Do we really observe the edges of galaxies at different epochs? (Advanced)

While observing M31 (the Andromeda Galaxy), it occurred to me that I was seeing the near edge of Andromeda as it was 2.2 million years ago, however the photons from the rear of the galaxy had left on their journey about 150,000 years earlier than those at the near edge! So, do we observe an unrealistic juxtaposition of time, considering that we are seeing the far side as it was earlier than what we are seeing on the near side? It seems to me that we are observing an evolution "transition" and 150,000 years from now, the near edges of Andromeda will have evolved to geometrically accommodate what we are seeing now on the far edges! Or is the 150,000 light-year diameter for some reason inconsequential? This is very confusing to me and I would appreciate any clarification.

It is true that we observe the "near" side of M31 at a different epoch than the "far" side; it's a consequence of the finite speed of light, and something that we simply cannot avoid when studying galaxies.

However, it isn't as great a problem as you might think. The reason is that 150,000 years is actually a fairly short timescale when it comes to processes that affect the evolution of a galaxy: for instance, it takes more than 1000 times longer than this for the Sun to circle once around the Milky Way, and most stars evolve on timescales longer than this as well. So, though astronomers worry about it, they don't worry about it that much: it is a decent approximation to assume that all of the light that you see left M31 at the same time, just like on Earth it's okay to assume in most cases that all the light that you see left their respective objects at the same time.

This page was last updated on January 28, 2019.

About the Author

Kristine Spekkens

Kristine Spekkens

Kristine studies the dynamics of galaxies and what they can teach us about dark matter in the universe. She got her Ph.D from Cornell in August 2005, was a Jansky post-doctoral fellow at Rutgers University from 2005-2008, and is now a faculty member at the Royal Military College of Canada and at Queen's University.

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