If the dinosaurs ever looked skyward, they might have been treated to a rare spectacle. About 210 million years ago, a small galaxy plunged into Andromeda—the spiral galaxy closest to the Milky Way. Streamers of stars created by the collision would have been visible for million of years. Although the minor galaxy moved on, Andromeda still holds signs of the encounter. These include a newly discovered ring of glowing dust surrounding the inner part of the galaxy.
Astronomers had previously found other features that suggested a collision: an outer dust ring, some warping of Andromeda's spiral disk, and loops and r