Ursa Major III

{{Short description|Extremely faint candidate satellite galaxy of the Milky Way}}

{{Infobox galaxy

| image =

| caption =

| name = Ursa Major III / UNIONS 1

| epoch = J2000

| type = dSph

| ra = {{RA|11|38|49.8}}

| dec = {{DEC|+31|04|42}}

| dist_ly = {{val|32.6|3.3}} kly
({{val|10|1}} kpc)

| z =

| appmag_v =

| mass_light_ratio = 6500

| stars = {{val|57|21|19}}

| half_light_radius_pc = {{val|3|1}} pc

| half_light_radius_arcminsec = 0.9′

| constellation name = Ursa Major

| absmag_v={{val|+2.2|0.4|0.3}}

| names = UMa III, UNIONS 1

}}

File:Ursa Major III.webm

Ursa Major III / UNIONS 1 (UMa III / U1) is a possible dwarf satellite galaxy of the Milky Way. If confirmed as a galaxy, as opposed to a star cluster, it would represent the smallest and faintest galaxy ever discovered, containing only about 60 star systems as opposed to hundreds of billions.{{cite news|last=Kuthunur|first=Sharmila|title=Group of 60 faint stars orbiting the Milky Way could be new type of galaxy never seen before|url=https://www.livescience.com/space/cosmology/group-of-60-ultra-faint-stars-orbiting-the-milky-way-could-be-new-type-of-galaxy-never-seen-before|website=Live Science|date=3 April 2024|accessdate=10 April 2024}} 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) 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 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 total luminosity of {{Solar luminosity|11.4}}.

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 halo, suggesting that Ursa Major III could indeed be a true dwarf galaxy, albeit one with an extremely low stellar mass. Alternatively, it may be the faintest globular cluster ever discovered, and the observed velocity dispersion could be the product of stellar binarity. In either case, Ursa Major III / UNIONS 1's total stellar mass is only about {{val|16|ul=Solar mass|fmt=commas}}, making it the least massive Milky Way satellite known.