Science
June 5, 2026

Magnetic Field Helps Binary Star Systems Form

Visualization of gas flows around a binary protostar system calculated by ATERUI III. The gas shown in red orbits around one of the two protostars. The gas shown in blue orbits around the combined binary system. The gas shown in green is being expelled from the system and is carrying away angular momentum. The present research shows that the magnetic field plays an important role in expelling gas and angular momentum. (Credit: Matsumoto, Hotokezaka, Inayoshi 2026) Image (1.7MB)

New simulations show that interactions with a magnetic field can work to decrease the distance between still forming binary protostars. These results can help explain the characteristics of the binary star systems observed in the Milky Way. These results can also be extrapolated to binary black holes, giving insights into how super massive black holes evolve.

Stars form from clouds of interstellar gas that collapse into dense regions known as molecular cloud cores. Multiple stars form close together simultaneously, and in some cases two stars will become gravitationally bound to each other, forming a binary star system. Observations suggest that these binary systems form early on, before the stars are even fully formed. Astronomers have struggled to explain how these still forming “protostars” can pull together into binary systems so quickly.

New simulations using multiple supercomputers including the ATERUI III supercomputer for astronomical simulations and its predecessor ATERUI II, both at the National Astronomical Observatory of Japan, have shown that interactions between an interstellar magnetic field and the gas around the protostars can remove angular momentum from the protostar pair, allowing the binary systems to form within a realistic time period. In the simulation run with zero magnetic field performed as part of this research, the protostars actually moved farther apart, indicating the importance of the magnetic field in the process.

The simulations also suggest that the same process could work on massive binary black holes in the gas-rich heart of a new galaxy formed from the merger of two smaller galaxies. This would help explain how massive black holes can move close enough to merge and form a supermassive black hole. Direct simulation of massive binary black holes over the timespans required to spiral towards each other is still computationally challenging, so rigorous investigation of the effects of magnetic fields on massive binary black holes remains a topic for future investigation.

Visualization of gas flows around a binary protostar system calculated by ATERUI III. The first half of the video shows a close-up view around the binary protostars. The second half shows a wide-field view of the system. You can see how the outflow escaping from the disk around the binary system carries angular momentum far away. (Credit: Matsumoto, Hotokezaka, Inayoshi 2026)

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