Tracing the Heartbeat of the Milky Way: Bursts of Star Formation Revealed by Gaia

The Milky Way’s center is a bustling region shaped by a bar of stars and a boxy, peanut-shaped bulge. Yet, understanding how these inner regions formed has long been difficult because of heavy dust and crowded stars. In this study, Ruiz-Lara et al. use data from the European Space Agency’s Gaia mission to reconstruct how stars formed in these regions over time. Rather than observing the inner Galaxy directly, they study super metal-rich stars near the Sun, stars that are unusually high in elements heavier than helium ([M/H] ≈ 0.5). Since such stars likely originated near the Galaxy’s center and migrated outward, their ages can act as “fossil records” of the inner Milky Way’s past.

Mapping Metal-Rich Stars with Gaia

The authors begin by assembling a clean, high-quality sample of stars from Gaia’s third data release. They select those within 1 kiloparsec (about 3,000 light-years) of the Sun, correcting for dust extinction using two independent 3D maps to ensure reliability. With these stars, they construct colour–magnitude diagrams (CMDs), plots of stellar brightness versus color, that reveal patterns in stellar ages and compositions. Using a custom tool called CMDft.Gaia, part of the larger ChronoGal project, the team fits synthetic CMDs to Gaia data to extract precise star formation histories. This approach enables them to derive when stars of different metallicities were born across multiple regions near the Sun.

Discovering Bursty Star Formation

The resulting age–metallicity maps reveal a striking pattern. Instead of forming continuously, the super metal-rich stars appear in several distinct bursts, roughly 13.5, 10, 7, 4, 2, and less than 1 billion years ago. These events were strongest near the plane of the Galaxy and fade with increasing height, suggesting they trace genuine episodes of star formation rather than measurement errors. Each burst likely represents a time when the inner Milky Way experienced an intense period of stellar birth, possibly triggered by cosmic interactions.

Clues from Simulations and Migrations

To interpret these findings, Ruiz-Lara and collaborators turn to the Auriga Superstars computer simulations, which model galaxies similar to the Milky Way. These simulations reproduce populations of metal-rich stars with the same bursty age patterns, all originating from the galaxy’s inner bar and later migrating outward. The simulations indicate that such migration can be caused by the bar’s gravitational influence, which stirs up stellar orbits and redistributes stars over great distances. This connection suggests that the Milky Way’s bar has played a crucial role in both mixing and shaping its stellar populations.

Linking Bursts to Galactic Events

The timing of the star formation bursts aligns intriguingly with major events in the Milky Way’s history. The oldest peaks correspond to the Galaxy’s early assembly and its merger with the Gaia–Enceladus–Sausage galaxy about 10 billion years ago, an encounter that may have even triggered the bar’s formation. Later bursts appear around the times when the Sagittarius dwarf galaxy and the Magellanic Clouds interacted with the Milky Way, likely compressing gas and igniting new star formation. The 4-billion-year-old burst, however, remains mysterious: no known merger coincides with it, raising the possibility of an internal trigger, perhaps a period of strong bar activity.

Implications for the Milky Way’s Evolution

The study presents a compelling scenario: the Milky Way’s inner regions have undergone episodic, rather than steady, star formation throughout their history. These bursts, often linked to galactic interactions, enriched the Galaxy’s core with metals and periodically reshaped its structure. Some of the resulting stars, pushed outward by the bar or other dynamic effects, now reside near the Sun, allowing astronomers to study them up close. Ruiz-Lara and colleagues conclude that these “messenger stars” carry the imprint of the inner Galaxy’s past, providing an indirect yet powerful way to reconstruct how the Milky Way grew and evolved.

Conclusion

By analyzing Gaia data with the ChronoGal technique, Ruiz-Lara et al. reveal a vivid timeline of the Milky Way’s inner life, one marked by bursts of creation rather than calm continuity. Their work highlights how migration and interactions have shaped both the Galaxy’s structure and chemistry. As future Gaia data releases refine stellar ages even further, astronomers will be able to test and expand upon this emerging picture of a dynamic, evolving Galaxy whose history is written in the stars themselves.

Source: Ruiz-Lara