The galaxy's kitchen has a recipe for stellar soup. The main ingredient is hydrogen, but the flavor depends on a pinch of metals. Astronomers have now peered into the pots where that pinch is almost absent, and found that the smallest stars are missing.

Using data from the Gaia space telescope, they selected about 53,000 stars with halo-like orbits and low metal content (less than one-tenth of the Sun's). For each star, they measured its mass and built the initial mass function — a plot showing how many stars of different weights are born. In environments with a moderate metal deficit (from −1.5 to −1 on a scale where the Sun is zero), small stars with masses between 0.2 and 0.5 solar masses are more common than in our neighborhood: the slope of the plot is steep, around −1.6. But in even poorer environments, the picture flips: the plot becomes nearly flat, with a slope close to zero. This means that lightweight stars are catastrophically rare there.

Why does this happen? There's no definitive answer yet. Perhaps in metal-poor clouds, gas fragments into larger clumps, so instead of a sprinkle of dwarfs, we get rare giants. Or the collapse conditions themselves prevent small stars from gaining mass. Importantly, the result doesn't depend on the chosen model of stellar evolution: it's a real property of nature, not an artifact of calculations.

The discovery changes our view of the early Universe. The first stars formed from nearly pure hydrogen and helium, with thousands of times fewer metals than today. If lightweight stars were almost absent under those conditions, then the first generation of stars consisted mostly of massive, short-lived giants. They exploded quickly, seeding the cosmos with the first metals, and only then did stars like our Sun begin to appear. That very pinch of metals missing from ancient clouds turned out to be the key to the emergence of small, long-lived stars, and thus to the possibility of planets and life.