In its younger days, the Milky Way devoured a smaller galaxy, and stars from the hapless victim still roam the skies today to tell the tale, a new study finds.
“This is a major event in the history of the galaxy,” says astronomer Amina Helmi of the University of Groningen in the Netherlands. “We’re really starting to probe 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 about 33,000 light-years of the sun, using data from the European Space Agency’s Gaia space telescope (SN Online: 5/9/18). A group of about 30,000 stars seem to be moving backward, the team reports October 31 in Nature. Instead of rotating around the galactic center with the sun and the rest of the stars in the Milky Way’s bright disk, these stars travel in the opposite direction.