Dimmest galaxy companion isn't dark matter blob, but dead stars
Scientists have discovered that Ursa Major III, the dimmest known companion of our galaxy, isn't packed with mysterious dark matter after all. Instead, it's a tight-knit star cluster held together by a core full of black holes and neutron stars.
This twist comes from new research published in August in the Astrophysical Journal Letters.
Ursa Major III's mystery
Floating over 30,000 light-years away with just about 60 visible stars, Ursa Major III always seemed oddly heavy for its faint glow—leading many to suspect dark matter.
But new evidence shows its mass comes from a hidden core of dead stars and black holes.
How did it happen?
By combining super-detailed telescope data with computer simulations that tracked billions of years of cosmic drama, researchers found that gravity from the Milky Way stripped away most outer stars.
What's left is a dense core—composed of black holes and neutron stars—that keeps the cluster together without needing any dark matter.
A new era in astronomy
This discovery could change how astronomers label ultra-faint space objects.
Some clusters thought to be tiny galaxies full of dark matter might actually be "dark star clusters" like Ursa Major III—a reminder that there are still surprises hiding in our own galactic neighborhood.