SPACE/COSMOS
Declining Star Formation Not Caused By Simple ‘Fuel Crisis’
FAST and DESI reshape our view of cosmic evolution CREDIT: Image by National Astronomical Observatories of China and Shanghai Astronomical Observatory of CAS
Key Takeaways:
- FAST plus DESI stacking of ~2.5 million galaxies (Nature Astronomy, Sept. 1) finds that over 4.5 billion years the cosmic star-formation rate fell to about 1/2.5 of its old level while neutral atomic hydrogen (HI) density only fell to about 1/1.4.
- That mismatch undercuts the simple “the fuel ran out” story: HI, seen in the faint 21-cm line, is still a large reservoir between the cosmic web and the molecular clouds that actually make stars.
- Authors shift the question to conversion efficiency—weaker inflow and lower density may stall HI → H₂ even while HI stays relatively plentiful—setting a late-universe baryon-cycle benchmark.
The universe has been having fewer and fewer stellar “babies.” Over the past 4.5 billion years, the rate of star birth has crashed to less than half of what it once was. But here’s the twist: the most essential “fuel” for making stars has barely decreased.
This finding comes from an international team led by researchers from the Chinese Academy of Sciences (CAS), in collaboration with the Dark Energy Spectroscopic Instrument (DESI) project. They used China’s Five-hundred-meter Aperture Spherical radio Telescope (FAST) to make high-precision measurements of cosmic neutral atomic hydrogen over the past 4.5 billion years.
The study reveals that while cosmic star formation has declined dramatically over this period, neutral atomic hydrogen (HI)—a vital gas reservoir for galaxies—has only decreased slightly.
This finding was published online in Nature Astronomy on Sept. 1.
Why has it become harder for the universe to form new stars? That question has long been central to the study of galaxy formation and evolution. A natural explanation is that, as the universe ages, the cold gas that fuels star birth is steadily consumed, inevitably driving a decline in star formation. If that were true, however, the drop in the star formation rate should be matched by a simultaneous sharp depletion of the cold gas reservoir. But such depletion has not been detected.
HI is a key cold-gas reservoir in galaxies that links the large-scale cosmic gas cycle to internal star formation and is mainly detected via its extremely faint 21-centimeter radio emission line. Unfortunately, individual signals for distant galaxies are often swamped by background noise.
For this reason, astronomers long faced a frustrating dilemma: deep observations could not cover large areas, while wide-field surveys lacked the sensitivity to detect faint signals. As a result, the evolution of total HI mass in the low-to-intermediate redshift universe has remained difficult to measure directly and reliably.
To overcome this bottleneck, the new study integrated FAST’s ultra-high radio sensitivity with DESI’s massive optical spectroscopic survey. The researchers analyzed a vast sample of about 2.5 million galaxies, covering nearly one-third of the sky. Using an innovative HI spectral stacking technique, they aligned and stacked the faint, otherwise undetectable radio signals based on precise galaxy redshifts. This allowed them to extract the average HI signal from the noise, tracing the evolution of cosmic neutral hydrogen with unprecedented statistical precision and sample size.
The highly precise measurements revealed a striking disparity: 4.5 billion years ago, the cosmic star formation rate was about 2.5 times higher than today, while the corresponding neutral atomic hydrogen density was only about 1.4 times its current level. This indicates that while star formation activity plummeted, the cosmic HI reservoir did not dry up synchronously. This striking contrast shows that rapid hydrogen depletion cannot explain the decline in star formation.
Shifting focus
According to the researchers, this breakthrough effectively shifts the scientific focus from “whether the gas is depleting” to “why it is increasingly difficult to form stars despite abundant neutral hydrogen reserves.”
Stars are primarily born in denser molecular gas clouds, and neutral atomic hydrogen sits at a critical intermediate stage between the large-scale cosmic gas supply and the formation of molecular gas. The study suggests that the most important late-time changes may occur not in the total HI reservoir itself, but rather in the gas flow through the baryon cycle. As the gas supply from the cosmic web weakens and gas density declines, the efficiency of converting HI into molecular hydrogen likely drops. This mechanism allows the HI reservoir to stay relatively stable, even as the molecular gas that directly feeds stars gradually dwindles.
As a result, the significance of this work goes far beyond merely measuring how much hydrogen is in the universe. Instead, it provides crucial new clues for understanding why the massive stellar engines of the universe are gradually shutting down.
According to the researchers, the joint observation by FAST and DESI establishes a brand-new observational benchmark for unraveling the late-stage cosmic gas cycle, the decline of star formation, and the broader evolution of galaxies.
The research was led by scientists from the National Astronomical Observatories of China, the Shanghai Astronomical Observatory of CAS, and Shanghai Jiao Tong University, in collaboration with researchers participating in DESI. The study includes contributors from research institutions across Asia, North America, and Europe, highlighting the scientific power of combining sensitive radio observations with large-scale optical spectroscopy.
SwRI Study Helps Unravel How The Sun’s Corona Transforms Into Solar Wind
This illustration shows how the Sun’s magnetic field shapes and directs the heliospheric current sheet (HCS). A Southwest Research Institute study used data from the European Space Agency’s Solar Orbiter to help define the early-stage composition of the HCS. This will help scientists construct more accurate models of how the solar wind affects space weather that can impact the Earth. CREDIT: Southwest Research Institute
Key Takeaways:
- SwRI analysis of ESA Solar Orbiter data (Astrophysical Journal) gives the closest look yet at the heliospheric current sheet—the Sun’s north–south magnetic boundary, twisted into a “ballerina skirt” that divides the heliosphere.
- Crossing a fold ~26 million miles from the Sun (inside Mercury’s orbit), instruments saw a drop in the iron-to-oxygen ion ratio aligned with the polarity flip—so the HCS is a compositional sorter in the corona, not only a magnetic switch.
- That does not settle how the million-mph solar wind is launched, but it constrains models of current-sheet formation, heating, and magnetic connectivity that space-weather forecasts depend on.
A Southwest Research Institute (SwRI) study of data from the European Space Agency’s (ESA) Solar Orbiter gives the most detailed view of the heliospheric current sheet (HCS) to date. The HCS is a sprawling undulating surface emanating from the Sun to beyond the solar system, which serves as the boundary between the Sun’s north and south magnetic field hemispheres.
Anchored deep in the solar surface, the HCS acts as a high-speed pipeline, carrying crucial data and information from the raw corona straight into space. “For decades, scientists have faced a cosmic paradox: how does the Sun blast a continuous supersonic stream of charged particles into space at over a million miles per hour,” asks SwRI’s Dr. Keiichi Ogasawara, lead author of the study.
“Known as the solar wind, this invisible torrent shapes space weather, powers auroras on Earth, and can disrupt modern satellite technology,” Ogasawara said. “The key to solving this puzzle lies in magnetic connectivity. By tracing magnetic field lines to the solar surface, scientists can link local surface activity directly to solar wind gusts. The HCS offers a prime connection point thanks to its distinct, oppositely directed magnetic signatures.”
The new study, published in The Astrophysical Journal, provides scientists with a better understanding of the origin and composition of the HCS and will help to define its relationship with the solar wind. As the Sun rotates, the HCS is twisted like a huge “ballerina skirt,” spiraling outward through the solar system and dividing the heliosphere into separate hemispheres where the Sun’s magnetic field points in opposite directions. In one hemisphere, the field pushes away from the Sun, while in the opposite hemisphere the field pulls toward the Sun. The heliosphere is a vast bubble of plasma created by the solar wind that streams out in all directions from the Sun. It surrounds the entire solar system and shields it from much of the high-energy galactic radiation found in interstellar space.
The Solar Orbiter recently passed through a fold of the HCS at about 26 million miles from the Sun, which is closer than the innermost planet Mercury, allowing researchers to study the youngest version of the solar wind ever observed. Solar Orbiter used its high-quality field, plasma and composition instruments to study how particle populations behave in this mysterious region.
SwRI researchers studying the Solar Orbiter’s observations found a clear and distinct change in the composition and makeup of ions within the HCS. An ion is an atom that has either lost electrons and become positively charged or gained electrons and become negatively charged.
“Within the HCS region, we identify a decrease in the ratio of iron and oxygen ions that lines up closely with the magnetic sector boundary itself,” said Ogasawara. “While the overall plasma on both sides is similar, we find a clear compositional change that is tightly aligned with where the magnetic polarity flips. This suggests that the HCS is not just a magnetic feature, it is also linked to how the Sun sorts ions in the corona and releases them into the solar wind.”
While this new data does not define HCS origins, the researchers believe that it helps to set boundaries for future models and theories.
“We have provided a detailed, multi-aspect view of an HCS crossing close to the Sun, and we showed that it includes organized, measurable variations of the types and amounts of particles within the plasma. It’s not just a simple flip of the magnetic field,” Ogasawara said. “This offers clear constraints that future models of current sheet formation, solar wind heating and magnetic connection must satisfy. Our work is less about proving any one theory and more about defining what any successful theory has to explain.”
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