The Milky Way refers to a luminous Milky band spanning the sky.

The beautiful and vast Milky Way in the night sky has sparked countless imaginations and endless exploration since ancient times. Our Milky Way, where we reside, is an ordinary barred spiral galaxy among countless others in the universe.

Like other similar galaxies, it has integrated over hundreds of billions of stars over the past more than ten billion years. These stars are mainly distributed in the Galactic halo and Galactic disk of the Milky Way, with the Galactic disk consisting of a geometrically thicker disk and a relatively thinner and more extended thin disk.

However, the origin of the diverse and colorful Milky Way has always been a scientific puzzle urgently needing to be solved by astronomers and is also a major scientific goal of large-scale astronomical observation programs worldwide, both ground-based and space-based.

Previous studies have typically suggested that our Milky Way experienced violent formation processes during its infancy (early stage), with a large amount of low-metallicity gas collapsing (in astronomy, elements other than hydrogen and helium are referred to as metals) or gas-rich galaxy collisions and mergers forming the stellar halo of the Milky Way.

Subsequently, the gas gradually cooled to form the early Galactic disk, namely the thick disk of the Milky Way. Finally, as time passed, the gas further cooled, initiating the formation of the thin disk of the Milky Way. The formation of the thin disk was a persistent and orderly process, extending from approximately 8 to 10 billion years ago to the present.

However, these images mainly come from numerical simulations and speculations based on fragmented observational evidence. Fortunately, the emergence of big data in astronomical observations is rewriting the picture of the evolution of the Milky Way, ushering in an era of unraveling the dusty history of the Milky Way.

The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) has released tens of millions of stellar spectra data, laying the foundation for digitalizing the Milky Way. The Gaia satellite launched by the European Space Agency provides a map of the positions and movements of one billion stars. This combination provides astronomers with unique advantages for retracing the assembly and evolutionary history of the Milky Way.

Dr. Xiang Maosheng and Professor Rix, based on LAMOST and Gaia data, constructed a high-quality data sample containing 250,000 subgiant stars and obtained their precise ages. Stellar age is one of the most difficult physical quantities to measure accurately and can be said to be one of the most challenging physical quantities to measure precisely in the field of astronomy.

Thanks to the progress in LAMOST Galactic surveys and other international survey projects, obtaining the ages of large samples of stars has gradually become a reality in the past few years. However, the typical age error of large sample stars obtained in previous studies was 20% or larger, and achieving a 10% age determination accuracy for stellar samples is challenging, with the spatial and parameter ranges of the samples also being highly limited.

Subgiant stars are stars transitioning from the main sequence stage to the red giant stage in stellar evolution. Their observable parameters, especially luminosity, are extremely sensitive to their initial mass and age, making their ages relatively easy to determine accurately. However, the evolution of stars in the subgiant stage is rapid, resulting in subgiant stars being relatively rare.

Utilizing LAMOST spectroscopic big data, Xiang Maosheng precisely determined the atmospheric parameters of 7 million stars and combined them with Gaia data to obtain high-precision stellar luminosity and kinematic parameters. From these 7 million stars, 250,000 subgiant stars were selected, and their precise ages were determined, with an average age accuracy of 7%.

The metallicity coverage range of the sample extends from -2.5 (1/300 of the solar metallicity) to 0.5 (three times the solar metallicity), covering a spatial range of 30,000 light-years.

This is the first time that such a large sample of stars with high-precision ages has been obtained within such a vast spatial range and stellar metallicity range in the Milky Way, successfully overcoming the limitations of data and marking a significant step forward in studying the formation and evolutionary history of the Milky Way.

Based on their motion characteristics and chemical DNA (elemental abundance) identification, the research team divided these 250,000 stars into two groups: one group represents stars forming in the relatively quiet dynamical process of the extended thin disk of the Milky Way, and the other group represents stars forming in the violent turbulent process of the Galactic halo and thick disk.

The research team found that these two groups of stars are distinctly divided by an age of approximately 8 billion years.

In other words, in terms of time, the assembly and evolutionary history of the Milky Way are divided into two clearly defined stages: an early stage from 13 billion years ago to 8 billion years ago and a late stage from 8 billion years ago to the present. The early stage formed the thick disk and Galactic halo of the Milky Way, while the late stage formed the thin disk of the Milky Way.