BOTB: Ask an Astronomer

(Photo courtesy of Jeff Mangum)

Why are Andromeda and the Milky Way on a Collision Course Rather than Moving Away from Each Other?

Posted on March 18, 2015 by Jeff Mangum

Question: If the universe is expanding and all galaxies are moving away from each other, how is it possible that the Andromeda and milky way galaxies are on a collision course? – Johnny

Answer: It is correct that on the largest scales that the universe is expanding such that all galaxies are moving away from each other. On smaller scales, though, there are so-called “peculiar motions” of galaxies, where one galaxy is found to be moving toward another galaxy due to local gravitational effects in the vicinity of the two galaxies.

How Do Astronomers Study How Things Evolve?

Posted on September 7, 2014 by Jeff Mangum

Question: We know that stars and galaxies we see are just fossil light as they were millions or billions of years ago. Is it possible to extrapolate the changes that we see today in those galaxies to determine their current state? – Vinod

Answer: In a way, yes. Since, as you point out, we see what amounts to the “fossil light” from stars and galaxies in the universe, we can piece-together how things evolve with time by sampling various times within this fossil record to study the evolution of these stars and galaxies. Note also that the timescales for the evolution of objects in the universe are, with few exceptions, much longer than a human lifetime, or even the total historical record of scientific measurements. This means that astronomers must study the evolution of just about every object in the universe by sampling its evolutionary state at different times in the cosmological record.

From Ask An Astronomer.
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