Stars at the Milky Way's Edge Could be Remnants of "Galactic Invasions"

Out on the fringes of the Milky Way are clusters of stars that drift in a region of space astronomers call the galaxy's "halo." For years, researchers believed that these stars were the remains of smaller galaxies that were torn to shreds when they collided with our own. But a closer look at the chemicals that make up these stars suggests that they born a bit closer to home.

According to a new study published in the journal Nature, researchers at the Max Planck Institute found that several groups of stars found on the furthest edges of the Milky Way have the same kinds of chemical composition as our own galaxy. Columbia University astronomer Kathryn Johnston co-authored the study and told WNYC that if the stars had come from outside the Milky Way, they would have been made up of different combinations of elements.

"In different types of galaxies, they get spread in different ways and incorporated into the next generation of stars in different ways. And that leads to different chemical compositions for the stars of big galaxies versus small galaxies," Johnston said.

Before, scientists believed that these stars got caught up in the Milky Way's gravitational pull as it tore through other, smaller neighbors. However, the composition of these stars suggests that the smaller galaxies had a much stronger impact on the Milky Way's rotation than astronomers once thought. According to Johnston, the impact of these galactic collisions was strong enough to tilt our galaxy and affect its orientation in space, marooning these stars on the Milky Way's frontier.

"We've gone from thinking, 'Okay, this is evidence of a little galaxy being ripped apart, oh the poor little galaxy got ripped apart,' to that little galaxy actually invading our galactic disk and causing our disk to wobble," Johnston said.

So far, the researchers have identified two different star groups that they believe were evicted from the Milky Way and relegated to its fringes. Johnston said future research could reveal when these collisions happened and how much they made our galaxy wobble.