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The history of our galaxy was much more turbulent than we thought. In a new document published in Nature This week, astronomers discovered that hundreds of millions of years ago, the Milky Way suffered a near-collision that caused the movement of millions of stars on its disk.
From our point of view here on Earth, the Milky Way looks like a series of bright stars against the night sky. But its actual shape is a more complicated spiral, with curved arms extending into a disk around a domed center. The majority of the stars of the Milky Way are on this disk and not on its central bulge. Astronomers generally assumed that the stars of the disc were moving in a relatively boring and symmetrical manner around the galactic core, having long been settled in a compound equilibrium for over 13 billion years since the birth of the galaxy .
To learn more about the origin, the evolution and structure of the Milky Way, the Gaia Space Observatory of the European Space Agency creates a 3D map of more than one billion stars across the galaxy with unprecedented accuracy. In the new study, scientists focused on the positions and speeds of six million stars.
The researchers unexpectedly detected a spiral pattern resembling a snail shell in the movements of these stars. Where other stars in the galaxy have relatively stable patterns of motion, they have been disrupted, like dust particles caught in a sudden breath of air. "At first we thought that there was a problem in the data, so we spent quite some time checking if this was real," said Teresa Antoja, lead author of the study and astrophysicist at the University of Barcelona in Spain.
Previous studies have failed to see this model as they were less accurate and looked fewer stars. "While these results are staggering, it's a minimal effect that they've found," said astrophysicist Kathryn Johnston of Columbia University, who did not participate to this research. "It's a testimony to Gaia's amazing data that we can see just about anything."
These new findings suggest that this model began between 300 and 900 million years ago, when previous work revealed that the Milky Way had met closely the Sagittarius dwarf galaxy in orbit. "Looking at the speeds of the stars that we measured today, we can travel to the past of our galaxy and see very strongly the disruption of our galaxy," said Antoja.
The dwarf galaxy of Sagittarius has a few tens of millions of stars (the Milky Way has 100 billion to 400 billion) and is currently cannibalized by our galaxy. His last close passage with the Milky Way would gravitationalize the stars of our galaxy. "We had already hinted that dwarf galaxies orbiting our own galaxy had an influence on our own galaxy, but I think it's the clearest evidence that this influence can be quite strong," he said. Antoja.
"We tend to think that the galaxy has a relatively quiet history," added Johnston. "What these results show us more about its history of interactions, which could help us better understand galaxies as a whole."
Much remains unknown about this ancient close encounter. "We do not know which parts of the galaxy were most affected or if the sun was also affected by this disturbance," said Antoja.
Future research on this spiral model could also shed light on hidden aspects of the Milky Way, said astrophysicist Lawrence Widrow of Queens University in Kingston, Canada, who did not participate in this event. study. For example, this spiral model is supposed to evolve over time, to curl more and more in the past and to examine the speed with which this spiral spiral can illuminate the gravitational field of the Milky Way. This in turn can help to obtain information about his mass, which may reveal details about the nature of his dark halo of dark matter, he said.
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