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Galactic orbits

  1. Feb 8, 2016 #1
    I just read that our solar system takes 225 million years to orbit the center of the galaxy. Is it possible that the other nearby stars in the Milky Way are orbiting faster or slower or have a more or a less circular orbit than does our Sun? If so, would the stars in the night sky as viewed from Earth look completely different 225 million years from now?
     
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  3. Feb 8, 2016 #2

    mfb

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    Typical relative velocities between nearby stars are about 20 km/s, or 1 light year per 15000 years. To have a completely different night sky, it is sufficient to wait a few hundred thousand years, as most bright stars are nearby (<20 light years away). To just see some notable deviations, even 2000 years are sufficient (and we have astronomic records that are older than that).

    Over the course of ~10 orbits, the positions of (now) nearby stars become completely scattered over the galaxy. This also works for the past: all the stars that formed together with sun are quite scattered around the galaxy now.
     
  4. Feb 8, 2016 #3

    phinds

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    @Alltimegreat1 something to keep in mind is that the spiral "arms" of the Milky way (and other spiral galaxies) are NOT coherent "arms" at all. They are "waves" of conglomerations, the contents of which change over time, as mfb explained.
     
  5. Feb 8, 2016 #4

    tony873004

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    Many of the brightest stars are very far. They're just BRIGHT. And many of the closest stars are dim, too dim to see without a telescope, including the closest star, Proxima Centauri.

    I don't know what the range of luminosities is, but it wouldn't surprise me that if the brightest stars were represented with a bank of stadium lights, the dimmest ones would be represented by the glowing tip of burning incense.
    .
    When I view Orion through binoculars, 10x more stars became visible. I used to think of these dim stars as far, while the bright ones that make up the asterism were close.

    On this trip to Orion, notice how many dim stars fade into view and whiz by you before the asterism starts distorting.


    So although all constellations change very little with time, some are extremely slow to change such as Orion.
    You can watch all the constellations change with time, past and future, in this simulator I made. It was used to make the above youtube video.
    http://orbitsimulator.com/gravitySimulatorCloud/properMotionHome.html
     
  6. Feb 9, 2016 #5
    It is indeed fascinating that everything in the universe is on the move and nothing stays put. Shouldn't we be concerned in the long term about a collision with another star? How close could we get to a star or binary system like Centauri before gravity would cause a crash?
     
  7. Feb 9, 2016 #6

    phinds

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    Chances of that happening are essentially zero. When galaxies, each having billions of stars, collide it is estimated that there will be maybe one or two star collisions. The distance between stars is HUGE.
     
  8. Feb 9, 2016 #7
    So what happens when galaxies collide if nothing actually crashes into anything else? Would the galaxies eventually pass through each other or would they remain together? Some claim the Milky Way and Andromeda galaxies are on a collision course.
     
  9. Feb 9, 2016 #8

    phinds

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    The gases interact (collisions) and gravity causes the galaxies to merge. They pass through each other but slow down a lot and come back towards each other. Graphical simulations are readily available on the internet. It is not a "claim" that the Milky Way and Andromeda will collide/merge, it's a simple fact.
     
  10. Feb 9, 2016 #9

    Janus

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    If we just consider those stars that are naked_eye visible, it works out that The dimmest star compares to the brightest star like a single mini Christmas light compares to 800 1500W metal Halide lights (in terms of absolute luminosity) .

    If we are talking about known stars, then the dimmest (normal)star is 1/16,000 as bright as the Christmas light and the brightest star is some 1,250,000 times brighter than the 8000 metal Halide lights.
     
  11. Feb 9, 2016 #10

    mfb

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    Okay, my 20 light years estimate was too low. 17 stars in that range are visible to the naked eye (using 6 mag as limit), 4 of them are brighter than 2.5 mag.
    In total, roughly 5000 are brighter than 6mag, and 92 of them are brighter than 2.5 mag.
    The list of brightest stars can also be sorted by distance.
     
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