Stars in Milky Way moving ‘like ripples on pond’, says European Space Agency

The Milky Way galaxy is enduring effects of a collision that took place nearly 300-900 million years ago and set millions of stars moving like ripples on pond, the European Space Agency (ESA) has said. An analysis of the motions of six million stars in the Milky Way disc - one of the major components of our Galaxy - by ESA's star mapping mission Gaia, revealed an interesting pattern.

"At the beginning the features were very weird to us," said lead researcher Teresa Antoja from the Universitat de Barcelona in Spain. "I was a bit shocked and I thought there could be a problem with the data because the shapes are so clear," she added.

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Researchers performed many tests on the data to look for errors that could be forcing such shapes on the data.  The reason they had not been seen before was because the quality of the Gaia data was a huge step up from what had come before.

In the study, appearing in the journal Nature, Antoja and her team investigated what had "hit" the Milky Way to cause this behaviour in the stars.

They found that the unexpected rippling movements of stars in the Milky Way took place when the Sagittarius dwarf galaxy sailed past it between 300 and 900 million years ago.

This small galaxy contains a few tens of millions of stars and is currently in the process of being cannibalised by the Milky Way.

"It looks like suddenly you have put the right glasses on and you see all the things that were not possible to see before," said Teresa.

"It is a bit like throwing a stone in a pond, which displaces the water as ripples and waves," she said.

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Unlike the water molecules, which settle again, the stars retain a 'memory' that they were perturbed. This memory is found in their motions.

Gaia is one of ESA's cornerstone missions and was designed primarily to investigate the origin, evolution and structure of the Milky Way. Gaia measures a three-dimensional movement of stars, using a combination of their position in space and their velocity.