Globular clusters are dense stellar archives, where the youth of the Galaxy is recorded. But reading this chronicle was hindered by imprecise chronometers: age estimates diverged by a billion years. The JWST telescope fixed this, turning brown dwarfs into a clockwork — failed stars lacking their own thermonuclear energy source.
Brown dwarfs cool and dim uniformly, like the hands of an ideal clock.
Computer modeling gives cluster ages with an error under 200 million years — for objects that have lived 10–13 billion years, that's like telling time to the second over a century.
The main source of error is imperfect cooling models, the influence of binary systems, and chemical composition. Yet the method is valuable as an independent cross-check for other dating methods. A surprising fact: these ancient dwarfs are so faint that their light is millions of times weaker than the sun's. Only the ultrasensitive JWST could spot them in distant clusters.
The surface temperature of old brown dwarfs can drop to hundreds of degrees Celsius, like a hot oven, but for a star that’s bone-chilling cold.