Astrophysicists have reclassified Ursa Major III, previously thought to be the Milky Way's most obscure dwarf galaxy dominated by dark matter. New research, published in August 2025, presents strong evidence that Ursa Major III is, in fact, a compact star cluster with a core composed of black holes and neutron stars.
This groundbreaking discovery, led by an international consortium including the University of Bonn and the Iranian Institute for Advanced Studies in Basic Sciences, suggests that Ursa Major III's substantial observed mass is not due to dark matter, but rather the gravitational influence of its dense stellar remnant core. This reinterpretation implies that prior observations may have misinterpreted the gravitational effects at play, potentially requiring a re-evaluation of numerous celestial bodies currently cited as evidence for dark matter.
Sophisticated computer simulations were employed by the research team, led by Ali Rostami-Shirazi, to model Ursa Major III's evolutionary trajectory. These simulations indicate that the object's current configuration can be fully accounted for without invoking dark matter. The findings, published in The Astrophysical Journal Letters, offer a novel framework for understanding Ursa Major III and may contribute to a deeper appreciation of the intricate processes governing the formation and evolution of star clusters and galaxies.
The University of Bonn highlighted that the research suggests gravitational interactions with the Milky Way over billions of years have stripped away Ursa Major III's outer stars. This process left behind a concentrated core of dense stellar remnants, mimicking the gravitational signature previously attributed to dark matter. This new perspective could significantly alter our understanding of galactic formation and evolution, as well as how gravitational interactions shape satellite galaxies.