Galaxies carry the evidence of their past. When the Milky Way captures a dwarf galaxy or tears apart a globular cluster, it leaves behind long, ghostly ribbons of stars called stellar streams. These streams preserve the history of ancient collisions, and their shapes may trace the invisible dark matter surrounding a galaxy.

This week, we follow the stellar wreckage left behind by galactic cannibalism, from the enormous Sagittarius stream wrapped around the Milky Way to Oyashio, the first suspected globular-cluster stream discovered beyond our galaxy. We’ll explore how astronomers use these delicate structures as cosmic windsocks and seismographs, and why gaps or kinks in a stream might reveal an encounter with something made almost entirely of dark matter.

We’ll also look at how the newly launched Nancy Grace Roman Space Telescope cou ...