For several years, astronomers have observed small dense gas clouds falling toward the supermassive black hole Sagittarius A* at the center of our Milky Way. Until recently, their origin remained unclear. But a research team led by the Max Planck Institute for Extraterrestrial Physics (MPE) proposes a compelling explanation: a nearby massive binary star could be the source of these clouds.
The galactic center is a very active environment, with a supermassive black hole surrounded by stars and gas. In recent years, infrared observations have detected several dense, compact gas clouds near Sgr A*. How these clouds form and feed the black hole has long remained an open question for scientists.
Astronomers have found evidence that a massive binary star near the center of the Milky Way could generate gas clumps that drift toward Sagittarius A*.
Credit: NASA, ESA, CSA, Ralf Crawford (STScI) Astronomers first identified a compact cloud of ionized gas, now named G2, in 2012. It contains the equivalent of a few Earth masses and emits light characteristic of hot gas mixed with dust. G2 follows an elongated orbit around Sgr A* and has a faint tail called G2t. By examining older data, researchers found a similar object, which they named G1, following a comparable trajectory. Together, these objects form a connected structure that traces the movement of gas in the galactic center.
Detailed study of the orbits of these clouds revealed that they are almost identical in shape and orientation. It is extremely unlikely that these independent objects share such specific orbital properties by chance. This strongly suggests that these clouds come from a common source. By tracing the gas stream backward in space and accounting for radial velocity, researchers identified a plausible candidate: the massive binary star IRS 16SW.
IRS 16SW is a contact binary located in the disk of young stars orbiting Sgr A*. Hydrodynamic simulations indicate that collisions of stellar winds in this binary system can create shocks where gas accumulates, compresses, and then separates into distinct clouds. These clouds then head inward, following orbits similar to those observed in this stream.
These discoveries link stellar evolution, gas dynamics, and black hole feeding into a single scenario. Massive stars near the galactic center could provide a continuous flow of matter toward the black hole via their stellar winds. Models indicate that an absorption of one cloud every ten years, carrying the equivalent of Earth's mass, would be enough to sustain Sgr A*'s current activity.
Stellar winds from massive stars
Massive stars, whose mass exceeds several times that of the Sun, produce powerful stellar winds. These are streams of charged particles ejected at high speed from their surface. These winds can reach speeds of several thousand kilometers per second (thousands of miles per second) and carry away a significant amount of matter.
When two massive stars orbit very close to each other, as in a contact binary, their stellar winds collide. This collision creates a shock zone where the gas is strongly compressed and heated. Simulations show that this process can generate dense, compact gas clouds.
These clouds, once formed, can be stripped from the binary system by surrounding gravitational forces. They then move through space, sometimes toward the central supermassive black hole. This mechanism offers a plausible explanation for the origin of the clouds observed near Sagittarius A*.