An interloper from another galaxy scoots low across the south on October evenings. It’s a tight family of stars – hundreds of thousands of them. The stars probably belonged to another galaxy that was consumed by the Milky Way in the distant past.

Messier 30 is low in the south at nightfall, in Capricornus. The sea-goat’s brightest stars form a wide triangle. M30 is on the lower left side of the triangle

Messier 30 is a globular cluster – a ball of stars about 90 light-years wide. Most of the stars are concentrated in the cluster’s dense core. The numbers tail off as you move toward the cluster’s edge. Anything that wanders too far from the center gets yanked away by the gravity of the rest of the galaxy.

The Milky Way is home to more than 150 globular clusters. But several of them appear to have come from other galaxies. And that includes M30.

The main clue to its origin is its orbit. As it circles the center of the galaxy, M30 moves in the opposite direction from most of the stars and star clusters.

The only way for such a massive cluster to move against the traffic is if it came from outside the galaxy. So Messier 30 isn’t a native of the Milky Way. Instead, it was pulled in by the Milky Way’s powerful gravity – making it a refugee from another galaxy.

We’ll talk about an individual star that might be a refugee from another part of the galaxy tomorrow.

Script by Damond Benningfield