# If the universe is expanding, how can 2 galaxies collide?

Sorry if this has been answered before, I am very new to this forum.

So, if the universe is expanding, how is it possible that Andromeda galaxy, and our Milky Way will collide? Should they not be moving away from each other, given that the distance between them is big enough for space expansion to occur (not sure it is really) ?

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mathman
The expansion of the universe is a very large scale phenomenon. Galaxies which are relatively nearby have their own proper motion.

The expansion of the universe is a very large scale phenomenon. Galaxies which are relatively nearby have their own proper motion.
Thank you.

And is it a common thing for 2 galaxies to collide?

davenn
Gold Member
2019 Award
No, I wouldn't say its common, considering the large distances between galaxies
There are some examples of colliding galaxies imaged by the Hubble Space Telescope and others

do some google searching for " images of colliding galaxies"

What mathman was hinting at was there are many galaxy clusters / groups and those clusters are gravitationally bound to each other regardless of the overall expansion of the universe.

The Andromeda and the Milky Way galaxies are part of a cluster called the Local Group of galaxies

Dave

Thanks you both, much appreciated

There are many scales of structure and the actions of the whole do not preclude local interaction. I'm going to post a link for a truly mind blowing video that while it has a bit less impact on a computer screen than it did when I first saw it at a Goddard Space Flight Museum demonstration where it was projected on a square on the floor exactly 1 meter on a side, it's impact is rather undeniable on one's perspective of the macro and the micro scales.

Enjoy!

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Are galactic mergers common?

And is it a common thing for 2 galaxies to collide?
If we include collisions and mergers of galaxies much different in mass - big galaxies "absorbing" smaller ones, know as minor mergers - likely yes. Much of the complicated structure and irregular composition of spiral galaxies is believed to be due to such events. Some astronomers conclude that our own Milky Way galaxy is currently in various stages of absorbing 2 or 3 small galaxies.

Major mergers - collisions and mergers of galaxies around the same mass - are also a common feature of current popular theories. About 10-15% of galaxies that have been categorized by shape are categorized as elliptical. Some, perhaps a consensus of astronomers in the specialty support theories that conclude that all or nearly all elliptical galaxies are the result of major mergers. One reason, which doesn't rely on computationally intense modeling, is that elliptical galaxies are more common in the near, and thus the more recently, than far away, and thus the ancient, universe, suggesting that, as the universe ages, elliptical galaxies become more common as primordial spiral and lenticular galaxies collide and merge. Another is that galaxies have been observed that appear to have proportionally much more massive central supermassive black holes than the one in our Milky Way or nearby Andromeda galaxies (For example, M87 is about 2 times as massive as the Milky Way, but appears to have a SMBH about 300,000 times as massive). A possible explanation for anomalously large SMBHs like this is that they are the result of many galactic mergers.