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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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    $\begingroup$ 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. $\endgroup$
    – Draco18s no longer trusts SE
    Jan 8, 2016 at 19:03
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    $\begingroup$ 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. $\endgroup$
    – SudoSedWinifred
    Jan 8, 2016 at 19:11
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    $\begingroup$ Boy, I really wish we could try this... We don't need Andromeda, right? $\endgroup$
    – AndyD273
    Jan 8, 2016 at 19:39
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    $\begingroup$ Note that the one in our galaxy is pretty small. In a more typical spiral galaxy that would have a more profound effect. $\endgroup$
    – JDługosz
    Jan 10, 2016 at 5:12
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    $\begingroup$ Well for one thing, the Pierson's Puppeteers could go home and open up General Products again... $\endgroup$
    – VBartilucci
    Jul 13, 2018 at 18:26

4 Answers 4

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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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    $\begingroup$ 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. $\endgroup$
    – Jim2B
    Jan 8, 2016 at 19:17
  • $\begingroup$ 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 $\endgroup$
    – AndyD273
    Jan 8, 2016 at 19:20
  • $\begingroup$ 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.) $\endgroup$
    – DaaaahWhoosh
    Jan 8, 2016 at 19:21
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    $\begingroup$ @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. $\endgroup$
    – Cort Ammon
    Jan 8, 2016 at 19:34
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    $\begingroup$ @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.) $\endgroup$
    – type_outcast
    Jan 9, 2016 at 1:16
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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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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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    $\begingroup$ 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. $\endgroup$
    – type_outcast
    Jan 9, 2016 at 0:46
  • $\begingroup$ 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. $\endgroup$
    – AndyD273
    Jan 9, 2016 at 0:49
  • $\begingroup$ @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. $\endgroup$
    – type_outcast
    Jan 9, 2016 at 1:38
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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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  • $\begingroup$ In Zones of Thought this would be super weird. Or maybe its along the same lines as what Countermeasure did (or rather, the reverse). $\endgroup$
    – Draco18s no longer trusts SE
    Jan 11, 2016 at 19:51
  • $\begingroup$ I'm not sure the gravity wave detectors would even notice, as the event is spherically symmetric. Usually those don't generate gravity waves. $\endgroup$
    – John Dvorak
    Mar 3, 2018 at 20:50
  • $\begingroup$ Gravity waves propagate at the speed of light. It would be quite a few centuries before they reached Earth. $\endgroup$
    – David Thornley
    Aug 16, 2018 at 22:34

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