An unusual group of stars in the constellation Orion has revealed its secrets. FU Orion, a binary star system, first came to the attention of astronomers in 1936 when the central star suddenly became 1,000 times brighter than normal. The behavior expected from dying stars has never been seen in young stars like FU Orionis.

The strange phenomenon inspired a new classification of stars of the same name (FU, or stars). Or stars suddenly burst into flames, only to go out again years later.

It is now understood that this illumination originates from stars receiving energy from their surroundings through gravitational deposition, the main force that forms stars and planets.

But how and why this happens has remained a mystery until now, thanks to astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA).

“FU Ori absorbed material for almost 100 years to continue exploding. We finally have an answer to the question of how these young stellar explosions replenish their mass,” explains Antonio Hales, deputy director of the ALMA North American Regional Center, a scientist at the National Radio Astronomy Observatory, and lead author of the study. . Astrophysical Journal.

“For the first time, we have direct observational evidence of the material that caused the explosion,” says Hales.play.

ALMA observations showed that FU Orionis fell on a long thin stream of carbon monoxide. This gas did not appear to have enough fuel to maintain current emissions. Instead, this accretion band is thought to be the remnant of an older, much larger structure that fell into this young star system.

“It is possible that interaction with larger gas flows in the past caused the system to become unstable and the brightness increased,” Hales explains.

Astronomers used various ALMA antenna configurations to collect different types of radiation from FU Orionis and detect mass flow into the star system. They also combined new numerical methods to model the mass flow as an accumulation line and predict its properties.

Ph.D. “We compared the shape and velocity of the observed structure with what would be expected from a falling gas trail, and the numbers were significant,” says Aashish Gupta. candidate of the European Southern Observatory (ESO) and co-author of this paper, who developed the methods used to model accumulation flows.

“The range of angular scales we can examine with a single instrument is truly impressive. ALMA gives us a complete picture of the dynamics of star and planet formation, from large molecular clouds where hundreds of stars are born to the more familiar scales of solar systems,” adds Director Sebastian Perez of the University of Santiago de Chile (USACH). The research team of the Young Exoplanet Millennium Core and its satellites (YEMS) in Chile and co-author of this study.

These observations also revealed the emission of slow-moving carbon monoxide from FU Orionis. This gas has nothing to do with the final emission. Instead, it is similar to the outflow observed around other protostellar objects.

Hales adds: “Understanding how these strange FUor stars form confirms what we know about how different stars and planets form. We believe that all stars undergo flares. These flares are important because they affect the chemistry of the accretion disks around newborn stars and the planets they eventually form.”

“We have been studying Orionis FU since the first ALMA observations in 2012,” adds Hales. “It’s exciting to finally get some answers.”