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. This is 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.

The youngest stars in Andromeda move in a relatively ordered way around its centre while older stars display much more disordered motion

Puragra Guhathakurta, from the University of California at Santa Cruz, US, who led the research using observation data from the Keck Observatory in Hawii and the Hubble Space Telescope, said: “In this context, the motion of the stars in Andromeda’s disk is more normal, and the Milky Way may simply be an outlier with an unusually quiescent accretion history.”

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.

The scientists found that the youngest stars in Andromeda moved in a relatively ordered way around its centre while older stars displayed much more disordered motion.

Previous studies have found evidence of galactic mergers in tidal streams of stars in Andromeda’s extended halo. They appear to be the remnants of cannibalised dwarf galaxies.

The findings were presented at the winter meeting of the American Astronomical Association in Seattle.

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