Astronomers have mapped an important sequence of events that shaped our galaxy 10 billion years ago. Our galaxy once swallowed another galaxy.

A violent collision shaped the milky way we see today. Astronomers are using the European Space Agency’s Gaia space telescope to understand how our galaxy swallowed another galaxy in the early days of the universe.

“Until now all the cosmological predictions and observations of distant spiral galaxies similar to the Milky Way indicate that this violent phase of merging between smaller structures was very frequent,” Matteo Monelli, an astronomer at the Institute for Astrophysics in the Canaries and a co-author of the article, said in a statement.

We know that two separate sets of stars comprise our galaxy. But exactly how or when they came together was a mystery.

Now, using the Gaia telescope, astronomers were able to take more precise measurements of the position, brightness, and distance of roughly one million stars.

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One of the sets is dominated by blue stars. And their motion told astronomers that they are the remnants of a dwarf galaxy that merged with the Milky Way. The other group is made of red stars.

Both groups live in the Milky Way’s halo, a spherical region that surrounds the entire galaxy.

Data from Gaia determined that the two populations of stars are identical in age, with each being no younger than 10 billion years old. All of these stars are older than the ones in the Milky Way’s disk.

The new research also suggests that one set of older stars are predominantly hydrogen and helium. Meanwhile, the other one contains larger amounts of heavier elements.

So, the story goes like this: Roughly 10 billion years ago, the ancient Milky Way and a smaller galaxy, called Gaia-Enceladus, slammed into each other. Over the course of millions of years, the Milky Way consumed the dwarf galaxy. About 4 billion years later a spurt of stars and gas from that activity settled to form the “thin disk” that runs through the center of the Milky Way.

Scientists published the new research in a paper published July 22 in the journal Nature Astronomy.

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