Ancient cosmic encounter’s ripples in Milky Way

LONDON: The Milky Way galaxy is still enduring the effects of a near collision that took place sometime in the past 300-900 million years and set millions of stars moving like ripples on a pond, scientists say.

The European Space Agency (ESA)'S star mapping mission, Gaia, discovered the pattern of movement in stars in the Milky Way disc -- one of the major components of our galaxy.

The pattern was revealed because Gaia not only accurately measures the positions of more than a billion stars but also precisely measures their velocities on the plane of the sky.

For a subset of a few million stars, Gaia provided an estimate of the full three-dimensional velocities, allowing a study of stellar motion using the combination of position and velocity, which is known as 'phase space'.

In phase space, the stellar motions revealed an interest- ing and totally unexpected pattern when the star's positions were plotted against their velocities.

Teresa Antoja from Universitat de Barcelona in Spain said that she was surprised to see a snail shell-like pattern in the graph that plotted the stars' altitude above or below the plane of the Galaxy against their velocity in the same direction.

Researchers performed many tests on the data to look for errors that could be forcing such shapes on the data.

Yet no matter what they checked, the only conclusion they could draw was that these features do indeed exist in reality.

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.

"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.