By Ron Cowen
Orbiting above and below the bulged center of the Milky Way is an elongated, wispy contingent of stars. It could be considered a mere footnote amidst the galaxy’s stellar hordes, but astronomers are reading it as if it were an entire chapter early in the galaxy’s autobiography. That’s a surprising view, given that these stars didn’t even start out in the Milky Way. Discovered over the past year, the orbiting contingent appears to be the tattered remnants of a galaxy that collided with ours billions of years ago. If this were confirmed, the intruder galaxy would have had about 15 percent of the stellar mass of our galaxy during that early epoch. So, it packed enough of a wallop to change the overall shape of the Milky Way. This collision about 10 billion years ago was probably the most consequential event in our galaxy’s 13-billion-year history. Since then, the galaxy has enjoyed a relatively peaceful existence.
Now, astronomers are reconstructing what happened during the Milky Way’s big collision and wondering if many other galaxies across the universe have a similar history. New evidence suggests that this may be the case.