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Motions of stars inside elliptical galaxies?

  1. Jan 28, 2012 #1
    I recently read an article about elliptical galaxies. The article mentioned that the stars have basically radial motions. What does that mean ? Do these stars oscillate between the center and some mid-radius? Stars in spiral galaxies have on average orbital motions in one direction around the central bulge which I understand.

    Scratching my head, Doug Ettinger
  2. jcsd
  3. Feb 2, 2012 #2
    What motions do the stars inside an elliptical galaxy have since they do not orbit a central point similar to spiral galaxies? Please tell me if not enough data is available to support any general conclusion.

    Still pondering, Doug Ettinger
  4. Feb 2, 2012 #3


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    I cannot state this for sure but it seems they will follow elliptical orbits similar to spiral galaxies.

    Despite the fact that the stars in an elliptical are not in a plane, they still have a centre of mass, and it is this centre of mass that each star orbits.

    [EDIT] Then again, this page suggests to me that it is much more complicated than I had any idea...
    Last edited: Feb 2, 2012
  5. Feb 2, 2012 #4
    I am not trying to be difficult. I am only seeking the best, currently accepted answer. Dave, can you cite a reference for your statement? In Wikipedia, it was mentioned that the stars generally have radial motions and I am inquiring what was meant. Thank you for your reply.

    Why radial, Doug Ettinger
  6. Feb 2, 2012 #5


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    Most humble apologies. When I made my last post, I completely forgot to actually include the link I was referring to, resulting in you misunderstanding my meaning. I've added the page I was referring to as a link in the post above. According to that page, stellar motion seems quite complex.
  7. Feb 2, 2012 #6


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    I cannot cite a reference for it, but I do propose the following explanation, after a little bit of thought.

    1] Radial motion is essentially an elliptical orbit with very high eccentricity. Elliptical galaxies characteristically have little rotational motion, so in general you end up with orbits that are just straight out and straight back in.

    ...but it's not a typical ellipse with the centre of mass at the focus...

    2] Ever heard of the thought experiment where you dig a tunnel through the centre of the Earth and fall though it? You would come out the other side, reverse direction and fall back again, oscillating forever (if we ignore rotation).

    One of the things that makes this different from a regular orbit is that, while you are falling, the gravitational force is steadily dropping to zero. It reaches zero when you get to the centre and then the force begins to increase again. So it's not a normal ellipse-about-the-focus orbit. You essential have two apogees and two perigees.

    What you do when through the Earth, is make passes back and forth through its centre. That's radial motion. It's identical to being inside an elliptical galaxy.

    Attached Files:

    Last edited: Feb 2, 2012
  8. Feb 3, 2012 #7
    Thanks for the excellent clarification. Perhaps this is also the reason why these galaxies have an elliptical shape. All the stars generally formed elliptical orbits. I will add three complications. 1) Is there any hypothesis for why their motions were initiated. 2) Over long periods of time these orbits should cross and become chaotic - unless there is a reason for their orbits remaining concentric. 3) Most elliptical galaxies supposely have large black holes at their centers; hence, your idea of a radial path oscillating through the center comparable to a hole through the Earth might not apply.

    Always thinking, Doug Ettinger
  9. Feb 3, 2012 #8
    whoah, so all the stars are either falling towards the center or away from it at all times? I wonder if they ever run into each other?
  10. Feb 3, 2012 #9
    Hello Shiska,

    The stars over time may collide with the central black hole to keep increasing its mass. Do you have any ideas?

    Looking for solutions, Doug Ettinger
  11. Feb 3, 2012 #10
    I would think the odds of this happening would be very small, less than 1%. Even in galactic collisions the odds of two stars smashing together is very small.
  12. Feb 3, 2012 #11
    Thats true, even with galactic mergers the odds of stellar collisions are EXTREMELY low due to the immense distances between stars, I would estimate this to be much less than 1%.
  13. Feb 3, 2012 #12

    I am well aware of the low probability of stars colliding within normal star fields. What are the chances of stars inside elliptical galaxies with their radial motions colliding with the central massive black hole?

    Coming together, Doug Ettinger
  14. Feb 3, 2012 #13
    I think that, even though the orbit as seen in Dave's picture gets really close to the center, on a galactic scale the distance from some black hole at the center and the periapses would still be large enough that it would be unlikely for there to be a collision.
  15. Feb 5, 2012 #14

    The word "radial" as used here, has nothing to do with "radius" as understood geometrically or in relation to measuring central distance from perimeters. As applied in the context it merely means that as we see them from earth, such stars are either moving away from us or toward us along our line of sight as indicated by either the red or blue shift in their light's wavelength. Eliptical galaxy stars are subject to the same laws requiring centrifugal force in order to prevemt orbital deterioration as are the spiral galaxy stars and all other stars which are members of stellar aglomerations which are gravitationally bound together.

    Last edited by a moderator: May 5, 2017
  16. Feb 5, 2012 #15


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    If your interpretation is the case then the original statement doesn't make sense (assuming it's been transcribed from the original text accurately).

    It would make sense if it were saying we can measure the radial motion, but it doesn't make sense to say the stars primarily have radial motion.
  17. Feb 5, 2012 #16

    You are right, it doesn't make sense from a contextual and and astronomical terminology viewpoint. What do you consider that I transcribed?
    Last edited: Feb 5, 2012
  18. Feb 6, 2012 #17
    I know now what "radial" meant in the context that I read it. Thank you.

    I am still left pondering the motion of the stars inside an elliptical galaxy. Let's take one star on the outside perimeter and assume it follows an elliptical path that closely approximates the galaxy's shape. Its orbit also bisects the shape of the galaxy into two similar volumes. Next, chose many more stars the same way. Their three-dimensional orbital paths must necessarily intersect. So how are the billions of stars in a massive elliptical galaxy prevented from having collisions over the life of the universe? Elliptical galaxies are considered to occur very early in the 13 billion year lifetime of this universe.

    Still pondering, Doug Ettinger
  19. Feb 6, 2012 #18


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    Well, lets look at the Sun. It is 1,392,000 km in diameter. The distance to the nearest star is over 4 light years away. That's 37,842,921,890,323.2 km. That means the distance to the nearest star is over 27,186,007 times as far as the Sun is across. So what we have are VERY VERY VERY small objects at HUGE distances from each other all moving around. On top of that, to have an actual collision would require the two stars to be approaching at very specific vectors, velocities, etc. The overwhelming majority of encounters between objects in space results in both objects simply passing by each other. Collisions do occur, but it is pretty rare.
  20. Feb 6, 2012 #19
    May I summarize and please express any opposing variation or opinion.

    The majority of stars in an elliptical galaxy are highly organized and move in elliptical orbits not necessarily in the same direction about a concentrated central mass of the galaxy. Orbital perturbations are fairly random but statistically homogeneous. This in turn maintains a fairly homogeneous distribution of stars that tend to become denser toward the center.

    Thanks for all your help, Doug Ettinger
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