Shell structures in our Milky Way are evidence of a huge collision between it and another dwarf galaxy, astronomers have found.

The crash happened nearly 3 billion years ago, when the dwarf galaxy fell into the middle the Milky Way and was torn apart by the collision.

But the merger has left behind evidence in the shape of formations, the first such "shell structures" ever found in the Milky Way, that shed more light on the collision as well as potentially explaining other phenomena.

The research not only sheds light on the history of the vast crash known as the Virgo Radial Merger, but also the “Gaia Sausage”, a clump fosters that is thought to have arrived in the Milky Way following a merger with another dwarf galaxy.

Usually, a cluster of stars of that kind would move together – and so astronomers proposed that the unusual behaviour was the result of a radial merger, where another galaxy had crashed into our galaxy side on.

Astronomers now say that the same merger left behind the shell structures that appear to have been formed as the dwarf galaxy was torn apart and bounced up and down through the Milky Way as it was sucked into our galaxy.

They found that the dwarf galaxy appears to have first passed through the centre of the Milky Way 2.7 billion years ago.

As the team continued to work through the possibilities, they realised that our Milky Way’s shells could be the result of a radial merger – just one that looks a little different from normal.

It just looks different because, for one thing, we're inside the Milky Way, so we have a different perspective, and also this is a disk galaxy and we don't have as many examples of shell structures in disk galaxies.".

The research suggests there is also more to be understood about the Gaia Sausage, another formation of stars in the Milky Way, which is thought to have come about after a merger with a dwarf galaxy some 8 to 11 billion years ago.

Researchers had previously thought that the two clumps results from the same event – but the fact that the Virgo Radial Merger is much younger suggests either they are actually separate, or that the Gaia Sausage also happened much more recently than expected?