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Abt seeing the Andromeda Galaxy with the naked eye in future

  1. Dec 4, 2015 #1
    According to what i've read, the andromeda galaxy is approaching the milky way galaxy.
    Also, i've just recently discovered (on the net) that andromeda has a much larger subtended angle than the moon or sun. Hence, as it approaches it's angle subtended will keep increasing.
    I've also known that the andromeda can be seen as a fuzzy something in the night's sky at a remote place at sea level to the naked eye., but i don't think its as large as the moon.
    So, my question is (could not put the full question in the heading):
    How many years in the future will the present "naked eye" of a human be able to see a fuzzy silhouette of the andromeda galaxy as big as the moon? I can only see 2 factors to be considered, the increasing angle subtended of the andromeda galaxy and the decrease of the distance light has to travel, but i do not know the math. The state of earth's atmosphere in the future cannot be determined, i guess, so you can use its present state.
  2. jcsd
  3. Dec 4, 2015 #2


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    I've just read in Wiki that M31's virtual diameter is already 6 times as big as the moon's. It should be an easy trigonometric task to calculate it: 140,000 lyrs in diameter, 2.5 million lyrs away, at 114 km/s approaching Milky Way, collision in 4-10 billion years.
  4. Dec 4, 2015 #3
    You seemed to have mis understood my question. The collision part i understand, and also i understand the virtual diameter thing. What i'm asking about is how many more years down the line will we start seeing the andromeda bigger than the moon. All we see right now (with the naked eye) is some fuzziness much smaller than the moon.
  5. Dec 4, 2015 #4

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    Depends on where you are. In very dark places, you can see a greater extent of it. My memory from my younger days, a time when very dark skies were available to me, it looked like it might have been as long as the moon. (A naked-eye comparison is impossible)
  6. Dec 4, 2015 #5
    So if you were on the back side of the moon, perhaps you could see much more of it naked eye.
  7. Dec 4, 2015 #6


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    Right now with the naked eye we can only see the central nucleus of Andromeda as it is the brightest part of the galaxy. The rest is just too dim. As it gets closer, the rest will begin to become visible. However, at the same time, the apparent size of the visible nucleus will increase. The brightness decreases sharply outside the nucleus area and then falls off more slowly. As it gets closer the nucleus will increase in apparent size and we will see more of the galaxy outside of the nucleus. Exactly when these two effects will result in an naked eye visible image of the galaxy depends on just how the brightness of the galaxy decreases with distance from the nucleus.
  8. Dec 6, 2015 #7
    Because there's no blanket of gas to conceal it, yes and perhaps. One should assume equatorial atmosphere over the pacific ocean?

    You have understood my question exactly, but is it too much to ask for the subsequent math and number? I do not know the math.
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