When a star like our sun begins to form, something strange happens deep inside clouds of dust and gas in space. Tiny charged particles and neutral particles start moving at different speeds — and for the first time ever, scientists have watched this happen in real time.
A team from Kyushu University in Japan and the Max Planck Institute for Extraterrestrial Physics in Germany spotted a process called ambipolar diffusion inside a prestellar core called L1544. That's a cold, dense ball of gas and dust in the Taurus molecular cloud — one of the closest star-forming regions to Earth. Their findings were published in the journal Astronomy & Astrophysics.
"Prestellar cores are fascinating stellar bodies," said Doris Arzoumanian, an associate professor at Kyushu University's Institute for Advanced Study and the study's lead author. "They are dense and cold, and a source of a lot of complex chemistry."
The team wanted to answer a big question: how do these cores overcome the strength of magnetic fields that push outward, preventing them from collapsing into new stars? When a prestellar core is young, charged particles called ions are locked to magnetic field lines, while neutral particles — the bulk of the gas — drift more freely. As the core grows denser and more shielded from radiation, the ions and neutral particles begin to decouple. The neutral particles then start falling inward under their own gravity, while the ions stay bound to the magnetic field. This drift between them is ambipolar diffusion.
To observe it, the researchers needed to find molecules that wouldn't freeze solid in the extreme cold. They tracked two specific molecules: Diazenylium-d1, an ion, and para-monodeuterated ammonia, a neutral molecule. Using the IRAM 30-meter telescope — a massive radio dish in Spain — they measured the speed of both.
The data revealed a clear difference. The neutral particles moved about 0.05 kilometers per second faster than the ions, a tiny but telling gap that proves the two groups were drifting apart.
"This process is known as ambipolar diffusion. Until now, observing this phenomenon in a prestellar core was a major challenge," Arzoumanian said.
Over time, as more neutral particles pile inward, the magnetic field weakens. Eventually, gravity takes over completely, and the core collapses into a baby star called a protostar.
The team now hopes to observe more prestellar cores with sharper instruments to confirm the finding. Understanding how stars form helps scientists trace the origins of planets — and possibly life — in other solar systems.
"Understanding star formation addresses a fundamental question about the origin of life in planetary systems and helps us better understand the universe as a whole," Arzoumanian said.