Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How will the pattern of stars in the milky way change over time?

  1. Dec 22, 2012 #1
    how will the pattern of stars in the milky way change as the galaxy rotates, will they be rotated in some direction or drifted or something?
    and what is the timescale for that to happen?

  2. jcsd
  3. Dec 22, 2012 #2


    User Avatar
    2017 Award

    Staff: Mentor

    The relative motion of nearby stars (so nearly everything you can see in the sky) is nearly random. They will head towards some random fixed point in the sky, and get dimmer, while other stars come from some random position and get brighter. Typical relative velocities are ~50km/s or ~2 light years per 10000 years. For stars 100 light years away (see the brightest stars), this gives a timescale of ~500,000 years until they are at completely different positions. Some stars are much closer, and some stars are quicker, so they moved by a visible amount (for the naked eye) in the last ~2000 years. 61 Cygni, for example, is visible to the naked eye and moves with ~5arcseconds per year or 1° in ~700 years. This is two times the diameter of the moon.
    Last edited: Dec 22, 2012
  4. Dec 24, 2012 #3
    Eventually, due to an accelerating expansion of the universe, it will be impossible to see any stars!
  5. Dec 25, 2012 #4
    That is not correct. Our local group of galaxies are gravitationally bound and the expansion of the universe does not effect the stars contained within them.

  6. Dec 25, 2012 #5


    User Avatar
    2017 Award

    Staff: Mentor

    It would need a big rip to pull our galaxy apart*, and there are no signs of this. But even if with a big rip, our sun will reach the end of its life long before that. All other stars won't live forever either.

    *well, the collision with Andromeda in ~4 billion years will significantly change its shape as well.
  7. Dec 25, 2012 #6


    User Avatar
    Staff Emeritus
    Science Advisor

    One would need to find the proper motion components of many stars.

    Here are insights.
    http://outreach.atnf.csiro.au/education/senior/astrophysics/proper_motion.html [Broken]




    Atlas of stars - http://www.atlasoftheuniverse.com/12lys.html
    Last edited by a moderator: May 6, 2017
  8. Dec 25, 2012 #7
    Ah yes, but even introductory books like Brian Greene explain that it isn't everything moving randomly, but space itself is expanding. So the gravitational boundedness only matters so long before the space between all of us expands and we can see no stars locally either.

    A more scientific source: http://smithsonianscience.org/2011/04/big-bang-model/
    Last edited: Dec 25, 2012
  9. Dec 25, 2012 #8


    User Avatar
    2017 Award

    Staff: Mentor

    This is irrelevant on the scale of our galaxy.

    The milky way does not expand.

    This just confirms that you cannot measure expansion of the universe within a galaxy - you need stars which get ejected so quick and travel so far away that they are not bound to the galaxy any more.
    See Astronuc's links for scientific sources.
  10. Dec 26, 2012 #9
    If you re-read that source, it states that after the merger of The Milky Way and Andromeda galaxies (he didn't bother to name the other galaxies in our local group as they are much smaller in comparison, but they will merge with the big 2 also), "The universe’s ever-accelerating expansion will send all other galaxies rushing beyond our “cosmic horizon,” sending them forever out of view."
    Not our local group!

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook