Complete astronomy noob over here who would be happy if he get a simple answer (and who is also aware that this may be not possible)...

I've learned from a tv documentary that the stars at the edge of the galaxy are not traveling more slowely than the ones closer to the center.

Does this also mean that all stars in a galaxy do not change their positions relatively to each other?

To put it simply: If I note the relative position of a star compared to our sun, and then do it again 10 (100 / 1000) years later: Are the coordinates of that star identical or will the star be in a different relative position?

I would assume that the star is in a completely different (relative) place given the speed star systems are travelling because the galaxy is rotating AND the "fact" (read: I don't know if this is really a fact) that the solar system is oscillating through the galactic plane.

  • $\begingroup$ AN interesting question. Others have answered it much better than I can, but as an example, I can suggest this link: en.wikipedia.org/wiki/… $\endgroup$
    – HDE 226868
    Aug 5, 2014 at 19:38

1 Answer 1


Stars do in fact move relative to one another within galaxies of all types. The orbital period of stars in a typical spiral galaxy (at around the same distance as the Sun is from the center of the Milky Way) is on the order of hundreds of millions of years. For the Sun it's something like 230 million years (source). A particularly old person (~100 years old), will have been around long enough to see something like one-tenthousandth of 1% of the orbit of stars in this region of the galaxy! Most of these stars simply do not move much relative to us in that amount of time.

The exception to this view of the motions of the stars is the Sagittarius A* (which is not a star at all, but is in fact a supermassive black hole at the center of the Milky Way). The period of stars traveling around it are on the order of 10's of years.


Also, to give you a rough idea of how galaxies look dynamically, here's a link to a particularly nice galaxy simulator implemented in the browser (created by Adrian Price-Whelan).

  • $\begingroup$ Hi astromax. Thanks for your answer! According to your answer and the (very interesting) link you provided, stars are indeed changing there relative positions to one another but these changes occur over long periods of time (although star systems are traveling at high speeds). Is it save to assume that therefore these changes are not relevant for, lets say, future space navigation? Like the continental drifts are not relevant for today's navigation? $\endgroup$
    – MarMun
    Feb 5, 2014 at 18:13
  • $\begingroup$ @HamburgerTS Right - the vast majority of the stars in a galaxy are not observed to move much on human time scales, though they are absolutely moving relative to one another. If you're wondering if we would have to account for their relative motion in order to travel to them, the answer is with current technology, absolutely yes. $\endgroup$
    – astromax
    Feb 5, 2014 at 18:17
  • $\begingroup$ Thanks for clarification. As far as I understand you in a "star trek" like setting with technology like the warp drive the relative motion of a destination star would not be a problem due to the short amount of time needed to reach it. $\endgroup$
    – MarMun
    Feb 5, 2014 at 18:32
  • $\begingroup$ But still. If the sun is indeed oscillating relative to the galactic plane: Would that not indicate that other stars are doing the same? Isn't it likely that other stars are oscillating with a deviant frequency and thus are constantly travel away or towards our sun? Hmm... A possible question may be: What do you mean with "not much"? Is "not much" enough to create the need to constantly update the star maps (because "not much" on a cosmic scale could mean a lot)? $\endgroup$
    – MarMun
    Feb 5, 2014 at 18:41
  • 1
    $\begingroup$ The maps need to be updated for close-by stars, most famous example is Bernard's star: en.wikipedia.org/wiki/Barnard's_Star $\endgroup$
    – Gerald
    Feb 5, 2014 at 20:58

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