Question

What causes galaxies to collide?

Answer 1

When the galaxies collide, it causes vast clouds of hydrogen to collect and become compressed, which can trigger a series of gravitational collapses. A galaxy collision also causes a galaxy to age prematurely, since much of its gas is converted into stars

Answer 2

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Gravitational Collisions:

This is often the result of a galaxies drifting too close to one another, to the point where the gravity of the satellite galaxy will attract one of the giant galaxy's primary spiral arms. In other cases, the path of the satellite galaxy may cause it to intersect with the giant galaxy.