Astronomers have uncovered the origin of an unusual star stream in the Milky Way.

The stellar stream is a group of stars that has not integrated into the overall "architecture" of the galaxy. In the case of the Milky Way and other spiral galaxies, these stars have not become part of its arms and instead orbit the center independently, typically not lying within the plane of the galactic disk. Often, they are almost perpendicular to this plane.

Within the Milky Way, which is under its gravitational influence, around two dozen stellar streams have been identified. Some consist of thousands of stars, while others contain hundreds of thousands, and in some cases, even up to a million. These streams originate either from absorbed or captured dwarf galaxies and globular star clusters.

In 2020, the Gaia space observatory helped to identify another small but very unusual structure, containing only 81 stars. This structure is intriguing because it is positioned almost in the galactic plane.

Scientists managed to determine the age of this stellar stream, revealing that it is as old as the galaxy itself—at least 12 billion years. Consequently, two hypotheses regarding the stream's origin were considered: it could be remnants of an absorbed dwarf galaxy or the result of a complex evolutionary process of the Milky Way's disk.

The stellar stream has been poetically named Icarus. This name conceals the hypothesis that has been favored from the outset: Icarus is the hero of an ancient myth who gained wings but acted recklessly by flying too close to the Sun, leading to his demise. In the case of this stellar stream, a similar fate is suspected: at one time, a small galaxy approached the Milky Way so closely that it "crashed" directly into it.

According to rough estimates, its mass was equivalent to a billion solar masses. The mass of the Milky Way is estimated to be from hundreds of billions to over a trillion solar masses.

Astronomers from Italy and Chile, whose article is available on the preprint server arXiv.org, sought to determine the origin of Icarus by comparing the chemical composition of its stars with those of other celestial bodies in our galaxy. It turned out that they differ significantly. This implies that the stars in Icarus are indeed not "local," validating the poignant name of the stream.