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Are black holes bound to galactic revolution?

  1. May 16, 2013 #1
    A couple of quick questions after watching a video on the helectic model that the solar system follows on it's course around the galactic center. Please bare with me these maybe idiotic questions.

    A) Are black holes bound to the spin of the galaxy or do they sit in place on the galactic plane?
    B) If not black holes does anything do this?
    C) Can an object travel in the opposite direction of the galactic spin or is there some sort of tidal force?
    D) At any point in our movement do we come closer to other stars?
    E) Do certain stars travel faster in orbit?
    F) If so are there any objects that go faster then the "arms" thus going through them?
    F) If so is it possible that a star could "catch up" to us?

    Again I hope these questions aren't dumb. I really am fascinated by the idea of the helectic travel through the galaxy. Thanks.

  2. jcsd
  3. May 16, 2013 #2


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    Hi CowedbyWisdom! :smile:

    (wot's "helectic"? :confused:)

    A) a black hole behaves the same as an ordinary star of the same mass … they have exactly the same gravitational field (outside the radius of the ordinary star)
    B) no
    C) F) there's no reason why something shouldn't go the wrong way, and not everything goes at exactly the same speed
    D) E) F) we don't come closer to other stars generally, but an individual star could theoretically come closer to us, but it's very unlikely … any "rogue" star would probably have been perturbed so much over billions of years that it would either have started going with the crowd, or it would have been flung out (like a "rogue" planet in the early solar system)
  4. May 16, 2013 #3
    Given the context, my guess is this is supposed to be "helical".

    And it's worth emphasizing that this question is unrelated to tidal forces. CowedbyWisdom: tidal forces are due to non-uniform gravitational fields, they have nothing to do with rotational motion.
  5. May 16, 2013 #4


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  6. May 16, 2013 #5
    Black holes follow forces of gravity like all other stars.
    Stars cannot, for if they sat in place for a moment, whether on or off the galactic plane, they would be accelerated to fall straight into galactic centre.
    To sit in place, something would need to be supported by a gas pressure. Baryonic gas rotates with galactic disc... how about dark matter?
    There is a tidal force, namely dynamic friction. But it is fairly weak. After 10 milliard years, globular clusters still do not follow the spin of the galactic disc. Some objects do move opposite to galactic spin.
    We are getting closer and further from other stars all the time. For example we are now getting closer to Alpha Centauri.
    Yes, there are stars whose peculiar motion is turnwise.
  7. Jun 13, 2013 #6
    Thanks for the responses. I'm sorry my english is not the best. You have answered all of my questions basically. I have a couple more though if you don't mind.

    What determines the traveling speed of a star?

    If black holes are not bound to stillness does that mean that black holes can travel at high velocities? So fast that they travel faster than something, say our sun?

    If the escape velocity of the milky way is about 232 km/s and an object travelling towards the "walls" of the galaxy reaches that "wall" what happens if it is not traveling that fast?

    Thanks again you guys are champs! I would love to sit down and have a beer with some of you guys.

    Last edited: Jun 13, 2013
  8. Jun 13, 2013 #7


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    Hi John! :smile:
    good ol' newton's first law …

    every thing moves at the same speed in the same direction unless there is a force acting on it
    as i said earlier …
    … a black hole behaves in exactly the same way as an ordinary star of the same mass

    in particular, if a black hole (or ordinary star) somehow reached high speed relative to the galaxy (probably by a "slingshot collision"), it would follow either an ellipse (and therefore would return over and over again) or a hyperbola (and therefore would not return) :wink:
  9. Jun 13, 2013 #8


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    It goes like this:
    When you've got just two bodies orbiting common centre of mass(e.g.,Earth-Moon) then the velocities of both can be exactly predicted from laws of motion and the law of gravitation. The farther the bodies apart, the lower their orbital velocities.
    If the two bodies are sufficiently far away from any other gravitational influences, then treating them as isolated is a good approximation(so we don't really need to think about the Sun when calculating Moon's orbit).

    But when you try to do the same calculations with three or more gravitating bodies, the equations become unsolvable apart from certain special cases(e.g., the lagrangian points, resonances; see: http://en.wikipedia.org/wiki/N-body_problem). In general, though, the motion of the bodies is unpredictable in the long run, as the multiple masses tug on each other and perturb their orbits.
    The bottom line is, with many bodies, you may end up with a semi-stable configuration like our solar system, or with some bodies gaining enough speed from the gravitational interactions to be completely flung out of the system. The actual result is pretty much random.

    These unpredictable interactions are what determines the peculiar velocities of stars. Peculiar velocities are local velocites, relative to neighbouring stars, not directly related to the global orbital speed around the galactic centre.

    If you now take the whole galaxy, with its myriads of stars distributed in somewhat regular fashion across vast distances, then their effects on some particular star you're interested in become predictable again(to an extent). You can think of the galaxy as if it were a giant disc-like body made of an insubstantial soup of particles(stars) that collectively act on a given star.
    In particular, the distribution of mass in the disc causes the stars to be attracted to the centre of the galaxy - inducing orbital velocities, and to the plane of the disc - causing the orbits to "wobble" up and down as the stars travelling above the disc are pulled back in, pass throuh it, and are pulled in again.

    So, taking the Alpha Centauri(a binary system) for example:

    At the local level, the motion of the two stars αCenA & αCenB are determined by mutual gravitational attraction of the two binary components.

    The star Proxima Centauri is just enough removed from the other two to make it difficut to say if its motion is still bound to the two other stars, or if its motion is already outside their influence, being significantly tugged in random directions by its other neighbours, including the Sun.
    This is the level on which it begins to make sense to think in terms of peculiar velocities.

    As you move farther away, you begin to notice that the whol Alpha Centauri system, and its neighbours, despite moving somewhat randomly amongst themselves, have a common motion around the galactic centre.

    As an additional note, it is worth mentioning that the orbital motion in the galaxy does not conform to the Keplerian laws of planetary motion, in that the orbital speed remains roughly constant for all stars but those closest to the centre(while for planets it goes down with distance). A discrepancy which prompted the introduction of Dark Matter to explain it.

    It's best to think of black holes as just another kind of star. As far as motion goes, they behave identically in all respects. They just don't shine, and when you get very, very close to them, you get spaghettified(http://en.wikipedia.org/wiki/Spaghettification) instead of burning up.

    First of all, I'm pretty sure the escape velocity in the galaxy is not 232 km/s, but something closer to 500km/s(from where the Solar System is located). ~230 km/s sounds like the orbital speed of the Solar System around the galactic centre.

    As for the question, I've already touched on it earlier. The star that travels above the galactic disc will eventually get pulled back in, pass through it and repeat the same thing ad infinitum, unless it gains enough peculiar velocity from local interactions to exceed the escape velocity.
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