More precisely, the Milky Way collided with the second galaxy, absorbing many of its stars and spiraling out a chaotic tangle of stellar matter — birthing new stars, altering the orbits of others, and sending some in the opposite direction of the Milky Way’s own rotation.
That last bit provided the first clue to astronomers, who have long observed that some stars in our home galaxy are essentially going the wrong way, moving against the trailing arms of the galaxy. Other stars have been observed to spin in strange clusters, bolstering the theory that the Milky Way is actually the result of a number of previous mergers and meals.
New research published in the journal Nature confirms that a good number of these rogue stars have a common origin — a deceased galaxy dubbed Gaia-Enceladus. It also suggests that our galaxy was largely formed by a few big collisions, rather than a lot of smaller ones, which helps answer a question that has occupied astronomers for years.
The discovery was made by an international team of researchers led by Amina Helmi, an astronomer with the University of Groningen in the Netherlands.