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Does a Galaxy’s Distance from us Correlate with its Age?

-- Austin | April 18, 2020

Question:

Does how far away a galaxy (or other cosmic object) is correlate pretty closely to its actual age? For example, if a galaxy is 2,000 light years away, does that mean it couldn’t have been born more than roughly 2,000 years ago? How can you tell the age of these things if I’m doing research for what galaxies were around near the beginning of the universe?

-- Austin

Answer:

The observed expansion of our universe allows us to see galaxies as they were when our universe was younger.  You are correct in that if a galaxy is 2,000 light years away, it implies that it could not have been born more than roughly 2,000 years ago.  Distance is not strictly correlated to galaxy age, though,  as we are only seeing what a galaxy looked like at an earlier time.  Galaxy age is determined by a number of factors, including the local environment within which a galaxy lives.  One way that we can determine the age of a galaxy is to measure the ages of the stars in a galaxy.  Since we have a pretty good understanding of how stars evolve, we can take a census of the stars in a galaxy and look at the distribution of star ages in that galaxy.  This assessment of the distribution of stellar ages can tell us how old a galaxy is.

-- Jeff Mangum