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Closeness of Andromeda galaxy

  1. Nov 24, 2011 #1
    Two thoughts on this. If the Milky Way is one hundred light years in diameter and Andromeda is 2.5 million light years away that means Andromeda is 25 times the diameter of the Milky Way distant from us. That's not much. When will they merge? And I wonder how long it will take for the merging to take place. That is from the time the two just start to touch to complete merging. I ask because I wonder if in the process of merging and the resulting disruption if any of solar systems if there would be time for earthlike life to evolve. And then it's been found that the furthest galaxy is about 13,230 million light years away (according to Wikipedia) and that means it's about 5300 times further than Andromeda from us...I think. Having said that ( I finally used that worn out phrase) if you take one of those corkboard pins with the round colored head and call that 1/8 inch in diameter then Andromeda's pin would be 3.125 inches from the Milky Way's pin and the furthest galaxy's pin would be (1/8" = 100,000 light years. and 13,230,000,000 / 100,000 = 132,300/8= 16,538 inches...and 16,538 inches=1,378 feet or about a quarter mile away) from Earth's pin. Anybody want to check this?
  2. jcsd
  3. Nov 25, 2011 #2


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    Andromeda is expected to collide with us in about 4.5 billion years. Givin the vast distances between stars, I would imagine that it is very much possible for life to evolve even during this collision or merger, but obviously I have no real way of knowing.
  4. Nov 25, 2011 #3
    You might be interested to know that it looks like our galaxy is right now colliding with the dwarf Magellanic Clouds galaxies. While they are relative small, that smallness is only relative.

    They used to be considered satellites however, in 2008 Australian astronomers established two facts 1. They don't appear to be in orbit but rather passing through. 2. That passage already started with their gas streams being pulled inside the milky way.

    Given the vast distances and relatively very low speeds involved, you shouldn't expect to see anything spectacular any time soon however.
  5. Nov 25, 2011 #4
    In addition, there is a stream or jet of stars at the outer edges of the Milky Way which, because of their unusual orbits, are thought to be the remnants of a past collision like that. Other galaxies which we could study in enough detail also often show similar feature.

    In short - no need to panic just yet :-)
  6. Nov 25, 2011 #5

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    First, you are off by a factor of 1000. It's 13 billion, not 13 million. Second, that 13+ billion light years figure is a bit (more than a bit) deceiving.

    It's more or less OK to use time in years = distance in light years * speed of light for short distances such as the mere 2.5 million light years that separate the Milky Way and the Andromeda galaxy. It is not OK to do that for longish time spans because the universe is expanding.

    At the time those remote galaxies emitted the light that we are just now seeing, the distance between that remote galaxy and the Milky Way was much less than 13.5 billion light years. Thanks to the expansion of space, the distance between that remote galaxy and the Milky Way "now" (the comoving distance) is much greater than 13.5 billion light years.

    Bottom line: It is far better to say that the light emitted by those far away galaxies took 13.5 billion years to reach us and just leave it at that.
  7. Nov 25, 2011 #6
    First, the OP was correct with his original number. 13,200 million light years IS 13 billion!
    Secondly, the OP was incorrect with his statement about the diameter of the Milky Way.
    The Milky Way is more like 100,000 light years in diameter, NOT 100 light years!
  8. Nov 25, 2011 #7


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    Maybe DH is European and uses commas to function as decimal points, rather than periods. Or maybe he just didn't read it closely enough. In any case, it doesn't change the truth of his original point, which is that 13 billion light years is still the wrong distance value: the distance to the object is not equal to the light travel time, due to the expansion of the universe. So, if light from that object took 13 billion years to reach us, then that object is now much farther than 13 billiion light years from us.

    I think this was a typo. It's pretty clear that the OP meant 100,000 light years, because he/she knew that 2.5 million light years was about 25 times the diameter of the Milky Way.
  9. Nov 26, 2011 #8

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    I've been reading too many posts by European as of late. Sorry for the invalid accusation.

    As cepheid noted, my main point still stands. Multiplying the 13 billion years for the light from some remote galaxy to reach us by the speed of light is not a proper thing to do.

    Getting back to the original post,
    What exactly do you mean by this?
  10. Dec 9, 2011 #9
    I heard this on the Discovery Channel also, and i don't get it. what is space? how come it can expand?
  11. Dec 9, 2011 #10


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    Disc. Channel may present things in a misleading or dumbeddown way to get viewers' attention. I don't know since i don't watch.

    I don't say "space expands" because it gives some people a mistaken idea of space as a material substance or "fabric".

    What has been gathered from observations over the years is a pattern of increasing distances between widely separated stationary objects.

    This is not like ordinary motion where somebody gets somewhere. In this pattern of increasing distances nobody gets anywhere. The distances between stationary objects just get larger in a kind of uniform way. Longer distances increase more rapidly.

    The current rate is about 1/140 of one percent every million years. Of course that is really tiny. 1/140 of a a percent is almost nothing, it would seem, and a million years is a long time needed for such a little change to happen. You would only notice if you had a very large distance to start with.

    The basic point is that geometry is not a fixed static thing. Geometry can change. That was Einstein's contribution. His 1915 "general relativity" equation is both a law of how gravity operates and a law of how geometry changes. Distances and angles can evolve according to his 1915 law. They aren't fixed eternal the way the Greeks thought. (no disrespect to the Greeks they had some fine scientists.)

    If geometry did not change, gravity would not work the way it is observed to work. The 1915 is much more accurate at predicting things that we can measure (on or around Earth, in the solar system, in the greater world at large). We get 6 decimal accuracy using the 1915 equation and so far nobody has been able to come up with something better. So it is just something to accept. Angles and distances can change over time in certain predictable ways, as material objects move around and influence geometry. Or just as a result of continuing some very gradual change that was begun in the past.

    The theory does not postulate "space" as a "substance". It is about geometrical measurements.
    Last edited: Dec 9, 2011
  12. Dec 9, 2011 #11
    Thanks Marcus, now it makes sense. I thought it was just a mind invention so that The Big Bang Theory would still stay up (i'm referring to the inflation, the fast expanding space, as they say, or in the light of your explanation, fast changings in geometry)
  13. Dec 24, 2011 #12
    Video didn't embed
  14. Dec 31, 2011 #13
    Can we measure a distance of nothing? Words can be so inaccurate and suggestive I suspect. It is just difficult to wrap your mind around something of nothing. We observe nothing in a vacuum and we can measure the distance so it must be true.
  15. Dec 31, 2011 #14


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    We observe real matter in the form of galaxies at very large distances, not empty space. As you say, we cannot measure "nothing".
  16. Jan 4, 2012 #15
    In the original thread post I was wondering about the time it would take for two galaxys to integrate in a collision. I found a video at space.com which I can't seem to pass on here but it showed about 400 million years for the process. It shows the stars etc being seriously spun out of place whereas some give the opinion that no contact would occur because of the vast space between the elements involved. Just guessin but is that long enough under those disrupting conditions for intelligent life to develop?
  17. Jan 4, 2012 #16
    Given our own evolution, I would say that 400 million years is time for life to develop elsewhere. The state of the 2 galaxies would be chaotic though surely. Apparently no contact would take place, but even so, orbits would change, sytems would be reconfigured.
    From what I understand, Milkomeda will be a large eliptical galaxy.
  18. Jan 4, 2012 #17
    While the structure of the galaxy may be seriously disrupted, it's still unlikely that anything will pass close enough to the sun to disrupt the solar system.
  19. Jan 4, 2012 #18
    If it takes 4.5 billion years for the collision, and another 4 billion to merge, that's 8.5 billion years, our sun would be a white dwarf by then, and my rent overdue XD
  20. Jan 5, 2012 #19
    Also a glaxay merger will cause a burst of star formation. This is a prerequist for life.
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