Cover Story: When Galaxies Collide

Mercury, July/August 2001 Table of Contents

Image courtesy of Brad Whitmore (STScI) and NASA

Though galaxies may appear isolated and serene, they frequently swoop past one another, crash into their neighbors, and transform each other in close encounters.

by Charles Liu

We live in the Milky Way Galaxy, a vast collection of gas, dust, over 100 billion stars, and a huge halo of dark matter. If we could fly out of the Milky Way and look back from a distant vantage point, we’d see that our home is a beautiful spiral galaxy, like countless others in the cosmos. Within our galactic neighborhood, we’d also see other spiral galaxies nearby, as well as elliptical galaxies and smaller, irregularly shaped galaxies. But a select group – perhaps one in a hundred – look downright peculiar, sporting rings, loops, and long tails of stars trailing away in odd directions. These are usually the result of galaxy collisions and interactions – some of the most awesome and astonishing events in the cosmos.

Not long ago, astronomers regarded galaxy collisions as rare occurrences of little consequence. We now suspect, however, that just about every galaxy in the universe has probably experienced such a close encounter. The details of these collisions, then, reveal vital secrets of galaxy birth, life, aging, and death – and the evolution of the universe itself.

The lure of these secrets draws me to study colliding and merging galaxies. I must admit, though, that the curiosity of a rubbernecker plays a part too. After all, who can resist slowing down to take a look at the grandest traffic accidents in the cosmos?

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