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How do galaxies collide in an expanding universe?

I was at the APOD page, and today's picture is of two galaxies colliding.

My question is how do galaxies collide? If things started in the Big Bang - moving away from every thing else - what would cause a mass the size of a galaxy to change direction and move into another galaxy?

It's interesting that you should ask a question about that picture. It was taken with the Wide-field Infra Red Camera (WIRC) at the Palomar Observatory. That camera was built by astronomers here at Cornell and a good friend of mine was involved in the observations in which that picture was taken.

Anyway it's quite natural for galaxies to collide even though the universe is expanding - although I could see why you might get confused about it. What happens is that there is a battle between the forces of gravity between the two galaxies (which is trying to pull them together) and the expansion of the universe (which is trying to pull them apart). With galaxies that start out quite close together, it is almost always gravity that wins, so in the end the galaxies will collide. This will most likely happen to the Milky Way and the Andromeda galaxy (our nearest large neighbour) in a few billion years.

Thank you Karen for your time, one quick follow up if you would. When Galaxies do collide is that the end of them or do they intermingle to make one large Galaxy.

It is thought that when two large spirals collide the end result is an elliptical, and that interactions with small galaxies are actually what make the spiral patterns in large spiral galaxies. It is becoming the idea that the morphology of a galaxy changes a lot over its lifetime as it interacts with other galaxies. Note that in galactic collisions stars do not collide.

April 2002, Karen Masters (more by Karen Masters) (Like this Answer)

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