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Effects of super massive blackhole on galactic movement

  1. Sep 23, 2013 #1
    Has anyone studied the effects of the central black holes in the galaxy's, on the rotation of the galaxy's spiral arms. When the black hole spins, do the stars in the surrounding galaxy spin in the same direction, does the speed of the spinning black hole affect the rate of rotation in the outer parts of the galaxy. Can the speed of rotation of the black hole affect the speed of the galaxy's movement through intergalactic space. When a black hole is feeding does this speed up or slow down the stars orbiting it's centre, or do the stars orbit at a constant speed.
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  3. Sep 23, 2013 #2

    Vanadium 50

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    They rest of the galaxy moves exactly as if the central black hole were replaced by an equivalent mass of rock. Or feathers. Or iron. It doesn't matter - mass is mass.
  4. Sep 24, 2013 #3


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    Yes. It has no effect.

    The rotation and feeding of the black hole has no effect on the galaxy at large, nor does it affect the nearby stars.
  5. Sep 25, 2013 #4

    Your answers to my questions make's no logical sense, we are talking about an object that could have a mass of 1 million or a billion times that of our sun's, but has the same spacial area of our sun, and could be spinning at anything up to the speed of light, but you are saying it has no effect on the surrounding Galaxy. If this is true why do Galaxy's spin, why do they spin in certain directions, why do they move in intergalactic space. If the dopler red shift theory is correct why do the stars in the outer part of our galaxy move faster than they should, why do the stars closest to a black hole orbit at tremendouse speed. If my understanding of astrophysics is correct it was the discovery of these fast orbiting stars that pinpionted the exact position of the super massive black hole at the centre of our own galaxy, but according to your answers this same black hole has no effect whatsoever on our galaxy. If I remember correctly Einstiens special theory of relativity was proven when the suns magnetic field bent light,and this same magnetic field holds our solar system in place, but you say a super massive black hole that has millions of times the mass of our sun, and therefor millions of times the magnetic field has no effect upon it's surrounding galaxy.
    I asked some what I thought, simple astrophysics questions; because I have an interest in the subject, but the answers I got back seem to be purile and condecending, if you dont know the answers, or there hasn't been a study into my original questions, then just say so, but to give me this guff about mass rocks and feathers, and "no effect at all " seems nonesence to me I consider these answers an insult to anyone's intellegence, show me the proof of what you are saying,sothat I can understand.
    The facts are that galaxy's do spin, they do move through intergalactic space, they do spin in certain dirrections. The stars closest to central black holes are effected by their magnetic fields and have hyper speed orbits.
    If what I have been told about galactic magnetic field attraction is true then our galaxy and the Andromida galaxy wiil collide in 4-5 billion years time,but of course this will have nothing to do with the super massive black holes at the centre of our two galaxy's. They are just along for the ride, according to you, so what is driving all these unusal things to happen
    If these facts are not in dispute, then what is causing them, I know lets call it somthing dark, dark matter, somthing we can't find, dark energy that we can't detect, Dark intentions somthing to cover up the fact that we dont know exactly whats going on, but we can act like we do.
  6. Sep 25, 2013 #5


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    There's no need to get all angry here. The answers so far provided were accurate, even if not exactly on the exhaustive side.

    You basically asked two questions:
    1. what does the presence of a supermassive black hole mean for the orbital motion of stars in a galaxy
    2. what does the rotation of the black hole mean for the same

    The orbital speed of a small object(like a star) depends on the mass of the object being orbited and distance from it, as per the Newton's laws. You get Kepler's laws from this.
    It can be shown mathematically that, as long as its density is spherically symmetrical, it doesn't matter how big(spatially extensive) is the central object(http://en.wikipedia.org/wiki/Shell_theorem). Only the mass matters. For the purposes of calculating orbits, all massive objects can be treated as if all of their mass was concentrated in a point at their centre.
    That's why the fact of there being a 1 million solar masses black hole in the centre of the galaxy is irrelevant outside its immediate neghbourhood. Its gravitational field looks exactly the same as if there were 1 million stars packed in a roughly spherical distribution.
    And that's not even such a huge number, considering ~100 billion stars the Milky Way contains.
    As Vanadium said, it could be even 1 million solar masses of feathers, and the orbits of far away stars would look just the same.
    The only difference it makes is for the stars very close to the black hole, which do orbit faster than if they were surrounded by other stars - this is also explained by the shell theorem(the second part in the wiki article) - and which are affected by tidal forces raised by the concentrated mass of the black hole.

    For the rotation of a massive body to have any effect on other bodies, it'd have to somehow drag the space around it. General Relativity predicts such an effect: http://en.wikipedia.org/wiki/Frame_dragging . However, it is very tiny, so from the point of view of the stars in the galaxy it's negligible. For all they care, the black hole may just as well not spin at all, or spin in the opposite direction.

    Re: the post above this one:
    - Whenever you say "magnetism" in that post, you ought to say "gravity". It doesn't make sense otherwise.
    - Galaxies spin because of the conservation of momentum of the collapsing gas from which they coalesced. One direction emerges during the collapse and is further magnified the further it goes. Just as with stellar sytems or water going down the drain.
    Here's a simulation of such a collapse, made by the guys from Univ. of Zurich:

    - Galaxies move in the intergalactic space according to the conservation of linear momentum and gravitational attractions with other galaxies. Whether there is or isn't a black hole in the centre doesn't matter.

    Finally, as an interesting aside, the measured velocities of stars in galaxies appear to conflict with the velocities predicted by analysing the distribution of visible mass. This has led to the introduction of the concept of Dark Matter to explain the discrepancies.
    Last edited by a moderator: Sep 25, 2014
  7. Sep 25, 2013 #6


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    The fact that you do not understand the correct answers does not make them less correct. Your attitude will not serve you well on this forum.
  8. Sep 25, 2013 #7


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    To be clear, it acts exactly as a million mass object should act. That is, it exerts a gravitational force on the entire galaxy, with the strength falling off according to the inverse-square law. Even with a million solar masses, its gravity is VERY weak on most of the galaxy.

    They spin because the angular momentum of the gas and dust clouds that collapsed to form the galaxy is conserved. They move through space because of the same reason. Momentum is conserved, and various interactions throughout the history of the universe have given them their velocity through space.

    That is NOT what anyone said, nor did you even ask this. Look closely at what you asked and how we replied. You asked about the the rotation of the black hole's effect on nearby stars, of which there is none. Frame dragging takes place VERY close to the event horizon, much closer than any stars get. And you asked about the effect the black hole has on the rotation of the spiral arms, which again there is none unless you want to include the small additional gravitational force it exerts on the arms.

    I assume you mean gravity, in which case yes, its gravity does affect the galaxy as I explained above.

    Text does not contain the normal verbal/nonverbal cues used to convey the subtleties of conversation. As such, please do not think that someone is condescending just because their message appears that way to you. I can almost guarantee you that they are not.

    The gravitational force from both galaxies is pulling us towards each other. That's it. There is a LOT more mass in each galaxy as a whole than in the supermassive black hole at their centers.

    You seem to have a problem with the scientific method. We observed things happening that didn't work according to known physics. We performed more observations, did lots of math, came up with models and theories that fit those observations, and came to the conclusion that the best explanation is that there is a type of matter out there that we cannot directly observe. This is exactly how science works everywhere.
  9. Sep 28, 2013 #8
    Let me say I'm sorry.

    To Bandsnatch, Phines and Drakkith,let me say how sorry I am, for the tone and content of my reply to your answers. I made an assumption that you had realise I was a layman with regard to atrophysics, and your answers were the exact opposite of what I had come to exspect. You were right to say that because I didn't understand your answers it didn't make them any less true.
    Now that I've read your reply's to my ignorent outpourings, I can only ask you to forgive my stupidity. I would also like to take this oppertunity to thank you for taking the time to reply to my tactless ramblings.
    I hope in the future if I ask questions on this subject of astrophysics, you might take the time to explain and enlighten me further.

  10. Sep 28, 2013 #9


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    Hey, you're learning. That's what this forum is for. :smile:
  11. Sep 28, 2013 #10


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    The milky way is almost incomprehensibly gigantic with a diameter over 100,000 light years, and just the visible mass of the galaxy is between 200 and 600 billion solar. A few million solar black hole has about as much gravitational influence on the galaxy as a marble has in the middle of an Olympic swimming pool. None of the stars known to be directly orbiting the CBH are more than a handful of light years distant. To put things into perspective, the gravitation influence of a million solar black hole is about the same at 1000 light years as that of the sun at 1 light year.
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