Declining star birth not caused by gas fuel shortage, study finds

Star formation across the universe has been steadily declining, but the underlying cause is not simply a cosmic fuel shortage as previously assumed.

A breakthrough study shows that while star birth has dropped dramatically over the past 4.5 billion years, the universe's fundamental gas supply has decreased only slightly. The findings challenge long-held astronomical theories about how galaxies mature and age.

The research was led by scientists from the National Astronomical Observatories of the Chinese Academy of Sciences, the CAS Shanghai Astronomical Observatory, and Shanghai Jiao Tong University. The paper was published online in the journal Nature Astronomy on Tuesday.

To conduct the study, researchers combined data from two world-class astronomical tools: the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in Southwest China's Guizhou province and the Dark Energy Spectroscopic Instrument (DESI) at Kitt Peak National Observatory in Arizona in the United States. FAST is the world's largest single-dish radio telescope, while DESI maps the overall 3D structure of the universe.

For decades, astronomers believed that star formation was slowing down simply because galaxies were running out of cold gas — the essential raw material needed to build new stars. If this traditional view were true, the drop in star formation should have been matched by a sharp drop in cold gas reserves.

To test this, the team focused on neutral atomic hydrogen, a primary form of cold gas that acts as an intermediate link between raw cosmic gas and star production. However, measuring this gas over huge cosmic distances has always been notoriously difficult. The weak radio signals emitted by distant hydrogen gas are easily drowned out by background space noise, creating a technical bottleneck for astronomers.

To overcome this obstacle, the team used a technique called spectral stacking. By analyzing a massive sample of roughly 2.5 million galaxies mapping nearly one-third of the sky, they aligned and combined millions of faint radio signals based on each galaxy's precise redshift — a metric that measures how much light stretches as the universe expands, indicating a galaxy's exact distance and age. Stacking these weak signals together allowed the researchers to extract clear data from background noise.

The precise measurements revealed a striking contrast: About 4.5 billion years ago, the rate of star formation across the universe was roughly 2.5 times higher than it is today. However, the density of neutral atomic hydrogen gas was only 1.4 times higher than present levels.

"This indicates that while star formation activity plummeted, the cosmic hydrogen reservoir did not dry up at the same pace," said Guo Hong, a professor at the Shanghai Astronomical Observatory. "The strong contrast directly rules out the simplistic picture that the rapid depletion of neutral hydrogen is the main driver behind the decline in star formation."

Guo noted that the focus must now shift from whether the gas is running out to why galaxies are finding it harder to turn existing gas into stars.

Zhang Chuanpeng, an associate professor at NAOC and first author of the study, explained that stars are not made directly from atomic hydrogen. Instead, atomic hydrogen must first condense into much denser molecular gas clouds, which then collapse under gravity to ignite new stars.

The study suggests that the problem lies in this conversion process. As gas flowing into galaxies slows down over time, the density drops, making it much harder for neutral hydrogen to transform into dense molecular gas. As a result, the ultimate raw fuel reserves stay relatively stable, even while the active "fuel factories" capable of directly nurturing stars gradually shut down.

"The study provides crucial new clues for understanding why the massive stellar engines of the universe are gradually shutting down," said Yang Xiaohu, deputy director of the Tsung-Dao Lee Institute at Shanghai Jiao Tong University.

"The joint observation by FAST and DESI establishes a new observational benchmark for unraveling the late-stage cosmic gas cycle, the decline of star formation and the broader evolution of galaxies," Yang added.