The new Vera C. Rubin Observatory, installed in Chile, has just detected two groups of stars that had gone unnoticed until now. These discoveries come from its first test images, taken before the launch of its major observation campaign. They concern the distant regions of the Milky Way, our galaxy, whose contours are still poorly understood.
The first object is called Aquarius IV. It lies about 355,000 light-years from Earth, a distance that light would take just as many years to cross. Its stars are so few and faint that it almost disappears into the background sky. Researchers consider it a probable small galaxy, but this classification still needs to be confirmed.
The Vera C. Rubin Observatory in Chile, with its dome open beneath the star cluster Messier 41, in April 2025.
Credit: RubinObs/NOIRLab/SLAC/NSF/DOE/AURA/P. Horálek — CC BY 4.0.
The second, Rubin-GC1, is different. It is most likely a cluster, in other words, a group of stars gathered in the same region. Located about 100,000 light-years away, it was difficult to distinguish behind the dust of our galaxy. This dust obscures part of the light, like a veil in front of a lamp.
Why did these objects escape astronomers until now? A galaxy does not always look like a large, bright spiral. Some contain only a very small number of stars and emit extremely faint light. To find them, researchers look for tiny concentrations of stars that stand out from the surrounding sky.
The Rubin telescope has a camera with 3.2 billion pixels. It photographs vast portions of the sky with great precision. The observations used here covered only about 7% of the entire sky. Yet they have already made it possible to identify these two objects, whose nature was then studied separately.
These discoveries also matter for a less visible reason: dark matter. Astronomers think that a large amount of this invisible matter exerts a gravitational pull on stars and galaxies. The smallest galaxies could help test this idea, because their motion depends on the mass around them.
For now, about 65 satellite galaxies of the Milky Way are known, and others may still be missing. The two new objects mainly show that part of this galactic neighborhood is probably still unknown.
Future observations will need to clarify the nature of Aquarius IV and measure the motions of the stars in Rubin-GC1. The latter may come from an ancient small galaxy gradually torn apart by the gravitational pull of the Milky Way. The Rubin Observatory is expected to continue its sky surveys for ten years, making it possible to search for other, even more elusive galactic neighbors.