Striking differences have been found between our galaxy the Milky Way and its near-twin neighbour, Andromeda.
Both are spiral collections of stars with similar structures and look almost identical, despite Andromeda being about twice as large as the Milky Way with a diameter of more than 200,000 light years.
But a new study has found that appearances can be deceptive.
Observations of the motions of different populations of stars in the galaxy suggest a history of violence, in contrast to the relatively peaceful Milky Way.
Andromeda's stars are more disordered than those of our galaxy, probably as a result of clashes and mergers with smaller galaxies.
Such violent histories are thought to be common for large spiral galaxies, with 70 per cent experiencing at least one interaction in the last 10,000 years.
The apparent orderliness of the Milky Way marks it out as unusual.
Professor Puragra Guhathakurta, from the University of California at Santa Cruz, led the research. The findings were presented at the winter meeting of the American Astronomical Association in Seattle.
Although "neighbours", the Milky Way and Andromeda are separated by a distance of 2.5 million light years.
The two are the largest members of the Local Group, a collection of 30 galaxies.
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