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Are We Seeing the Andromeda Galaxy as it Was 2.5 Million Years Ago?

-- Sean | June 20, 2020

Question:

If we are currently seeing Andromeda as it was 2.5 million years ago, how do we know it really is 2.5 million light years away from us? Hasn’t it traveled quite a distance away from that ancient location we now perceive it to be in? Aren’t we looking at a “ghost” of its past and therefore a “ghost” of its actual distance and location? (Polite laugh) I’m quite puzzled. Hoping you might help me. Thanks

-- Sean

Answer:

You are correct. Due to the “universal speed limit” for the transport of information in the universe, the speed of light, we see objects in space as they were in the past. For the Sun, its light takes about 8.3 minutes to reach us, so we always see the Sun as it was 8.3 minutes ago. The Andromeda galaxy is about 2.5 million light years away, so we see it as it was 2.5 million years ago. Now, regarding the distance that Andromeda has traveled in the 2.5 million years that its light has taken to get to us, it turns out that this very question has been answered quite well on the Astronomy Magazine web site. As this web site indicates, Andromeda is moving toward us at about 110 kilometers per second. If we assume that Andromeda has been moving at that same speed and direction for the past 2.5 million years, then in that time it has traveled about 900 light years closer to us. This is a small distance, especially when one considers the fact that Andromeda is about 110,000 light years in size.

-- Jeff Mangum

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