Study reveals star formation decline driven by conversion failure, not gas shortage

For billions of years, the universe has been making fewer stars. Astronomers have long suspected they knew why: galaxies were running out of fuel. The cosmic gas that feeds star birth was drying up, the thinking went, and so the great stellar factories were simply starving to a halt. A new study upends that assumption entirely.

Researchers from China's National Astronomical Observatories, the Shanghai Astronomical Observatory, and Shanghai Jiao Tong University have found that while star formation has plummeted over the past 4.5 billion years, the universe's supply of cold gas has barely budged. The rate of new stars being born has dropped by roughly two and a half times. The amount of neutral atomic hydrogen—the primary form of cold gas and the essential ingredient for star-making—has declined by only 1.4 times. The mismatch is stark and definitive. Published in Nature Astronomy on September 1, the work challenges decades of astronomical orthodoxy about how galaxies age and evolve.

The breakthrough required combining two of the world's most powerful observatories. The team used FAST, the Five-hundred-meter Aperture Spherical radio Telescope in Guizhou province in Southwest China—the largest single-dish radio telescope on Earth—alongside DESI, the Dark Energy Spectroscopic Instrument at Kitt Peak National Observatory in Arizona. DESI maps the three-dimensional structure of the universe itself. But measuring neutral hydrogen across cosmic distances has always been brutally difficult. The radio signals emitted by distant hydrogen are faint, easily buried under the noise of space. The technical barrier has long frustrated astronomers trying to answer this question.

To break through, the researchers deployed a technique called spectral stacking. They analyzed roughly 2.5 million galaxies spread across nearly one-third of the sky. For each galaxy, they measured its redshift—the stretching of light that reveals how far away it is and how old the universe was when that light left it. Then they aligned all those millions of faint radio signals by redshift and stacked them together, layer upon layer, until the background noise cancelled out and a clear signal emerged. The precision of the resulting measurements was unprecedented.

What they found reframed the entire problem. Guo Hong, a professor at the Shanghai Astronomical Observatory, explained the implication: the universe's hydrogen reservoir has not dried up in step with the decline in star birth. The old theory—that galaxies simply ran out of gas—cannot be right. The question now is not whether gas is running out, but why galaxies have become so inefficient at turning the gas they have into new stars.

The answer lies in a conversion step that astronomers had perhaps taken for granted. Stars do not form directly from atomic hydrogen. First, that atomic hydrogen must condense into much denser molecular gas clouds. Those clouds then collapse under their own gravity and ignite into stars. As gas flows into galaxies more slowly over cosmic time, its density drops. Lower density makes the transformation from atomic to molecular gas far more difficult. The raw fuel remains available, but the machinery that converts it into star-forming material has gradually seized up. The universe's stellar engines are not starving; they are simply losing the ability to process what they have.

Zhang Chuanpeng, an associate professor at the National Astronomical Observatories and the study's first author, laid out the mechanism. Yang Xiaohu, deputy director of the Tsung-Dao Lee Institute at Shanghai Jiao Tong University, called the findings crucial for understanding why the universe's most massive stellar engines are gradually shutting down. The study does not answer that question fully—it opens it. But it does something equally important: it tells astronomers where to look next.