It was once thought that the Milky Way grew by gobbling up other galaxies. The outskirts of our Galaxy are littered with leftovers from such meals. The most prominent is the 'Gaia Sausage' cloud. For a long time, it was taken as the scar of a single powerful impact. But the DESI telescope, splitting starlight into colors, revealed four distinct streams within. Their ages range from 7 to 12 billion years, and their chemical makeup varies more than the recipes for a layer cake. Some are rich in carbon and oxygen, while others contain iron forged in supernova explosions and the disruption of stellar remnants — white dwarfs. Each stream is a separate layer in this cosmic pie, adding its own ingredient to the overall structure.

This discovery flips the picture: instead of one collision, a series of small mergers. The oldest group of stars predates our galactic disk: they ignited when the Universe was just 1.5 billion years old. The layered structure helps us understand the distribution of invisible dark matter that holds everything together. The James Webb Space Telescope will in the future discern even finer layers of this cosmic pastry. The foundations of knowledge were laid by Cecilia Payne-Gaposchkin, who deciphered the composition of stars; Fred Hoyle, who explained element synthesis; and Chandrasekhar, who found the mass limit for explosion.