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THE Milky Way’s twin galaxy had a more violent past than we thought, as pictures taken by the Hubble Space Telescope reveal.

The Andromeda Galaxy (M31) is the Milky Way’s nearest neighbour and is similar in size, shape and age. Both spiral galaxies are encircled by haloes of stars whose ages provide a vital clue to each galaxy’s history.

The stars surrounding the Milky Way are 11 to 13 billion years old. Now the age of the stars around Andromeda has been pinned down for the first time. The Hubble data reveals they are much younger, a third forming as recently as 6 billion years ago.

Tom Brown, who presented the finding to a Space Telescope Science Institute conference in Baltimore last week, speculates that the younger stars were ejected from Andromeda’s central disk when another galaxy crashed into it.