BEIJING -- Chinese astronomers have conducted observations and analyses of star clusters in the outer halo of the Andromeda Galaxy — the Milky Way's cosmic neighbor — hoping to uncover the growth story of this galaxy from these celestial "time capsules."

The study was recently published in The Astronomical Journal.

The Andromeda Galaxy is the nearest large galaxy to the Milky Way, located approximately 2.5 million light-years away, and is also one of the most distant objects visible to the naked human eye. Enveloping Andromeda is a vast galactic halo scattered with numerous globular star clusters. These clusters serve as "fossils" of the universe, preserving the history of the galaxy's evolution.

By analyzing the ages and chemical compositions of these star clusters, astronomers can piece together the "growth diary" of the Andromeda Galaxy — determining which clusters were "born locally" and which "immigrated" from other galaxies, said Gu Hongrui, the first author of the paper from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC).

Previous studies struggled to obtain high-precision parameters simultaneously. In this research, a team led by Zhao Gang, an academician of the Chinese Academy of Sciences, utilized spectroscopic data from the 2.16-meter telescope at the Xinglong Observatory of NAOC, along with multi-band photometric data, to conduct, for the first time, a combined analysis of 29 star clusters in the halo of the Andromeda Galaxy.

The study determined the ages and metallicities of all 29 star clusters, with three of them analyzed using this combined approach for the first time. The findings suggest that these star clusters are likely not "natives" of Andromeda but rather originated in smaller satellite galaxies that later merged and were accreted into the halo over vast cosmic timescales, according to Fan Zhou, a corresponding author of the study at NAOC.

"This research not only enhances our understanding of the Andromeda Galaxy's formation process but also provides important clues for future galactic archaeology. Looking ahead, the China Space Station Telescope to be launched in the future will offer higher-resolution near-ultraviolet data, continuing to unveil more cosmic stories for such studies," said Zhao.