Imagine a star as a simmering pot of soup. One ingredient, magnesium, is added early: it quickly appears after a supernova explosion. Another, yttrium, forms much later, during the slow process of nucleosynthesis inside old stars. The ratio of these 'spices' in the soup tells you how long it’s been cooking. By the mid-20th century, Cecilia Payne-Gaposchkin had figured out what stars are mostly made of, and Fred Hoyle and Margaret Burbidge explained how chemical elements are born. Now astronomers have used that knowledge to build a chemical clock for measuring age — one of the most crucial parameters in stellar evolution.

New research covered 736 stars, and using spectroscopy, the analysis of light broken into colors, astronomers precisely measured the amounts of yttrium and magnesium. It turns out the chemical clock isn’t universal. In the galactic disk, where we live, it runs faster on the outskirts: the farther from the center, the more dramatically the ratio shifts with age. In regions of active past star formation, like the thick disk, the clock practically freezes — stars there were born so fast that yttrium didn’t have time to build up. Moreover, metallicity (the fraction of elements heavier than helium) also plays a role: in metal-rich stars, the clock slows down.

These findings are important for galactic evolution. They show that the Milky Way formed unevenly: the inner regions were quickly 'seeded' with elements from supernovae, while the outer parts evolved slowly, allowing products of long nuclear processes to accumulate. Understanding such differences helps reconstruct the entire history of the Galaxy — from ancient thick disks to modern spiral arms. And, of course, it reminds us that even cosmic clocks need to be calibrated for the place they were made.