Ursa Major II Dwarf (UMa II dSph) is a

dwarf spheroidal galaxy A dwarf spheroidal galaxy (dSph) is a term in astronomy applied to small, low-luminosity galaxies with very little dust and an older stellar population. They are found in the Local Group as companions to the Milky Way and as systems that are c ...

situated in the

Ursa Major Ursa Major, also known as the Great Bear, is a constellation in the Northern Sky, whose associated mythology likely dates back into prehistory. Its Latin name means "greater (or larger) bear", referring to and contrasting it with nearby Ursa M ...

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

and discovered in 2006 in the data obtained by the

Sloan Digital Sky Survey The Sloan Digital Sky Survey or SDSS is a major multi-spectral imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project began in 2000 a ...

. The galaxy is located approximately 30 kpc from the Sun and moves towards the Sun with the velocity of about 116 km/s. It has an elliptical (ratio of axes ~ 2:1) shape with the

half-light radius Galaxy effective radius or half-light radius (R_e) is the radius at which half of the total light of a galaxy is emitted. This assumes the galaxy has either intrinsic circular symmetry, spherical symmetry or is at least circularly symmetric as vi ...

of about 140 pc. Ursa Major II is one of the smallest and faintest satellites 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 ...

—its integrated luminosity is about 4000 times that of the Sun (absolute visible magnitude of about −4.2), which is much lower than the luminosity of the majority of

globular cluster A globular cluster is a spheroidal conglomeration of stars that is bound together by gravity, with a higher concentration of stars towards its center. It can contain anywhere from tens of thousands to many millions of member stars, all orbiting ...

s. UMa II is even less luminous than some

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

s, like

Canopus Canopus is the brightest star in the southern constellation of Carina (constellation), Carina and the list of brightest stars, second-brightest star in the night sky. It is also Bayer designation, designated α Carinae, which is Rom ...

in the Milky Way. It is comparable in luminosity to Bellatrix in Orion. However, its mass is about 5 million

solar mass The solar mass () is a frequently used unit of mass in astronomy, equal to approximately . It is approximately equal to the mass of the Sun. It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxie ...

es, which means that the galaxy's mass to light ratio is around 2000. This may be an overestimate as the galaxy has a somewhat irregular shape and may be in the 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 , which means that they contain 300 times less heavy elements than the Sun. The stars of UMa II were probably among the first stars to form in the

Universe The universe is all of space and time and their contents. It comprises all of existence, any fundamental interaction, physical process and physical constant, and therefore all forms of matter and energy, and the structures they form, from s ...

. Currently, there is no star formation in UMa II. The measurements have so far failed to detect any neutral hydrogen in it—the upper limit is only 562 solar masses.

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{{DEFAULTSORT:Ursa Major II Dwarf Dwarf spheroidal galaxies Ursa Major Local Group Milky Way Subgroup ?

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