Ursa Major II Dwarf
Encyclopedia
Ursa Major II or Uma II is a dwarf spheroidal galaxy
situated in the Ursa Major constellation and discovered in 2006 in the data obtained by Sloan Digital Sky Survey
. The galaxy is located approximately 30 kpc from the Sun and moves towards the Sun with the velocity of about 116 km/s. 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 of the Milky Way
—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
s. Uma II is even less luminous than some star
s, like Canopus
in the Milky Way. It is comparable in luminosity to Bellatrix in Orion
. However, its mass is about 5 million solar mass
es, 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 , 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
. 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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Dwarf spheroidal galaxy
Dwarf spheroidal galaxy is a term in astronomy applied to low luminosity galaxies that are companions to the Milky Way and to the similar systems that are companions to the Andromeda Galaxy M31...
situated in the Ursa Major constellation and discovered in 2006 in the data obtained by Sloan Digital Sky Survey
Sloan Digital Sky Survey
The Sloan Digital Sky Survey or SDSS is a major multi-filter 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 was named after the Alfred P...
. The galaxy is located approximately 30 kpc from the Sun and moves towards the Sun with the velocity of about 116 km/s. It is classified as a dwarf spheroidal galaxy
Dwarf spheroidal galaxy
Dwarf spheroidal galaxy is a term in astronomy applied to low luminosity galaxies that are companions to the Milky Way and to the similar systems that are companions to the Andromeda Galaxy M31...
(dSph) meaning that it has an elliptical (ratio of axes ~ 2:1) shape with the half-light radius
Effective radius
The effective radius of a galaxy is the radius at which one half of the total light of the system is emitted interior to this radius. This assumes the galaxy is circularly symmetric...
of about 140 pc.
Uma II is one of the smallest and faintest satellites of the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
—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
Globular cluster
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is...
s. Uma II is even less luminous than some star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s, like Canopus
Canopus
Canopus |Alpha]] Carinae) is the brightest star in the southern constellation of Carina and Argo Navis, and the second brightest star in the night-time sky, after Sirius. Canopus's visual magnitude is −0.72, and it has an absolute magnitude of −5.53.Canopus is a supergiant of spectral...
in the Milky Way. It is comparable in luminosity to Bellatrix in Orion
Orion (constellation)
Orion, often referred to as The Hunter, is a prominent constellation located on the celestial equator and visible throughout the world. It is one of the most conspicuous, and most recognizable constellations in the night sky...
. However, its mass is about 5 million solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es, which means that galaxy's mass to light ratio
Mass to light ratio
In astrophysics and physical cosmology the mass to light ratio, normally designated with the symbol \Upsilon is the quotient between the total mass of a spatial volume and its luminosity...
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 , 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
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...
. 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.
See also
- Ursa Major I Dwarf
- Ursa Minor DwarfUrsa Minor DwarfThe Ursa Minor Dwarf dwarf elliptical galaxy was discovered by A.G. Wilson of the Lowell Observatory in 1954. It is part of the Ursa Minor constellation, and a satellite galaxy to the Milky Way...
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