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Where is andromeda?

  1. Jan 10, 2013 #1
    where is andromeda???

    I know that when i look at the andromeda galaxy i am seeing it as it was 2.5 million yrs ago. what i want to know is am i seeing it where it is physically in space now or has it moved closer to us in the 2.5 million yrs that the light has taken to reach us...
  2. jcsd
  3. Jan 10, 2013 #2
    Re: where is andromeda???

    Hi and welcome to the forum your seeing where it was not its current position.
  4. Jan 10, 2013 #3
    Re: where is andromeda???

    do you know how much closer to us it is actually????
  5. Jan 10, 2013 #4
    Re: where is andromeda???

    I myself don't nor know how to calculate its current position but I'm sure someone does lol
  6. Jan 10, 2013 #5


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    Re: where is andromeda???

    According to the wiki on it, it is moving toward us at 300 km/sec.
  7. Jan 10, 2013 #6
    Re: where is andromeda???

    Does that movement rate account for current spacetime acceleration of the region between andomeda and the milky way?
  8. Jan 10, 2013 #7
    Re: where is andromeda???

    I think it does. Andormeda and the Milky Way are gravitationally bound, dark energy effects doesn't play a big role at such scales.
  9. Jan 10, 2013 #8


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    Re: where is andromeda???

    Correct. The expansion of the universe has zero* effect since our galaxies are gravitationally bound to each other.

    *I say zero, but in reality it is...complicated. The model that predicts and explains expansion models the universe as homogenous when it is not. Whether the metric expansion of space actually happens for bound objects is still up in the air I believe. But either way for all practical purposes we can say it has zero effect.
  10. Jan 11, 2013 #9
    Re: where is andromeda???

    (according to sites)
    Adromeda galaxy is approaching (moving) at about 2 billion miles per yr, 100 to 140 km per second, distance of 2.5 million light years and 'predicted' to be in collision after an initial approx 4.5 billion yrs but the photon we're observing happened (approx) 2.5 million light yrs ago(limitation of light speed). Were getting an actual approximation of 2.5 billion yrs. Here is an (approx) diagram of distance getting closer.


    Visual approximation (view of the skies and simulation of collision).



    Just a few questions though. Being noob in cosmology. Is it pertaining to initial computation considering the factor of time/conditions/system/parameter/N-body (timing argument)? Or just an approximation base on movement(local group and getting bigger in our line of sight). And since, we were observing data (photons) that happened 2.5 million years ago. Are there any special consideration(computation) to time or is it included in DL,DA,DC,DT. How did we get such values and are those values referring to actual events(now). Anyone? Thanks in advance.
    Last edited: Jan 11, 2013
  11. Jan 11, 2013 #10
    Re: where is andromeda???

    according to my dubious maths, i think andromeda has moved 2.3562 to the 19 km closer to us in the last 2.5 million yrs. can anyone confirm this??
  12. Jan 11, 2013 #11
    Re: where is andromeda???

    How to compute distances below 2 billion light years? Are we supposed to include the tiny value of dark energy or not needed?

    Here is the current distance but i'm not sure if it is in realtime or limits of light/time are considered/deducted (I don't know if the values are prior 2.5 million years ago or in real time). Or Messier 110 has been devoured by 'now'(just a thought). Or it is closer than we previously thought (realtime).

  13. Jan 11, 2013 #12


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    Re: where is andromeda???

    Well let's see my maths also gets dubious with oh so many 0000000000000's, but at a closing speed of 300 km/sec per russ_waters and that web site which seems to agree by noting that Andromeda Galaxy is getting 301 km closer to us each and every second of each and every passing day, i get (300 km/s)(3600 s/hr)(24 hr/day)(365.25 day/yr)(2.5 million years) = 2.4(10^16) km. Relativistic effects are minimal at that speed.

    At cosmological scales, distances get to be mind-boggling. 2.4(10^16) km is about 2500 light years. So she's 2500 light years closer to us than her observed position. Only minutely closer in the grand scale of things. In another 2.5 billion years, she'll be on our doorstep, although other sites say it is more like 4 billion years, buying us more time to prepare.
  14. Jan 11, 2013 #13


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    Re: where is andromeda???

    Just FYI, Google is your friend.
    The above link gives 2.4 × 1019 meters, in agreement with PhanthomJay's result.
  15. Jan 12, 2013 #14
    Re: where is andromeda???

    An easy approximation is that 1 km/s for 1 million years is about 1 parsec.

    Andromeda Nebula is approaching Sun at a speed of 301 km/s, and that value is known with the precision of +- 1 km/s.

    Andromeda Nebula is at a distance of 778 kpc, known with +- 17 kpc.

    At that speed, Sun should arrive in 2,6 milliard years.

    But the thing is, this is the radial velocity relative to Sun.

    Sun is orbiting Milky Way.

    Milky Way happens to have rotation plane near Andromeda Nebula.

    And Sun happens to be orbiting Milky Way in direction closely approaching Andromeda Nebula.

    The orbital speed of Sun around Milky Way is less well known. It turns out that the radial approach speed of Milky Way and Andromeda, as of now, is 109 km/s - with errors +- 4 km/s.

    In 100 million years, Sun will have completed half an orbit around Milky Way, and Andromeda Nebula shall be receding at 100 km/h.

    Note that within 2,5 million years, the curve of Sun´s orbit will have little effect yet... so the Sun will have approached Andromeda by about 800 pc or 2500 lightyears.

    If the mass of Milky Way and Andromeda were negligibly small, then Milky Way and Andromeda should take 7 milliard years to collide - unless they possess transverse velocity and miss each other.

    But the masses are highly unknown. Therefore it is unknown how much Milky Way and Andromeda would accelerate on approach, and when they would have a collision or closest pass.
  16. Jan 12, 2013 #15
    Re: where is andromeda???

    Its right here!

    Attached Files:

  17. Jan 13, 2013 #16
  18. Jan 13, 2013 #17


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    Re: where is andromeda???

    If anyone gets the program Sandbox Universe, it has a collision between Andromeda and the Milky Way that you can easily load up and watch happen. Pretty neat.
  19. Jan 13, 2013 #18
    Re: where is andromeda???

    Universal Sandbox looks very interesting: Thanks for the lead.

    A couple of other interesting simulator programs are the Mars24 Sunclock, which tells you the real time on Mars (runs on Java), and Stellarium, which is open source 3D planetarium software program. Both are free.

    Download locations.



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