A two-decade-long riddle about the bizarre shape of the Milky Way’s nearest spiral-shaped galaxy, Andromeda, has been solved, suggests a new study. Instead of having the flat plane and outflung arms that are the hallmarks of a mature spiral galaxy, Andromeda has a warped plane and several rather chaotic, overlapping outer rings. The reason, according to an international team of astronomers, is that Andromeda suffered a head-on collision with a smaller galaxy some 210 million years ago. The evidence comes from infrared images taken by NASA’s Spitzer Space Telescope. They show a previously hidden, tilted second ring that protrudes from the heart of the galaxy. This ring is likely to be the shockwave of gas and dust from a colossal collision. The theory has been put to the test in a computer simulation. It suggests a dwarf galaxy called M32 probably drove straight into the heart of Andromeda.

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