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Nerull t1_jdx5z5i wrote

No, the black hole is an essentially insignificant portion of the galaxies entire mass. The galaxy is orbiting it's own collective mass not the black hole.

For reference, Sag A* is about 0.0007% of our galaxies mass. It's gravitational attraction on our solar system is completely insignificant compared to the rest of the galaxies mass.

Our solar system orbits the galaxy at about 230000 m/s. Sag A*'s escape velocity at our orbital radius is about 2090 m/s. If the rest of the galaxy vanished, we would go flying out into intergalactic space, since we aren't even close to being gravitationally bound to the black hole.


jaibhavaya t1_jdxnpmw wrote

So is it just some coincidence that super massive black holes tend to be at the center of spiral galaxies?


BailysmmmCreamy t1_jdy089s wrote

Almost certainly not a coincidence, but we don’t know whether the supermassive black holes are the ‘seed’ of the galaxy or whether the galaxy formed first and the stuff in the center collapsed into the black hole.


Kenshkrix t1_jdyapre wrote

I lean towards the "seed" idea, because black hole formation and growth as we understand it requires that supermassive black holes had to form already extremely massive relative to an 'ordinary' black hole.

A sufficiently energetic collapse to cause a supermassive black hole straight out is extremely unlikely to occur if things are already in any kind of orbit.

We still don't really know, but it's pretty interesting nonetheless.


jaibhavaya t1_jdz01j3 wrote

I guess that makes sense… if matter wasn’t already coalescing, then what would have collapsed to create the initial black hole? The black hole forms an accretion disk, then the accretion disk pulls other matter into orbit… and suddenly you have a galaxy? So really the “rest” of the galaxy ends up orbiting around its collective center of gravity, that just so happens to be close to the center of this initial big ol’ black hole.

Or something like that? I read that on the underside of a Snapple cap.


Kenshkrix t1_jdzvo4k wrote

The main issue with black holes is that you can't feed them too much stuff at once unless it falls directly into them, which won't happen in a galactic environment since everything has orbital momentum.

Once a black hole has an accretion disk, the disk itself has so much energy that it will shove away extra matter trying to fall into it.

Thus, one theory on the formation of supermassive black holes is the "black hole star".

Put simply, the idea is that in the early universe in areas where there weren't any particularly big things or galaxies it would be possible for light years worth of diffuse gas to begin accelerating towards the same area, which could collapse directly into a singularity.

It would still have enormous amounts of gas falling towards it, though, and the sheer gravity of all this gas could overcome the energy emitted by the relativistic accretion disk and continue to grow the black hole at a prodigious rate.

Eventually the balance would break and it would explode, but most of the remaining gas might not reach escape velocity, this would be the "seed" of a potential galaxy.


Makhnos_Tachanka t1_jdygrqt wrote

It's not a coincidence, it's just physics. Any rotating system like this will tend to sort objects by mass, with the heaviest at the middle. You can see this if you just take some sand and swirl it in a cup with some water. The largest grains will end up at the center. Why? They're the heaviest, and it takes the most energy to fling them around. If we simplify a galaxy to a two body system, you will imagine that a light star and a heavy black hole will orbit each other with the star making a much longer orbit than the black hole, which may be almost stationary. In a galaxy, you don't have a two body system, you have every body in the entire galaxy acting together, and relativistic effects and all that, but the same thing is happening. The distribution of masses in a galaxy broadly follows what's called the Einsato profile, which essentially says the densest objects will have the lowest radius. Of course, galaxies come in all shapes and sizes, and come in various states of development (a galaxy that has just collided with another may have a random distribution) but over time, a galaxy will organize itself with the densest objects at its center.


theboehmer t1_jdxxymc wrote

From what I understand, black holes are incredibly massive. They formed systems that added to the collective mass(gravity), which in turn led to more and more collected mass(stars/dust/what not). Over billions of years it's now the galaxy we know. Our galaxy is in a group of galaxies that will evolve and attract until they merge, growing further.


jaibhavaya t1_jdy8uyq wrote

Yea! That’s certainly how I understand it. I was curious how that fit into the black hole contributing little to holding the galaxy together as commented. Maybe it just plays a larger part in the early formation of the galaxy.


ktElwood t1_jdz0o5c wrote

Rather think of them as incredibly dense.

A black hole the mass of earth would be super small, but would have the same "gravitational pull"


theboehmer t1_jdzluqe wrote

True, it's a little counterintuitive thinking more mass equals smaller radius


EarwaxWizard t1_jdxcx4o wrote

Not to mention the dark matter that helps keep the galaxy together (gravitationally)


AncientMarinerCVN65 t1_jdxn2x5 wrote

I had heard our galaxy's super massive black hole was a small percentage of the overall mass, but I didn't know it was that small. That's amazing, considering it's over 4 million times the mass of our sun.

Another way to look at it, also, is that the matter in accretion disks around black holes is slowing down and spiraling in due to friction. We ain't! We're content to just keep orbiting way out here in the galactic boondocks, at least until we collide with the Andromeda galaxy in 5 billion years.


Head_Weakness8028 t1_jdxkr2h wrote

Excellent summation and examples. Technically, to summarize even farther for OP, I would say that the supermassive black holes at the center of galaxies are the accretions of the matter that coalesced into a galaxy. Edit: I suppose you could say that the galaxy is an “accretion disk” in the making. Edit: Which begs the question; Do stars inevitably spiral into the supermassive black hole in the center of the galaxy or does the gravity from all of the stars, orbiting the center of the galaxy, balance everything out? I’m assuming the latter, however, off to research!


thawed_froyo t1_jdxu7zh wrote

> If the rest of the galaxy vanished, we would go flying out into intergalactic space

What would the effect be if Sag A* suddenly vanished?


Nerull t1_jdy9585 wrote

For us, essentially nothing. Some stars extremely close to the galactic core might have their orbits disrupted, but we might not even notice.


thawed_froyo t1_jdyae82 wrote

Would that not have a ripple effect, even if it takes billions of years?


Nerull t1_jdyv679 wrote

Our orbital eccentricity would change slightly. I'm not sure the change would be large enough to even measure.


trustych0rds t1_jdx5wv8 wrote

No, since the black holes themselves don't generally have anywhere near the gravitational attraction to bind all of the galaxy's matter gravitationally itself.


Half-Borg t1_jdx5wv9 wrote

No. Galaxies and their black holes form together.


I-melted t1_jdx4v8a wrote

I want to know this too. Things have changed since I was at school.


wombat5003 t1_jdxd0wo wrote

I was reading the other day, and it was also verified by how the universe works (but don’t quote me exactly) that a galaxy is interconnected not just by gravity, but also magnetically through magnetic fields that interconnect everything…. So someone smarter than myself please explain that more, but that was the general gist… please forgive I think it’s basically the hardest thing to get your head around :) you know I took a small course on quantum computing recently and the concepts of quantum in relation to the universe is so hard to comprehend when you start getting down to sub atomic levels…


AncientMarinerCVN65 t1_jdxl5r4 wrote

That's interesting, I've never heard it put that way. All of the stars in our galaxy are bound to each other via gravity. And they would have similar magnetic fields if they are all rotating the same direction (adding together and making a cumulative magnetic field, similar to two waves in the ocean combining to make one big wave). But over long distances, gravity is much more powerful than electromagnetism. So the force of gravity wins out on large objects like planets and stars. But the galactic magnetic field would definitely have an effect on individual electrons or ions floating around in space, perhaps funneling them towards a certain point, the same way Earth's aurorae funnel solar wind towards the North and South poles.


SpartanJack17 t1_jdz8f4p wrote

Hello u/darthvadercock, your submission "Are galaxies just giant accretion disks around super massive black holes?" has been removed from r/space because:

  • Such questions should be asked in the "All space questions" thread stickied at the top of the sub.

Please read the rules in the sidebar and check r/space for duplicate submissions before posting. If you have any questions about this removal please message the r/space moderators. Thank you.


IntentoDeAstronauta t1_jdx5nmk wrote

Not all galaxies have super massive black goles.


Assembly_R3quired t1_jdy6fdz wrote

I for one have super massive black goals, but to reach his own, especially as universal evolution is concerned.


SomeKindOfAdult t1_jdxwxtl wrote

Ethan Siegel (Starts With a Bang) gives a good explanation in his article about how some galaxies don't have supermassive black holes. (

>It’s an eminently reasonable thought that every galaxy in the Universe should have a supermassive black hole, especially considering that the processes that we think lead to their formation:

  • early, very massive stars form,
  • some go supernova and some directly collapse,
  • their remnants dynamically interact with the surrounding matter,
  • causing them to sink to the proto-galaxy’s center,
  • where they merge,
  • and then these “seeds” of supermassive black holes accrete matter and grow,
  • leading to what we observe today,

>ought to occur everywhere a galaxy is present.

So the answer is "sort of". While the super massive black hole isn't what holds the galaxy together, it may be that it was the seed that pulled the initial cloud of gas together to start the galaxy.


theroadlesstraversed t1_jdy2d3x wrote

So like, super retardedly huge stars form after big bang. Star go boom, gases form clouds, cooling happens, electromagnetism makes elements form, clumps get big, some rocks and shit happen, some small boomies form, their reactions pull rocks and lava around them, "gravity" causes solar systems to form, all the while the original lite brights core is pulling stuff in and around itself... am I close?


rumblesnort t1_jdy6oyn wrote

Kind of like a big stellar RAID array? With parity.