
Does everything keep on like nothing happened?
Does it slowly unravel?
Does it quickly unravel?
Something else?

Why this question?
In the Void Trilogy by Peter F. Hamilton, an artificial black hole at the center of the galaxy is suddenly removed. In the story, nothing much happens, and it seemed weird that no one seemed to think it was much of a big deal.

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AndyD273
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    Well if its mass is no longer there, the galaxy would fly apart. But very...very...slowly. It would take 50,000 years for the systems along the edge to even notice. – Draco18s no longer trusts SE Jan 08 '16 at 19:03
  • @Draco18s Thats what I was thinking, though someone else told me that everything would just keep orbiting the center of galactic mass. I didn't have enough information to argue the point. – AndyD273 Jan 08 '16 at 19:06
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    Both are right. Most objects in the galaxy will continue to orbit, but some will be moving at the (newly lowered) escape velocity for the galaxy (at their particular distance from the center), and fly off. The galaxy would become more diffuse, though, as even objects remaining in galactic orbit will take a larger/longer orbit than they did before the subtraction of galactic mass. – SudoSedWinifred Jan 08 '16 at 19:11
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    Everything outiside the "light cone" of the black hole's disappearance would continue orbiting it. Gravity propagates at the speed of light, so things are still pulled towards it until that propagation reaches the object in question. The black hole would poof, the nearby stars would fly off, and the rest of the galaxy would slowly catch up. Cort is likely correct that largely speaking nothing would notice (due to the relatively slight mass difference), but the effect over what time scale is a bit fuzzier. – Draco18s no longer trusts SE Jan 08 '16 at 19:11
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    Boy, I really wish we could try this... We don't need Andromeda, right? – AndyD273 Jan 08 '16 at 19:39
  • I won't say it's a good riddance but we're still in a collision course with Andromeda. – user6760 Jan 09 '16 at 09:00
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    Note that the one in our galaxy is pretty small. In a more typical spiral galaxy that would have a more profound effect. – JDługosz Jan 10 '16 at 05:12
  • @JDługosz I wonder if there is a way to determine how much of the core would need to fall into the plot hole to make a difference here. I'd love to see this as an answer – AndyD273 Jan 10 '16 at 05:26
  • Must say that would be one very appropriate usage of a plot hole. – Selenog Jan 11 '16 at 13:06
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    Well for one thing, the Pierson's Puppeteers could go home and open up General Products again... – VBartilucci Jul 13 '18 at 18:26
  • I'm always happy to see some mention of one of my favorite sci-fi authors, PFH! –  Aug 16 '18 at 21:37
  • @VBartilucci How else would we get our (not quite) indestructible ships and (almost) fail-proof technology? – In Hoc Signo May 30 '20 at 19:58
  • @VBartilucci At first I agreed with you, but they're fleeing supernovas, not the black hole. Also, I don't think they would go back even with the core explosion gone, they would fear discovery. On the other hand, I never understood why they went out of business. – Loren Pechtel May 31 '20 at 03:38
  • @LorenPechtel They didn't go out of business, they just shut down their operations. Big difference. – In Hoc Signo Jun 15 '20 at 22:10
  • @TheDaleks Huh? They ceased selling GP hulls and I can't see a reason for it. – Loren Pechtel Jun 16 '20 at 00:37
  • @LorenPechtel In-universe they were leaving the galaxy, which was putting more and more distance between them and Known Space. Since hyperspace travel is (relatively) slow, their ability to fulfill warranty claims decreased proportionally. Puppeteers always fulfill their contracts, so they were forced to stop selling stuff in order to keep up with warranty claims. – In Hoc Signo Jun 16 '20 at 00:55
  • @TheDaleks They were leaving by normal space. They were going to be close enough to human space to continue to provide warranty service for a long time to come. – Loren Pechtel Jun 16 '20 at 01:45
  • @LorenPechtel Due to the durability of GP hulls, it usually took after the initial sale for warranty claims to start coming in. Also, I think it is worth noting that their primary purpose in running GP was to acquire the necessary funds so that they could exert political influence. Since they were withdrawing from politics they did not need those funds anymore. – In Hoc Signo Jun 16 '20 at 11:42
  • @TheDaleks Good point about the lack of need. The time for warranty claims is not a problem, the Puppeteers that do it have to travel by hyper anyway, it will be a long time before the Fleet of Worlds is far enough away the hyper time matters. – Loren Pechtel Jun 17 '20 at 14:41

4 Answers4

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Not all that much

Sagittarius A* is big, but not that big. Its mass is estimated to be around 4,200,000 (four million two hundred thousand) solar masses. That's a lot of gravity! But consider the Milky way is estimated to be around 1,000,000,000,000 solar masses! In all, the total gravitational effects would be minimal. The largest effect would be on stars near the center (for whom most of the gravity from the Milky Way's stars cancel out rather equally, so they feel mostly the pull of the center). However, once you get a short distance away from the center of the galaxy, the effect of Sagittarius A* herself is actually quite a small player in the grand scheme of things.

Of course, you ask if anything unravels. Certainly physics just unraveled, right through your plot hole. Many years later, some intelligent species might notice that something funny happened.

EDIT: type_outcast was kind enough to work through the numbers to see how fast a star would have to be orbiting to achieve escape velocity of the galaxy, initiating an "unraveling" like effect. He used the escape velocity equations, $v_e=\sqrt{\frac{2GM}{r}}$, where M is the mass of the galaxy and r is the distance between the center of the galaxy and the escaping star. For a reasonable star, like S0-102, which is close enough to the center to be noticeably effected by the loss of nearby mass, that escape velocity was over half the speed of light! This means, unless the star is traveling at relativistic speeds already, it will not escape the galaxy. Thanks type_outcast!

EDIT: This questions is actually quite fascinating if you think about it. An entity labeled "super-massive black hole" vanishes from existence, and we hardly even notice because the galaxy is just that mind-numbingly big! I figure this might be a good chance to plug the Universe Factory, the WorldBuilding.SE blog, which has an article on why it can be so hard to fathom these scales. It's worth a read, if I may say so myself!

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Cort Ammon
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    This is the right answer. Only objects close to the center would notice the absence. The mass for everything outside of a given object's orbit cancels out (it only sees the mass inside its orbit), so a few objects close to the center would be greatly affected. But for everything beyond a certain range (probably calculable - but I'm not doing it), the difference would be negligible. – Jim2B Jan 08 '16 at 19:17
  • By unravel, I'm picturing the stars in the immediate vicinity of the BH suddenly having nothing to orbit, and so proceed to head out in a straight line. As they leave, the void is now slightly larger, and more stars have nothing to orbit, and so on. This is where I'm wondering if I'm missing something – AndyD273 Jan 08 '16 at 19:20
  • When you say 'planets near the center', you mean stars, right? And what exactly would happen to them? If all the stars in the center of the galaxy start acting oddly, wouldn't the stars a little bit further away be affected by it? Wouldn't there be some sort of chain reaction? (EDIT: I see I was ninja'd, but my first question still stands.) – DaaaahWhoosh Jan 08 '16 at 19:21
  • Planets and stars orbit around centers of mass, not entities themselves. You may have removed the black hole, but not all of the mass. The stars near the center would go roughly straight out, until they were far enough out to be in a well described orbit around the galactic center. It may be a rather eccentric orbit, but it would be an orbit. – Cort Ammon Jan 08 '16 at 19:25
  • @DaaaahWhoosh correction made. Good catch. Silly me and my solarcentric viewpoint =D – Cort Ammon Jan 08 '16 at 19:26
  • For exploring cases like this, it may be worth pointing out that the sun is effectively in orbit around the center of mass of the solar system, which is close enough to the center of mass of the sol-Jupiter system that we can get away with claiming the sun is orbiting around the center of mass between the sun and juipter. – Cort Ammon Jan 08 '16 at 19:27
  • Ahh, and one more edit, the outer stars would be effected, but negligibly. For every star that is moving away due to the loss of the black hole, there will be one moving towards you, so the center of mass of the entire galaxy wont change much. – Cort Ammon Jan 08 '16 at 19:30
  • What about the gravity relationships among the star systems themselves? This is an n-body problem after all. I'd be interested to know about those finer details. – The Anathema Jan 08 '16 at 19:32
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    @TheAnathema In theory, every orbital mechanics problem is a n-body problem. In theory every object is influenced by the gravitational pull of every object within its light cone. Every star in the milky way will get affected once light propagates far enough. However, for all intensive purposes, we can often solve an approximation with far fewer details. For the bulk of the galaxy (say 99.999% of the galaxy, give or take), we can approximate it as though the stars were orbiting the galactic center of mass with acceptable fidelity. – Cort Ammon Jan 08 '16 at 19:34
  • You really lost me with the Sun/Jupiter thing... So, if the sun was removed the planets would keep going around in their orbits (though very frozen of course)? – AndyD273 Jan 08 '16 at 19:35
  • For the remainder, we'd have to solve the n-body problem to really know for sure, but consider that the "worst case" is that they escape outwards. As they escape outwards, the approximation of "orbiting the center of mass of the galixy" will become more and more valid. – Cort Ammon Jan 08 '16 at 19:36
  • @AndyD273 The full version is the n-body problem, as TheAnathema points out. However, we can approximate the sun/planets as "everything orbits around the center of mass of the solar system," which is roughly the center of mass of sun+juptier, because everythign else is so much smaller than that. If I may pull in some notation now, each planet has a set of Keplarian coefficients which describe their orbit. You can always calculate Keplarian coefficients from any position and velocity (this also means everything is technically in orbit!). The most meaningful coefficients right now are those – Cort Ammon Jan 08 '16 at 19:38
  • describing the orbit around the center of the mass of the solar system (or roughly the center of the jupiter/sun center of mass). However, if the sun vanished, we could easily calculate Keplarian coefficients around the new center of mass of the solar system (which is basically Jupiter). The rest of the planets would orbit around Jupter. These orbits would be very large, because Jupiter is much smaller than the sun, and they'd be really eccentric because they weren't going in the right direction around Jupiter, but they would orbit. – Cort Ammon Jan 08 '16 at 19:40
  • This model would hold up unless the orbits of the other plants get to be so wide that the effects of other stars start to come into effect. For example, if Pluto ever passed close to Alpha Centauri, Alpha Centarui would "Capture" it... but ignoring those bigger picture effects, an orbit around Jupiter (or really close to the center of Jupiter) is exactly what you'd get! – Cort Ammon Jan 08 '16 at 19:41
  • (Pluto getting "captured" by Alpha Centauri is an example of one of those n-body mechanics TheAnathema mentioned. In that case, the oversimplified model of "everything orbits the center of mass" breaks down because my assumptions start to get ugly. However, if we start playing with that question, you start having to ask "which year did the sun vanish," because the results will actually differ based on the location of the planets at the time it vanishes.) – Cort Ammon Jan 08 '16 at 19:43
  • Ok, That all makes sense now. The center of mass is still probably pretty close to the center of the sun, otherwise orbits would start to get weird pretty quick. I guess that might partly be why our orbit is slightly elliptical? – AndyD273 Jan 08 '16 at 19:44
  • @AndyD273 You made me look up a factoid =) The "barycenter" of the solar system is roughly 500,000km above the sun's surface (which sounds like a lot, until you note the sun is nearly 700,000km in radius!) – Cort Ammon Jan 08 '16 at 19:49
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    @CortAmmon If the sun disappeared, we would go our separate ways. Looking at your given Earth-Jupiter relationship, for instance: $M_J = 1.898\times 10^{27},kg$, and the distance (r) at its closest is 591 million km. That means its escape velocity is $v_e = \sqrt{\frac{2GM_J}r} = 0.65,km/s$, but the Earth's orbital velocity is $30,km/s$. Since that far exceeds the Jovian escape velocity, in an otherwise empty universe, the Earth would follow a parabolic path away from Jupiter, forever. (Technically it would approach zero relative velocity as $t \to \infty$, never returning.) – type_outcast Jan 09 '16 at 01:16
  • @type_outcast blast, you're right about that. I failed to take that into account. Fortunately, that will only affect the solar system discussion we've had in comments. In the real answer, I think the logic will still apply because there will be other stars pulling things back toghether. – Cort Ammon Jan 09 '16 at 01:19
  • Yep, no problems with your bigger picture logic! To wit, I did that math in a comment to João da Silva's answer. Feel free to steal it for your answer if you want! I'm not going to answer this question, as you've already done a fine job. – type_outcast Jan 09 '16 at 01:48
  • @AndyD273 I think you've spotted a real problem. The void in the center will grow, and the whole galaxy disintegrate within the next ten million years. Depending on the size of the vanished mass, and the spatial inhomogeneiety of the galaxy, local fluctuations will however at some point stop the process. – Karl Aug 16 '18 at 20:50
  • @Karl No, because they do still have something to orbit: the center of mass of the galaxy. But the orbit around that will have a much larger radius than the orbit around Sag.A*. The stars that were orbiting close in will shift to different orbits, but as far as anything far away is concerned, the specific orbits don't matter. The solar system, for instance, can be treated as though it's orbiting around a point mass at the center of the galaxy with a mass equal to the total mass of everything in smaller orbits than we are. The stars that shift around will still contribute to that. – Ray Apr 08 '19 at 20:34
  • @Ray No. The outer parts still feel the remaining gravity, but the innermost stars not. If you climb into a hole, all the earth mass further away from the center than yourself does not contribute to you local gravity. – Karl Apr 11 '19 at 21:20
  • @Karl True, though the effects should be rather negligible. If I am reading my own answer correctly, something like 0.00001% of stars will be close enough to the center of the galaxy to see a noticeable change in gravitational pull. Those stars will certainly leave their existing orbit, though they will not achieve escape velocity from the galaxy. – Cort Ammon Apr 12 '19 at 02:27
  • @Karl The innermost stars would keep moving at whatever their instantaneous linear velocity was at the moment Sag A* disappeared. But eventually they would get far enough out that there's enough mass closer to the center than they are to cause them to reenter some orbit. While there would then exist a new set of innermost stars that don't have anything pulling them into an orbit, they have a much smaller linear velocity, so their orbits would shift by much less. Eventually, stars would stop moving into larger orbits. (continued...) – Ray Apr 12 '19 at 05:00
  • (...continued) Once we reach that point, anything that started out significantly farther away from the radius of that orbit will be largely unaffected. And since in most galaxies, the black hole doesn't make up that large a percentage of the mass, the radius shouldn't be too far out from the center. As for what happens to the stars inside that radius, we'd probably need to actually approximate the n-body problem to say for sure. But it seems likely that the barycenter of all the stars that shifted orbits would still be somewhere near the center of mass of the galaxy. – Ray Apr 12 '19 at 05:03
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The answers so far have assumed that the galaxy in question is a spiral galaxy - and if we're talking about the Milky Way, then that's all well and good. But galaxies are pretty diverse, both in shape, size, mass and composition. Most look nothing like our own. It turns out that if you're willing to set your story in a different galaxy, you can get some pretty interesting effects from the removal of a large black hole.

I'll look at the ratios between the mass of a certain black hole in a galaxy/star cluster and the mass of the galaxy itself: $M_{\text{BH}}/M_{\text{galaxy}}$. For reference purposes, the black hole at the center of the Milky Way, Sagittarius A*, has a mass of $\sim4\times10^6$ solar masses, while the Milky Way itself has a mass of $\sim1\times10^{12}$ solar masses, giving us $M_{\text{BH}}/M_{\text{galaxy}}\approx0.000004$. That's small; removing Sagittarius A* from the Milky Way won't do squat.

Globular clusters and intermediate-mass black holes

Globular clusters are dense, gravitationally bound sets of stars, gas and other objects, usually of around . They're usually quite old - in the case of the Milky Way's globular clusters, as old as the galaxy itself. Now, what's interesting for our purposes is that there's not really a firm dividing line between certain globular clusters and dwarf galaxies, which may contain up to $\sim10^8$-$10^9$ solar masses. In fact, a few globular clusters, such as Mayall II and Omega Centauri, may contain intermediate-mass black holes, a putative class of objects with masses of up to $\sim10^6$ solar masses.1

In the case of Omega Centauri - where the existence of the black hole is disputed - the maximum mass is $\sim10^4$ solar masses. The mass of the globular cluster itself is $\sim4\times10^6$ solar masses, meaning $M_{\text{BH}}/M_{\text{galaxy}}\approx0.0025$. Mayall II gives a ratio that's roughly the same, maybe a bit lower. If the black hole in one of these two globular clusters was removed, it would influence the orbits of the innermost stars. This is perhaps more dramatic than in the case of a normal galaxy, because globular clusters have density distributions strongly peaked towards the center. In other words, yes, many orbits would be disrupted, although I doubt that it would be enough to disrupt the cluster. Remember, the mass ratio is still less than 1%.

Massive elliptical galaxies

Some supermassive black holes have masses on the order of $\sim10^9$ to $10^{10}$ (1 billion to 10 billion) solar masses, three of four orders of magnitude greater than Sagittarius A*. These black holes yield much better mass ratios than smaller supermassive black holes. One issue, unfortunately, is that some of these ultra-high mass supermassive black holes are found in very massive elliptical galaxies, which can be up to several trillion solar masses in size.

Consider NGC 1600. Its central supermassive black hole likely has a mass of $\sim2\times10^{10}$ solar masses, while the galaxy itself has a mass of $\sim10^{12}$ solar masses. That's not bad; we get a mass ratio of $M_{\text{BH}}/M_{\text{galaxy}}\approx0.02$. NGC 4889, a supergiant elliptical, has a central black hole of similar mass; its total mass is $\sim10^{13}$ solar masses, yielding $M_{\text{BH}}/M_{\text{galaxy}}\approx0.002$ - possibly smaller, if non-luminous matter exists there in large quantities.

Dwarf galaxies and supermassive black holes

Omega Centauri (and certain other high-mass globular clusters) may be the cores of dwarf galaxies, stripped apart by tidal forces from the Milky Way. As I said before, the dividing line doesn't really exist. However, a high-mass dwarf galaxy is certainly different from a low-mass globular cluster.

Now, consider a set of dwarf galaxies called ultra-compact dwarfs (UCDs). Their masses are on the order of $\sim10^8$ solar masses. One UCD that particularly excites me is M60-UCD1. This galaxy has a mass of $\sim10^8$ solar masses, and might house a supermassive black hole of $\sim2\times10^{7}$ solar masses - five times the mass of Sagittarius A*! This leads to a mass ratio of $\sim0.15$, which is enormous! The orbits of many stars in the galaxy - which is only about 200 light-years across - are quite strongly influenced by the black hole. Removing it would certainly disrupt a number of orbits.

There ultra-compact dwarf population continues to grow, as does the population of supermassive black holes in UCDs. It was recently announced that UCD-3, a galaxy with a mass of $\sim9\times10^7M_{\odot}$, likely contains a black hole of $3.5\times10^6M_{\odot}$, giving us $M_{\text{BH}}/M_{\text{galaxy}}=0.038$. This is lower than M60-UCD1 by a factor of four, but that's not much, and it's quite encouraging.

I will say that I don't think you can get any better than this. Compared to the Milky Way, M60-UCD1 is an excellent candidate for this sort of setting. It's also extremely dense, and quite massive for an ultra-compact dwarf. The high density means that, just like in a globular cluster, you can probably find plenty of exotic objects inside, from blue stragglers to Thorne-Żytkow objects.

1 As of July 2018, no intermediate-mass black holes have been confirmed, but there are a number of candidates:

If some of these exist, they could be reasonable decent choices for you. Also, a recent search of Chandra data indicates that there may be a substantial population. I'll update this list if any of these are verified in the future.

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HDE 226868
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Everything's relative. If the force pulling the stars to the center ceases, the stars' orbit velocity will shoot them out from the center in a straight line (obviously - there are no circular forces) but not perpendicular to the center. If you think it does this slowly, well, the Sun is travelling at 720.000kmh. That's fast. Relatively speaking. And the closer to the center, the highest the speed.

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    This doesn't quite tell the whole story. There's enough other mass in the galaxy that the stars will not just fly off on their tangential velocities, but even if they did, 720,000 km/h is fast compared to my '64 Ford Galaxie, but is actually quite slow in Milky Way Galaxy terms: if the tangentially-unbound star keeps going on a straight path, it would take about 76 million years to reach the outer rim of the galaxy. Actually, other masses along the way (and the dark matter that dominates the average density) would have a slowing effect. – type_outcast Jan 09 '16 at 00:46
  • I wonder what the escape velocity of the galaxy is, and if a star was suddenly released, if it would escape, or be pulled back toward the center. If the later, would enough stars make a break for it fast enough to make a difference, or would they become like a cloud of commits around the core area. – AndyD273 Jan 09 '16 at 00:49
  • @AndyD273 That's an easy one, sort of! $v_e = \sqrt{\frac{2\mu}r}$. $\mu$ is proportional to the mass of the galaxy: about a trillion times the mass of our sun ($1 \text{ trillion } M_{\odot}$), and $r$ is the radius (in this case, a tiny fraction of the Milky Way's radius, for stars that are close to the galactic centre like S0-102, far less than 1ly). We get $v_e \gt 0.5 c$, which is not surprising, as the black hole was only $4.3 \text{ million } M_{\odot}$, compared to the total galactic mass of $1 \text{ trillion } M_{\odot}$. S0-102 doesn't orbit that fast, so it would be re-captured. – type_outcast Jan 09 '16 at 01:38
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A flippant answer. Every gravitational wave detector on earth would glitch off the scale and a lot of physicists would be thinking "WTF now??". At least until they compared notes with other gravity wave detectors and with the astronomers.

The rest of the world would first not notice and then not care a jot.

Setting this event in Vinge's zones of thought universe would be interesting.

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