Three star clusters in the Milky Wayโs chaotic inner bulge act as time capsules: one young (11 million years old), one middle-aged (240 million years), and one ancient (1.1 billion years). Gaia DR3 revealed their strikingly different orbits; some plunging on eccentric, bar-driven paths, while one follows a calm, near-circular local orbit. The galaxyโs central โdowntownโ is a melting pot of heated, accreted, and in-situ stars, reshaping our view of bulge formation.
Using Gaia DR3 and UPMASK, researchers confirmed 44, 55, and 112 probable members for HSC 25, 37, and 2878, deriving ages, distances (6โ7.4 kpc), near-solar metallicities, and masses (135, 755, 204 Mโ) with individual extinction corrections. HSC 37 (11 million years old) is youthful and massive; HSC 2878 (1.1 billion years old) is settled and calm; same neighborhood, different origins.
Clusters 25 and 37 follow high-eccentricity orbits (e โ 0.6โ0.86) with tiny perigalacticons (~0.2โ0.3 kpc), hinting at dynamical heating or accretion. HSC 2878โs near-circular planar orbit suggests it formed in place.
Non-axisymmetric simulations show the galactic bar disrupts inner clusters like a cosmic cocktail shaker; relaxation times confirm the youngest remains loosely bound, while the older two are evolved. This first detailed study of Hunt & Reffert (2024) candidates reveals bulge star formation as messy, multi-origin, and still ongoing.