Complete the galactic star map

By peering into the cosmic dance of stars, a team led by researchers from the Leibniz Institute for Astrophysics in Potsdam (AIP) has revealed the complex structure of our galaxy, the Milky Way. Assuming that each observed star represents a larger population of stars sharing the same orbit, they reconstructed the properties of these “hidden” stars, thereby filling in the gaps in the galactic disk that holds the secrets of the past, present and future of our galaxy.

Our understanding of the galaxy has progressed as the number of stars has increased. From early observations to increasingly advanced space and ground-based telescopes, each step has revealed new layers of the galaxy’s complex structure and motion. While stellar studies continue to grow in volume, our view of the Milky Way remains severely obscured, with the vast majority of stars we can study concentrated around the sun.

This discrepancy is largely due to a fundamental limitation of our observations, arising from our position in the central plane of the Milky Way disk. Our location limits the volume of potentially observable stars, based on their brightness, which is further affected by dust and gases that can block or attenuate their light, called interstellar quenching.

Researchers at the Leibniz Institute for Astrophysics Potsdam (AIP), in collaboration with the University of Vienna and the Paris Observatory, have developed an innovative method to fill gaps in our understanding of the structure of the Milky Way . They showed that instead of relying solely on observations of individual stars, the entire orbits of real stars can be used to represent galaxy structure and dynamics.

As stars move around the galactic center, they serve as a tool for mapping regions of the galaxy beyond the direct range of our telescopes, including areas on the other side of the Milky Way. Using a mass distribution model of the Milky Way and the observed positions and velocities of the stars, they not only calculated the orbits of the stars but, more importantly, measured the amount of mass that should be associated with each orbit.

Using a new technique applied to a large sample of stars with spectroscopic parameters from the APOGEE survey, part of the Sloan Digital Sky Survey, the researchers mapped stellar kinematics across the Milky Way. For the first time, they revealed the complex motion of stars in the bar region without being hampered by the uncertainties of distance measurements.

By reconstructing stellar orbits using real stars in the Milky Way with precisely determined parameters, the team quantified the mass-weighted chemical abundances and age structure of the galaxy. This approach circumvents the challenges posed by dense interior regions and the extinction of the interstellar medium, providing a comprehensive view of stellar populations, including previously unobservable areas on the other side of the Milky Way.

The study was published on the arXiv preprint server as a series of three articles.

Sergey Khoperskov, a scientist at AIP and first author of the studies, explains: “We can look at this approach from a different angle. Imagine that for every star we observe, there is a large sample of stars that follow the exact same orbit but, for various reasons, were not captured by the survey. What we do is reconstruct the positions, velocities and stellar parameters of these invisible stars, filling in the missing pieces of the galaxy structure.

The new data strongly suggest that the Milky Way formed in two distinct phases, manifested by different chemical abundance-age relationships. The inner disk, located well within the Sun’s radius, formed relatively quickly during the early stages of the galaxy’s evolution. About 6 to 7 billion years ago, the outer disk began to assemble, rapidly expanding the radial extent of the Milky Way and shaping its current structure.