Why do Black Holes in the middle of galaxies not suck up the whole galaxy?

  • As stated in several sources, it's supposed that in every galaxy there is a black hole in the middle.



    My question is, why do these black holes in the middle of galaxies not suck up all the surrounding matter in the galaxy?


    @Raidri There's also a good one about the relationship between mass and gravity in the *What-If* book. IIRC it discusses what effects a chunk of "neutron star material" would do locally on earth. Long story short: Don't touch it. If you haven't already, I highly suggest picking up a copy (of the book, not the neutron star).

    Please don't use the comment space for answers.

    Last I knew, it was not supposed that "in ***every*** galaxy there is a black hole in the middle." Notice the emphasis on *every*. Should edit "every galaxy" to "many galaxies"

    Matter in a galaxy are spread VERY far apart. A SMBH wouldn’t have the gravitational force to “suck” it all up.

    @Raidri Also relevant (especially the 3rd paragraph): https://what-if.xkcd.com/129/

    For the same reason that the star at the centre of the solar system doesn't suck up all the planets.

  • Phiteros

    Phiteros Correct answer

    4 years ago

    You shouldn't think of black holes as "sucking things in". Black holes interact with matter through gravity, just the same as any other object. Think of our Solar System. All the planets orbit around the sun because it has a lot of mass. Since the planets have some lateral motion (they're not moving directly towards or away from the sun), they circle around it. This is known as conservation of angular momentum.



    When talking about gravity, all that matters is the mass of the objects involved. It doesn't really matter what kind of object it is*. If you were to replace the sun with a black hole that had the same mass as our sun, the planets would continue on the exact same orbits as before.



    Now, the black holes at the centers of most spiral galaxies do accumulate mass. Some of these black holes have accretion disks around them. These are swirling disks of gas and dust that is slowly falling into the black hole. These gas and dust particles lose their angular momentum through interactions with gas and dust nearby and by radiating energy as heat. Some of these black holes have very large accretion disks, and can generate huge amounts of electromagnetic radiation. These are known as active galactic nuclei.



    So, long story short, black holes don't "suck". They just interact with things gravitationally. Stars, gas, and other matter in the galaxy has angular momentum, so it stays in orbit around the center of the galaxy. It doesn't just fall straight in. This is the same reason the Earth orbits around the Sun.



    *Disclaimer: When you talk about things like tidal forces, you do need to take into account the size of the objects. But for orbital mechanics, we don't need to worry about it because the distances between the objects are generally much larger than the objects themselves.


    To add, if the Sun became a black hole, and if it managed to do so without losing mass, Earth and the rest of the solar system will continue to orbit around it as they always have. You only end up getting "sucked into" the black hole if you get really close to it, at which point the gravitational force is tremendous. But you have to get really close. For illustration, the Earth would need to be compressed to ~1 cm wide to become a black hole. In real life, even if you tunnel to the Earth's core, you won't feel that kind of gravity since the mass is above you (see the shell theorem).

    "Some of these galaxies have very large accretion disks" I think you meant "Some of these *black holes*..."

    @jpmc26 Indeed I did, thanks for the catch.

    Possible pedantic caveat - on a long enough timescale one might expect the various objects orbiting a galactic black hole to eventually fall into it as friction with the interstellar medium gradually depletes their angular momentum. Of course, the density of the interstellar medium is so low that "a long enough timescale" is probably absurdly, ridiculously long (almost certainly longer than the lifespan of any object in the galaxy).

    @aroth Yes, I was considering adding some more footnotes about how things lose angular momentum and the fact that the galaxy isn't orbiting around the black hole per se, but about the center of mass of the entire galaxy. But adding too much detail can cause confusion. Generally I find that it's best to answer the question simply without adding too much extra information. If they want to find out more about a particular aspect, they can ask another question.

    Thanks a lot, now I understood. So if an object is not too close to the black hole so that the last one exerts a very strong gravitational force nothing happens.

    I have just realized that a galaxy is essentially just a giant accretion disk for the supermassive black hole in the middle...

    @OiRc That's essentially correct. Black holes aren't a cosmic vacuum cleaner, they're just a really massive celestial object that exerts gravitational force the same way as anything else does. Until you get close enough to start breaking General Relativity (ie: crossing the event horizon), you can think of them working the same as a really big star (because that's basically what they are, the star just died).

    @JohnDvorak Bingo. And the reverse is also true; Planets/asteroids are just the accretion disc of their respective star(s), moons are the accretion disc of planets... Everything started from big clouds of dust that slowly clumped up into bigger and bigger objects. (That's a bit simplified, but generally accurate.)

    @JohnDvorak Not the same thing at all. The black hole + accretion disk system is vastly dominated and is driven by the gravity of the black hole. Big as the supermassive black hole may be, it's just a speck of dust at the scale of the whole galaxy. The galaxy is a very different system from an accretion disk - it's held together by its own total gravity, as opposed to merely orbiting a black hole.

    If planets can orbits black holes , then can black hole act as source of energy like our sun ? if so then can these planets habitable ?

    I think this is where we go "Any sufficiently granular accretion disk is indistinguishable from orbiting bodies." lol

    @RahulRabhadiya Yes and no, but practically speaking far more no than yes. Ultimately, the sun is an energy source because it radiates colossal amounts of electromagnetic radiation (light), not because of its gravity. Black holes are literally defined by having gravity so strong that light cannot escape, which means they do not radiate light in the same way. That’s the big no: the sun’s gravity is not why it is a good energy source, and the black hole’s gravity actually makes it a *bad* energy source.

    @RahulRabhadiya On the other hand, there is a kind-of yes to your question. Black holes are hypothesized to emit “Hawking radiation,” which is also electromagnetic radiation (light). That’s complicated, but for a black hole the mass of the sun, its effective temperature would be 60 billion*ths* of a degree above absolute zero. It would absorb more energy from the cosmic background radiation than it emits, so it would be a net reduction in energy. Hawking radiation goes *down* with larger black holes, too, so to get more energy you need smaller black holes—which need smaller things orbiting.

    "Now, the black holes at the centers of most spiral galaxies do accumulate mass". Doesn't that mass *growth* change the surrounding orbits? Or is that growth insignificant when compared to the mass of the black hole?

    One could add that anything that *didn't* have sufficient angular momentum, i.e. was on a collision course, has fallen in long ago. What has survived had the properties necessary to survive. The cosmos, as anything else, is subject to the basic principles of evolution; that which is not sustainable tends to disappear (which is somewhat tautological, it's that basic).

    @NathanMerrill: The mass they're consuming was already there. No mass is created or destroyed when the black hole absorbs it. From the perspective of anything orbiting any meaningful distance away, the difference between "mass near the black hole" and "mass consumed by the black hole" is irrelevant.

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Content dated before 7/24/2021 11:53 AM