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The Question

(Submitted January 16, 1998)

After reading a book about galaxies and learning that galaxies can sometimes change shape when one galaxy brushes past another galaxy, one of the students in our Third Grade asked if part of a galaxy can break off and join another galaxy when they brush past each other?

The Answer

This is a very good question. Unfortunately I can't give a definitive answer. Here is what I can tell you: galaxies consist of 3 kinds of material: gas, stars, and 'dark matter' (material that we know must exist because its gravity is needed to hold the galaxy together, but we can't observe it directly). When 2 galaxies interact at a distance, they affect each other through their gravitational forces. These are of 2 types: first is the ordinary gravitational attraction which holds us onto the earth, and which holds solar system together; second is the tidal force, which is due to the fact that gravity decreases with distance. The tidal forces due to the moon and the Sun are responsible for the earth's tides. If they were very much stronger, they could actually rip the oceans off of the earth, or rip the earth apart. There is no danger of this, but it can happen in galaxy interactions -- the tidal forces can disrupt one of the galaxies, or remove the gas from one of them. This has been suggested as a way of explaining why some galaxies (ellipticals) have little gas while others (spirals) have a lot more. In this case some of the gas is probably transferred to the bigger galaxy. Other possible interactions include total disruption of one of the galaxies, or merging of the two galaxies. Which of these occurs depends on how closely the galaxies approach each other and the masses of the two galaxies.

I hope this helps,

Tim Kallman
for the Ask an Astrophysicist Team

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