Galaxy NGC 5584 is a vast stellar city, where some neighborhoods are just being built while others are already marked by aging giants. To see both, astronomers used two very different flashlights. The ultraviolet telescope Swift lit up hot blue patches — stellar nurseries where massive stars have recently ignited. Meanwhile, the infrared camera on James Webb revealed sprinklings of red dots — red supergiants, heavyweight stars that have already aged and cooled.

Comparing the maps, scientists found that 82% of the ultraviolet maternity wards lie where red supergiants have gathered. It might seem simple — born in the same place, aged in the same place. But there's no perfect overlap. The brightness of ultraviolet spots and infrared clusters is very tightly linked (correlation coefficient 0.96), yet the groups are offset from each other. Comparison with models shows that red supergiants are on average about 14 million years old. It's as if in a city, daycare centers and retirement homes were in the same districts but not door to door — an entire era passes between them.

This picture points to a wave of star formation. Gas compresses, new stars flare up, the most massive among them burn out quickly and become red supergiants. But over a few million years, the gas clouds drift, and the stars themselves wander a bit. So ultraviolet light shows us today's construction front, while infrared shows its recent past.