The Milky Way ate a galaxy less than 10 billion years ago

In its young days, the Milky Way swalloweda small galaxy, and the stars of his unfortunate victim are still wandering the sky, telling their stories, scientists say. “This is a big event for the history of the galaxy,” says astronomer Amin Helmi of the University of Groning in the Netherlands. "We are just beginning to explore the ancestors of the Milky Way." Helmi and her colleagues analyzed the speeds and positions of tens of thousands of stars in the Milky Way within 33,000 light-years from the sun using data from the Gaia space telescope, owned by the European Space Agency. A group of 30,000 stars seems to be moving backwards, which scientists wrote on October 31 in the journal Nature. Instead of revolving around the galactic center with the sun and the rest of the stars on the bright disk of the Milky Way, these stars move in the opposite direction.

“That was the first hint,” says Helmy. "When the stars move in the opposite direction, this suggests that they did not form there and not like most stars in our galaxy."

The second hint came from finding these stars inThe APOGEE experiment catalog, which takes the light spectrum of stars as a basis to understand their chemical composition and age. Moving in the opposite direction had fewer heavy elements than stars like the Sun. This suggests that they formed earlier in the history of the universe, even before the time came for massive stars and supernovae that propagate heavy elements throughout the galaxy.

Milky Way - the eater of galaxies

This chemical composition has strengthened the confidence of scientists. It immediately became clear that they had formed somewhere else.

Comparing observations with computer models,Helmy’s team concluded that 10 billion years ago, the Milky Way collided with a smaller galaxy, possibly four to five times smaller than the Milky Way at the time, containing 600 million stars with a mass of the Sun.

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