Astronomers reveal hidden black holes, not dark matter, as the glue of the Milky Way’s faintest satellite

Ursa Major III, the faintest known satellite of the Milky Way, may not be a dark-matter-rich dwarf galaxy as previously believed. Located over 30,000 light-years away and containing just 60 visible stars, this ghostly object has long puzzled astronomers.

For years, its unusually high mass-to-light ratio, a measure comparing how much mass an object has versus how much light it emits, led scientists to classify it as a dark-matter-dominated dwarf galaxy.

However, new research published on August 7 in the Astrophysical Journal Letters suggests that instead of dark matter, its gravity may be dominated by a dense core of black holes and neutron stars, reshaping our understanding of how such compact stellar systems form.

A ghostly satellite under the microscope

Ursa Major III, also called UMa3 or Unions 1, is one of the Milky Way’s faintest companions. Its tiny size and sparse star population made it an unusual object. With only around 60 visible stars, it is much smaller than typical dwarf galaxies, which often contain thousands of stars.

Scientists initially thought that its faint glow meant the object was dominated by dark matter, an invisible substance that makes up about 85 per cent of the universe’s mass and influences the formation of galaxies.

Black holes, not dark matter, keep it together

To investigate, the research team combined high-precision observations of UMa3’s stars with computer simulations that modelled the cluster’s evolution over billions of years.

The results suggest that repeated gravitational interactions with the Milky Way stripped away most of the cluster’s outer stars, leaving behind a dense, invisible core.

This core, composed of black holes and neutron stars, is now strong enough to hold the remaining stars together without requiring the presence of dark matter.

“Dark star clusters form when gravitational interactions with a massive galaxy like the Milky Way remove the outer stars from a star cluster,” Haghi explained. “What remains is a compact core that can mimic the properties of a dark-matter-dominated dwarf galaxy.”

This discovery offers a fresh perspective on how astronomers interpret faint stellar objects.

It suggests that some of the Milky Way’s satellites previously thought to be dominated by dark matter may, in fact, be these “dark star clusters,” composed primarily of remnants of massive stars.

Solving a cosmic mystery

In simple words, this discovery solves a mystery about what holds tiny star systems together.

Astronomers used to think Ursa Major III was a dwarf galaxy full of dark matter because it seemed too heavy for the few stars we can see.

But the new study shows that instead, it’s a normal star cluster, and its gravity comes from a hidden core of black holes and neutron stars, not dark matter.

So basically, it helps scientists understand the true nature of faint star clusters and avoids wrongly assuming that every weird, faint object around the Milky Way is full of dark matter.