Just two billion years after the Big Bang, massive galaxies are already exhaling their gas back into space. The JWST detected this by the yellow trace of sodium — a color familiar to everyone from streetlights. This trace appears when cold gas absorbs light. In galaxies, this gas is the primary fuel for birthing new stars.

To understand where the fuel goes, astronomers led by Matteo Sapori studied 811 distant galaxies with redshifts (stretching of light due to the expansion of the universe) from 3 to 7. They used spectroscopy — a method that splits light into colors, much like a raindrop creates a rainbow. In each spectrum, they searched for a dip at the wavelength of sodium. A confident signal turned up in only 20 galaxies, but these were the most massive and dusty. Among those whose star formation (the process of star birth) had already slowed, the fraction with such an exhalation reached 43%. Estimates show that gas flows out faster than it transforms into new stars — up to 12 solar masses per year.

So what pushes the gas out? Most likely, powerful radiation from young massive stars, and sometimes an active galactic nucleus — a supermassive black hole devouring matter at the center. This wind drains the reservoirs of cold fuel, and star formation fizzles out. Thus early galaxies transition from a tumultuous youth to a calm maturity. Two yellow streaks in the rainbow of light tell us how the universe learned to breathe, billions of years before our time.