Let's say if a galaxy is moving, shouldn't it be shaped like a funnel? I am saying this because it would take considerable time for the outermost star to know that the center of gravity has shifted. For example, if milky way's radius is 100,000 light years, there is possibly no way for the outermost star in our galaxy to know that the center of gravity has moved before 100,000 years. So taking a circular symmetry, the only thing that comes to my mind is a funnel-like shape.

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    $\begingroup$ What you're forgetting is that the star and the center of gravity are moving at basically the same speed, so from the star's reference frame, the center of gravity is not really moving. $\endgroup$ Nov 30, 2018 at 12:00
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    $\begingroup$ Why are you assuming that the galaxy's motion originates at its center of mass? $\endgroup$
    – tparker
    Nov 30, 2018 at 12:04
  • $\begingroup$ I guess you are probably right. But things will change in case of accelerated motions, right? Such as in the event of collisions of two galaxies? $\endgroup$ Nov 30, 2018 at 12:28
  • $\begingroup$ You should address your replies with the @ sign. Otherwise members are not notified. $\endgroup$
    – safesphere
    Nov 30, 2018 at 13:57

2 Answers 2


Let's say if a galaxy is moving, shouldn't it be shaped like a funnel? I am saying this because it would take considerable time for the outermost star to know that the center of gravity has shifted.

Moving with respect to what?

Suppose the galaxy's center of mass is moving at some velocity respect to some external object. You can always find some other external object for which the motion of the galaxy's center of mass is in exactly the opposite direction. Or yet another object for which the galaxy isn't moving at all.

This concept of relativity dates all the way back to Galileo. Galilean relativity is implicit in Newtonian mechanics. Einstein went one step further and made relativity a central concept in his theories of special and general relativity.

One thing that is absolute in Newtonian mechanics and special relativity is acceleration. This concept carries forward to some extent to general relativity. Suppose a galaxy is on a collision course with another galaxy. When the galaxies are far removed from one another, all of the stars in one galaxy experiences more or less the same (very small) gravitational acceleration toward the other. But when the galaxies get close to one another, the stars in one galaxy closest to the other galaxy experience a much greater gravitational acceleration than do the stars in that galaxy that remain well removed from the other galaxy.

These tidal accelerations have the potential to tear galaxies apart, but not into a funnel shape.

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    $\begingroup$ "Moving with respect to what?" I can answer this one. There is actually a rather special frame of reference, namely the frame of the cosmic background radiation. It is not relevant to the question at hand, though. $\endgroup$
    – my2cts
    Nov 30, 2018 at 12:50
  • $\begingroup$ If a smaller galaxy hits a larger one in tbe center with a high speed and momentum, this would create a funnel, would it not? And this would be a unique opportunity to study the galaxy rotation mystery (dark matter, etc.). So is there a search for such an occurrence? $\endgroup$
    – safesphere
    Nov 30, 2018 at 14:39
  • $\begingroup$ @safesphere: Sure, we've found many interacting galaxies. And yes, galactic interactions have been studied to try to understand dark matter; in particular, read up on the Bullet Cluster. $\endgroup$ Nov 30, 2018 at 16:21
  • $\begingroup$ @MichaelSeifert The Bullet does look like a funnel: upload.wikimedia.org/wikipedia/commons/e/ea/Bullet_cluster.jpg $\endgroup$
    – safesphere
    Nov 30, 2018 at 16:38

You are forgetting about Newton's 1st law. Objects in motion stay in motion unless an outside force acts on it. The stars in the galaxy are already moving relative to the center of mass, and so they will keep up with the center of mass by default. They don't need information about the motion of the center of mass of the galaxy because objects don't need forces to make them move at a constant velocity.

It's kind of like asking why I don't drag behind my car when I drive on the highway. Once me and my car are both going 60 mph, there doesn't even need to be a force keeping me going because of Newton's 1st law.


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