Why do astronomers say that the Milky Way and Andromeda Galaxy are supposed to collide, Then they also say that they are moving away from each other (which is proven by its redshift).
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ANSWERS: 3
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Because the calculations are difficult and based on very little data. Even one new observation can change a theory entirely. And because of our limited observation point, it's challenging to gather enough information to really tell much. Yet.
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The uniform expansion of the universe is indeed based on redshift observations. More distant objects are moving away faster than closer objects, at speeds roughly proportional to distance (Hubble's Law). But at a more local level, especially within galaxy clusters, objects have their own peculiar local motion. So based on observation, Andromeda (our next-door neighbor galaxy) is apparently approaching the Milky Way galaxy and will collide with it. This does not violate the fact of the universe's overall expansion. When galaxies collide, btw, the individual stars rarely collide -- they just gravitationally affect one another to disrupt the shapes of the galaxies.
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Actually they're saying that the Andromeda and Milky way galaxies will eventually collide. But they never said anything about them moving away. The Andromeda galaxy's redshift is outweighed by it's blueshift. [edit] Expansion of space Main article: Metric expansion of space In the early part of the twentieth century, Slipher, Hubble and others made the first measurements of the redshifts and blue shifts of galaxies beyond the Milky Way. http://en.wikipedia.org/wiki/Redshift
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