Astronomers Finally Photographed the Invisible Funnel That Launches a Newborn Star's Jet — 40 Years After It Was Predicted

For decades, the standard story of how a newborn star fires off a jet has rested on something nobody had ever seen: a magnetic field twisted into a donut shape around the outflowing gas, acting as a nozzle. Astronomers using the Atacama Large Millimeter/submillimeter Array have now imaged that structure directly, at high resolution, for the first time.

The target was NGC 1333 IRAS 4A, a young double-star system buried in the Perseus molecular cloud roughly 960 light-years from Earth. A team led by Tao-Chung Ching, a former Jansky Fellow at the U.S. National Science Foundation's National Radio Astronomy Observatory, reported the result in Nature Communications. "For the first time, these ALMA observations have captured this invisible funnel of magnetic fields," Ching said. "This is exciting because it proves a decades-old theory about how stars, like our own sun, are born and fire off powerful cosmic jets."

Stars grow by pulling gas and dust inward from a surrounding disk, and they simultaneously throw some of that material back out in narrow, fast jets and broader, slower outflows. Theory has long held that magnetic fields wound into a funnel do the throwing. Testing that required measuring the field itself at very small scales, which is not something a telescope can do directly. Instead the team measured the faint polarization of carbon monoxide gas glowing in the outflow — light whose orientation is nudged by the field it passes through — using ALMA's resolving power, about 30 times sharper than earlier instruments.

What emerged was a coil. The field ran perpendicular to the direction the gas was traveling and matched the outflow's rotation, wrapping around the stream at distances of only a few hundred astronomical units from the protostar. Its strength came out at a few thousandths of a gauss — trivial next to a household magnet, and enormous by the standards of interstellar space. That geometry is the signature of a toroidal field, precisely the configuration models have demanded for forty years without ever confirming at this scale. "This study represents the first and highest-resolution observation of milligauss-strength toroidal magnetic fields at a scale of several hundred astronomical units from a protostar," Ching said.

IRAS 4A was chosen because it had already behaved itself once. "We knew that IRAS 4A was a textbook case: 20 years ago, in work published in Science in 2006, we found that this region followed the theoretically expected magnetically driven collapse," said co-author Josep Miquel Girart of the Institute of Space Sciences (ICE-CSIC) and the Institute of Space Studies of Catalonia.

The team also came away with a tool they were not looking for. The twisting of the field turned out to track the electric currents flowing through the gas in a simple, linear way — a direct consequence of Ampère's law. Because that relationship is predictable, it gives astronomers a new route to infer magnetic field directions inside star-forming clouds, one of the hardest measurements in the field. The physics matters beyond one protostar: shedding angular momentum through a magnetically driven outflow is how a young star keeps growing instead of spinning itself apart, and the same mechanism is thought to operate at vastly larger scales around the supermassive black holes that power distant galaxies.