Ursa Major III

Ursa Major III is an incredibly faint and small satellite galaxy of the Milky Way, the faintest and smallest ever discovered. Ursa Major III was discovered in the deep, wide field Ultraviolet Near Infrared Optical Northern Survey (UNIONS), and its discovery was announced in November 2023. It contains a very old (over 11 billion years.old), metal-poor stellar population located about 10 kiloparsecs away, or about 32,600 light years away. Ursa Major III has a radius of only about 9.8 light years, and its absolute magnitude (brightness as seen from 10 parsecs) is only +2.2, making 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_☉. Only about 60 stars are predicted to exist within Ursa Major III.^[1]

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.^[1] 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.

References

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Milky Way

Location

Milky Way → Milky Way subgroup → Local Group → Local Sheet → Virgo Supercluster → Laniakea Supercluster → Local Hole → Observable universe → Universe
Each arrow (→) may be read as "within" or "part of".

Structure

Galactic Center	Sagittarius A Sagittarius A* Fermi bubbles Supermassive black hole
Disk	Carina–Sagittarius Norma–Cygnus Orion–Cygnus Perseus Scutum–Centaurus Near 3 kpc Arm Far 3 kpc Arm
Halo	Aquarius Stream Fimbulthul stream Gaia Sausage Helmi stream Magellanic Stream Monoceros Ring Palomar 5 stream Pisces-Eridanus stream Sagittarius Stream Virgo Stream

Satellite
galaxies

Magellanic Clouds	Large Magellanic Cloud Small Magellanic Cloud Magellanic Bridge
Dwarfs	Antlia II Boötes I Boötes II Boötes III Canes Venatici I Canes Venatici II Canis Major Carina Coma Berenices Crater 2 Draco Eridanus II Fornax Hercules Leo I Leo II Leo IV Leo V Leo T Phoenix Pisces Overdensity Pisces II Reticulum II Sagittarius Spheroidal Segue 1 Segue 2 Sculptor Sextans Triangulum II Ursa Major I Ursa Major II Ursa Major III Ursa Minor Virgo I Willman 1

Related

• Alternate names
• Andromeda–Milky Way collision
• Baade's Window
• In mythology
• Zone of Avoidance

• Astronomy portal
• Category

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