Ursa Major II Dwarf Galaxy

Ursa Major II Dwarf (UMa II dSph) is a dwarf spheroidal galaxy situated in orbit of the Milky Way Galaxy. It is classified as a dwarf spheroidal galaxy (dSph) meaning that it has an elliptical (ratio of axes ~ 2:1) shape with the half-light radius of about 140 pc.

Uma II is one of the smallest and faintest satellites, even less luminous than some stars, like Canopus in the Milky Way. It is comparable in luminosity to Bellatrix. However, its mass is about 5 million stellar masses, which means that galaxy's mass to light ratio is around 2000. This is may be an overestimate as the galaxy has somewhat irregular shape and may be in process of tidal disruption.

The stellar population of Uma II consists mainly of old stars formed at least 10 billion years ago. The metallicity of these old stars is also very low at [Fe/H] ≈ −2.44 ± 0.06, which means that they contain less heavy elements. The stars of Uma II were probably among the first stars to form in the Universe. Currently there is no star formation in Uma II. The measurements have so far failed to detect any neutral hydrogen in it.