Astronomers Watch Two Stars Join to Form a Binary System 60 Years Ago

Astronomers using the ALMA radio telescope have directly observed two massive young stars coming together to form a binary system, and traced the encounter to roughly 60 years ago — an instant on a timescale where stars live for billions of years. The system, called IRAS 07299−1651, lies about 5,300 light-years away and was studied by a team led by Yichen Zhang of Shanghai Jiao Tong University, whose findings were published Tuesday in the journal Nature Astronomy.

The team first observed the still-forming stars, known as protostars, in 2019, using ALMA (the Atacama Large Millimeter/submillimeter Array) alongside the Very Large Array, and infrared images from the James Webb Space Telescope and the Very Large Telescope. Over eight years, they mapped subtle shifts in the stars' positions, reconstructing the full three-dimensional structure of the system for the first time — including how the stars orbit each other, how their surrounding disks of gas and dust are aligned, and the angle at which they blast jets into space.

The observations surprised the team. They had expected the stars to circle each other in neat, near-circular orbits, but instead found highly flattened, almost parabolic orbits. The disks of material feeding each star were also tilted at a sharp angle to each other and to the stars' orbits — a pattern that is hard to explain if both stars had formed from the same disk of gas and dust that later fragmented.

Instead, the researchers propose that the two stars formed separately in their own individual clouds of gas and dust, before a chance encounter brought them and their surrounding material together. Tracing the stars' orbits backward, the team estimated that this encounter occurred around 60 years ago, and found that the disk-shaped pockets of material around each star have kept their well-defined structure despite the encounter.

The team said it is not yet clear whether the two stars are destined to stay together or eventually drift apart, since ongoing interactions with surrounding gas could still shape the system's future. Around half of sun-sized stars in the Milky Way have a companion star, a figure that rises to about 90% among massive stars like those in IRAS 07299−1651.