The true reason for the rapid fading of stars in the Universe has been revealed

An international team of astronomers led by specialists from the Chinese Academy of Sciences has revealed the true cause of the sharp decline in star formation in the Universe. It turned out that the main problem lies not in a deficit of hydrogen fuel, as previously believed, but in a drop in the efficiency of its conversion. The results of the study were published in the journal Nature Astronomy.

Over the past 4.5 billion years, the rate of new star formation has plummeted by more than half. Previously, scientists believed that the Universe was simply running out of "fuel," but data from the FAST radio telescope and the DESI dark energy survey instrument overturned these assumptions. It turned out that the reserves of neutral atomic hydrogen — the primary gas for galaxy formation — had decreased only slightly. Having analyzed data on 2.5 million galaxies covering nearly a third of the celestial sphere, the researchers established that 4.5 billion years ago, the intensity of star formation was 2.5 times higher than it is today, while hydrogen density exceeded the current level by only 1.4 times.

For many years, astronomers were unable to accurately track the evolution of cosmic hydrogen — the radio signal from individual distant galaxies proved too weak and was simply lost in background noise. To overcome this obstacle, researchers from the National Astronomical Observatories of China employed an elegant method of spectral stacking: the mathematical summation of hundreds of thousands of weak radio signals made it possible to extract an average hydrogen content measurement.

This breakthrough fundamentally shifts the focus of scientific inquiry — from the question of gas reservoir depletion to the problem of declining matter conversion efficiency. Stars are born in dense molecular clouds, and neutral atomic hydrogen serves as a key intermediate stage between the global cosmic gas supply and the formation of new stars.

The study demonstrates that as the inflow of gas from the cosmic web weakens, the efficiency of converting atomic hydrogen into molecular hydrogen also drops. It is precisely this mechanism that allows the overall gas reservoir to remain relatively stable, while the fuel that directly powers star birth gradually dwindles.