In a city park, trees grow faster than in a deep forest — not because the soil is better, but because branches collide and seeds are exchanged more often. Galaxies in the Spiderweb protocluster behave much the same way. This vast clump of matter will become a mature cluster billions of years from now. Astronomers pointed the NIRCam camera of the James Webb Space Telescope at it and, through a narrow filter, like looking through colored glass, captured the light of hydrogen in the Paβ line. The brightness of this line reveals how quickly stars are being born in a galaxy.

In this crowded environment, small galaxies, lighter than one billion solar masses, blaze brighter than their field counterparts. They produce more stars per unit mass, even when dust, which usually dims some of the light, is not taken into account. The effect is as steady as a streetlamp in fog: the dust veil only muffles it, not extinguishes it. Moreover, the hot spots of star formation are not huddled at the protocluster's center but are scattered across its entire web, so the cause is not some special "central heating" but the environment itself.

Massive galaxies, from one billion solar masses upward, behave the same whether in a crowd or in solitude. They seem to have found their own pace and are less sensitive to the jostling of neighbors. But for low-mass galaxies, the protocluster acts as an accelerator: frequent close encounters and mergers, along with a generous inflow of gas from the surroundings, spur bursts of star formation. Interestingly, in other protoclusters of the same age, the picture is sometimes different: there, even medium-weight galaxies can speed up. So stellar nurseries depend not only on mass but also on how vigorously the protocluster itself gathers its members. In the Spiderweb park, only the youngest saplings are still growing.