Do we really observe the edges of galaxies at different epochs?
While observing M31, 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 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 LY 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 150,000Ly 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.
Get More 'Curious?' with Our New PODCAST:
- Podcast? Subscribe? Tell me about the Ask an Astronomer Podcast
- Subscribe to our Podcast | Listen to our current Episode
- Cool! But I can't now. Send me a quick reminder now for later.
- How long does it take for the Sun's light to reach us?
- Why is looking out into space the same as looking back in time?
- Why doesn't light from distant galaxies reach us instantaneously?
- Can we make a movie of the Milky Way galaxy's rotation?
How to ask a question:
If you have a follow-up question concerning the above subject, submit it here. If you have a question about another area of astronomy, find the topic you're interested in from the archive on our site menu, or go here for help.Table 'curious.Referrers' doesn't existTable 'curious.Referrers' doesn't exist
This page has been accessed 17917 times since October 20, 2004.
Last modified: October 20, 2004 9:06:17 PM
Ask an Astronomer is hosted by the Astronomy Department at Cornell University and is produced with PHP and MySQL.
Warning: Your browser is misbehaving! This page might look ugly. (Details)