Ursa Major III

Location of Ursa Major III, a dwarf satellite galaxy near the Milky Way

Ursa Major III
Observation data (J2000 epoch)
Constellation	Ursa Major
Right ascension	11h 38m 49.8s
Declination	+31° 04′ 42″
Distance	32.6±3.3 kly (10±1 kpc)
Absolute magnitude (V)	+2.2+0.4 −0.3
Characteristics
Type	dSph
Mass/Light ratio	6500 M☉/L☉
Number of stars	57+21 −19
Half-light radius (physical)	3±1 pc
Half-light radius (apparent)	0.9′
Other designations
UMa III, UNIONS 1

Ursa Major III (UMa III) is a dwarf satellite galaxy of the Milky Way, the smallest and faintest ever discovered. 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 (two observatories in Hawaii), with additional data provided by the Keck Observatory's Deep Imaging Multi-Object Spectrograph (DEIMOS), which has 64 megapixels of resolution.

Ursa Major III's discovery was announced in November 2023, with a paper appearing in The Astrophysical Journal 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. This absolute magnitude corresponds to a stellar mass of 16 M_☉.

Ursa Major III is predicted to 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 galaxy. This very high value indicates the presence of a massive dark matter halo, indicating that Ursa Major III is indeed a true dwarf galaxy, albeit one with an extremely low stellar mass. As Ursa Major III's total luminosity is approximately 11.4 L_☉ its total mass is probably only about 74,000 M_☉, which would make it one of the least massive galaxies known.