Ancient Galaxy collision

Astronomers surveying the Andromeda galaxy have discovered an association of stars in its outskirts, which they believe to be part of a separate galaxy that merged with Andromeda about 700 million years ago.

The findings suggest that Andromeda's outer swathes of stars are from the same parent galaxy, and may help astronomers determine the Andromeda's total mass.

Large galaxies are believed to be built partly by the merging of smaller galaxies, an event that destroys the smaller galaxy.

The strong gravitational forces of a large galaxy can rip apart a small galaxy, producing loose streams of stars that astronomers call tidal debris. Measuring the amount of the tidal debris that exists in present-day galaxies allows astronomers to examine the role mergers play in galaxy formation.

(Image courtesy of University Of Massachusetts Amherst)
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