Interacting Galaxies

A common occurrence in the universe, and an essential ingredient for the formation of many types of galaxy, is galaxy interaction. This can happen on a smaller scale (for example, a satellite galaxy warping the spiral arms of a spiral galaxy) or on a much bigger scale (where entire galaxies collide). When galaxies collide, it can be devastating to their morphology, more often than not resulting in peculiar or even irregular galaxies. Galactic collisions are also thought to be behind the formation of elliptical galaxies and even lenticular galaxies.

Near-miss encounters can also significantly warp a galaxy's shape. In some cases, these galaxies may even exchange some material in their brief encounters.

When galaxies collide, the odds are extremely low that any two stars themselves may collide, as stars are very small compared to the spaces between them. However, clouds of gas and dust will be compressed during collisions and this will trigger lots of star formation.

Galactic interaction is a very slow process and  and is unlikely to affect a galaxy’s habitability significantly. The night sky might be warped significantly.

Examples of interacting galaxies include the Antennae Galaxies, NGC 520, the Mice Galaxies and NGC 2207/IC 2163. In about 4.5 billion years, Andromeda and the Milky way will collide.

Worldbuilding in Practice
"The Galaxy NSG 104 are two galaxies colliding and according to a computer simulator, NSG 104 is going to merge into an Elliptical Galaxy. Recently, large amounts of stars and planets are suddenly popped out between the Galaxy colliding area, possibly due to gas and dust forced to merge into stars and planets with the collision. The local civilization noticed the spectacular Galaxy changes and some panicked that star collisions will appear. However, there is a really rare chance to have a star collision."