Anonymous
Anonymous asked in Science & MathematicsAstronomy & Space · 3 weeks ago

Science question here. If the universe is expanding, how can astronomers tell the boundaries of different galaxies? ?

I understand that there is red shifting of light to different spectrums, but that still doesn't answer for me, how scientists can know....

"oh, ok, this is galaxy 1224a, and here is the boundary of galaxy xp7b" for example. It's not like there's a road sign demarcating them.

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  • 3 weeks ago

    We simply can not see that far

    Any thing that can be said is mere conjecture

    To make a Telescope that can look that far

    It would basically need to be the width of our Solar System

    And how could that be done ?

    • daniel g
      Lv 7
      3 weeks agoReport

      HST has been doing just fine, and the new Webb telescope is supposed to do better.

  • 3 weeks ago

    In general, galaxies are millions of light years apart... There's a few that are colliding, but the individual galaxies are pretty apparent. 

  • 3 weeks ago

    There's a set of receding light sources. The fact that they happen to be moving as a group is not the main issue.

  • Jon
    Lv 6
    3 weeks ago

    Sounds like you're not totally clear on what a galaxy is. The collections of stars we call galaxies are typically separated by distances much larger than their diameters, with few if any stars between them.

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  • 3 weeks ago

    Galaxies don't have definite boundaries. Is how. An entire galaxy has a red shift. Galaxies at different distances have different red shifts. Galaxies farther away from us have greater redshifts than galaxies closer to us. A few galaxies are blue shifted, Like the Andromeda I galaxy. . Gala dies occur in clusters and super clusters and sheets or walls with boss. Galaxies go through each other, "collide," but the spaces between stars is so great that stars rarely collide. Galaxies have Hill spheres of gravitational influence. So do planets and moons. Galaxies do.NOT have specific, fixed boundaries, .One major problem for the LST x40/ years add where are the dark matter halos around galaxies?. Scientists still have not found those dark matter halos that might define those boundaries, but the boundaries are not "fixed.". https://en.m.wikipedia.org/wiki/Hill_sphere.

  • 3 weeks ago

    The expansion of the universe is only about 7% per BILLION years, and that expansion is all in the space between clusters of galaxies.  What does that have to do with finding the boundaries of galaxies?

  • 3 weeks ago

    Surely you have seen photos of various galaxies, clusters, and nebula.

    You might note all the gasses, matter, stars seem to gather around what they call a barycenter of mass, sot of bound together in their own gravity field.

    Most are separated by a vast distance where galactic gravity has little influence on neighbor galaxies.

    They can estimate boundaries by viewing them and analyzing their sum mass.

    Our galaxy, Milkey way, is approximately 105,700 light years diameter. Not much matter outside this diameter.

    The Andromeda Galaxy (M31) , our closest neighbor, is similar at 220,000 light years diameter, but separated by 2 million light years. Even for this distance, the two are gravitationally locked, even getting closer at a rate about 110 kilometers per second.

    Some time in the future, the two will sort of do a docy-do and merge together.

    An event that does happen. Two galaxies with billions of stars together will make for one supermassive galaxy, the massive black holes at the centers likely form a binary black hole system. Not the first one out there.

    https://miro.medium.com/max/3150/1*Gatzf3rD088yg3O...

  • poldi2
    Lv 7
    3 weeks ago

    The edge of a galaxy is actually quite easy to recognize - its when the number of visible stars is almost zero.

    Each galaxy is separated by quite a distance from every other galaxy (other than interacting galaxies of course).

  • 3 weeks ago

    The Milky Way Galaxy is usually said to be about 100000 light-years across.  The Andromeda Galaxy is about 2.5 million light-years away.  That gives you an idea of why we don't need to define a boundary between them.  Many galaxies are MORE isolated than that.

    These two are indeed approaching one another, and are likely to collide.  Then the notion of a "boundary" will be meaningless, until or unless they pass through one another with each one retaining much of its original mass.

  • Anonymous
    3 weeks ago

    There is typically a LOT of space between galaxies, like millions of light years.

    If they have collided then yes, it's more difficult to say whether a group of stars came from galaxy 1 or galaxy 2.

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