Ursa Major III / UNIONS 1 (UMa III / U1) is a dwarf satellite galaxy of the

Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

or a stellar cluster. If confirmed as a galaxy it would represent the smallest and faintest galaxy ever discovered, containing only about 60 star systems as opposed to hundreds of billions. It was found by the deep, wide field Ultraviolet Near Infrared Optical Northern Survey (UNIONS), a collaboration between the Canada–France–Hawaii Telescope and

Pan-STARRS The Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1; List of observatory codes, obs. code: IAU code#F51, F51 and Pan-STARRS2 obs. code: IAU code#F52, F52) located at Haleakala Observatory, Hawaii, US, consists of astronomical ...

(two observatories in Hawaii) and spectroscopically confirmed as a real satellite based on data obtained with the Keck Observatory's Deep Imaging Multi-Object Spectrograph (DEIMOS). Ursa Major III's discovery was announced in November 2023, with a paper appearing in ''

The Astrophysical Journal ''The Astrophysical Journal'' (''ApJ'') is a peer-reviewed scientific journal of astrophysics and astronomy, established in 1895 by American astronomers George Ellery Hale and James Edward Keeler. The journal discontinued its print edition and ...

'' in January 2024. It contains a metal-poor stellar population, indicating an extreme age of 11 billion years. Located about 32,600 light years away, it has a diameter of just 19.6 light years and is thought to contain only about 60 stars. Combined with its absolute magnitude of only +2.2, this makes it by far the Milky Way's dimmest satellite, and only about as bright as

Altair Altair is the brightest star in the constellation of Aquila (constellation), Aquila and the list of brightest stars, twelfth-brightest star in the night sky. It has the Bayer designation Alpha Aquilae, which is Latinisation of name ...

. This absolute magnitude corresponds to a total luminosity of . Dynamical measurements of Ursa Major III / UNIONS 1's stars suggest it may have a mass-to-light ratio of about 6,500. However, this becomes only 1,900 with the removal of one of the stars suspected to be part of the system. This very high value may indicate the presence of a massive

dark matter In astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravity, gravitational effects that cannot be explained by general relat ...

halo, suggesting that Ursa Major III could indeed be a true dwarf galaxy, albeit one with an extremely low stellar mass of , making it the least massive Milky Way satellite known. A 2025 study found the previous estimate of the stellar mass to be highly underestimated, since it does not account for the large number of compact stars within the galaxy, which make 50 to 80% of Ursa Major III's mass and are invisible by UNIONS. Accounting for this the stellar mass would be . Simulations show UNIONS 1, without dark matter, can survive for 2.7 billion years with this higher mass. This is substantially larger than the 400 to 800 million years previously estimated, and as of such it can be a star cluster rather than a galaxy.

References

Astronomical objects discovered in 2023 Dwarf galaxies Local Group Milky Way Subgroup Ursa Major {{galaxy-stub