A filament of the cosmic web stretches through the young universe, and strung along it like beads on a necklace are galaxies. In the core of the SSA22 protocluster, which in a few billion years will become the center of a massive cluster, these beads are still hot and dusty: stars are being born inside them, and cold gas serves as fuel. Astronomers peered into this distant corner using ALMA, JWST, and JVLA to understand how the beads grow and why they eventually freeze.

It turns out the beads are not all alike. Some are still rich in gas and stretched out, like soft taffy: their stars are scattered over a large volume. Others have already contracted into dense clumps, and their gas is nearly exhausted. Between these extremes lies a sequence. The more massive the galaxy, the less gas it has and the more tightly packed its stars are. This is similar to how taffy, when kneaded for a long time, becomes harder and smaller.

What makes the beads contract? The filament they hang on. As long as the filament feeds the galaxy cold gas, star formation churns and the galaxy stays puffy. When the gas flow dries up, the fuel runs out, and the galaxy gradually calms down, while gravity pulls the stars closer together. So the bead transforms from soft taffy into a hard candy.

Today, giant elliptical systems that have not formed stars for a long time reside in the centers of galaxy clusters. Their ancestors might have looked like those dusty beads on the filaments. By tracing their evolution, astronomers have obtained an observational basis for a scenario in which the filaments of the cosmic web first feed galaxies, and then, when the feeding stops, leave them to freeze into compact spheres. So a necklace of soft taffy over time becomes a string of hard candies.