Segue 2 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 to systems that are com ...

situated in the constellation Aries and discovered in 2009 in the data obtained by Sloan Digital Sky Survey. The

galaxy A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar Sys ...

is located at the distance of about from the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared rad ...

and moves towards the Sun at a speed of . It is classified as a dwarf spheroidal galaxy (dSph), meaning that it has an approximately round shape with a

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 A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is de ...

of about 34 parsecs. The name is due to the fact that it was found by the SEGUE program, the

Sloan Extension for Galactic Understanding and Exploration 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 200 ...

. Segue 2 is one of the smallest and faintest

satellites A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotop ...

of the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked ey ...

—its integrated luminosity is about 800 times that of the Sun (absolute visible magnitude of about −2.5), which is much lower than the luminosity of the majority of

globular cluster A globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of memb ...

s. However, the mass of the galaxy—about 550,000

solar mass The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass o ...

es—is substantial, corresponding to a

mass-to-light ratio In astrophysics and physical cosmology the mass-to-light ratio, normally designated with the Greek letter upsilon, , is the quotient between the total mass of a spatial volume (typically on the scales of a galaxy or a cluster) and its luminosity. ...

of about 650. The stellar population of Segue 2 consists mainly of old

star A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Earth make ...

s formed more than 12 billion years ago. The

metallicity In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as ...

of these old stars is also very low at e/H< −2, which means that they contain at least 100 times less heavy elements than the Sun. The stars of Segue 2 were probably among the first stars to form in the

universe The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. A ...

. Currently, there is no star formation in Segue 2. Segue 2 is located near the edge of Sagittarius Stream and at the same distance. It may once have been a satellite of

Sagittarius Dwarf Elliptical Galaxy The Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph), also known as the Sagittarius Dwarf Elliptical Galaxy (Sgr dE or Sag DEG), is an elliptical loop-shaped satellite galaxy of the Milky Way. It contains four globular clusters ...

or its star cluster. In June 2013 ''

The Astrophysical Journal ''The Astrophysical Journal'', often abbreviated ''ApJ'' (pronounced "ap jay") in references and speech, is a peer-reviewed scientific journal of astrophysics and astronomy, established in 1895 by American astronomers George Ellery Hale and Ja ...

'' reported that Segue 2 was bound together with

dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ab ...

. Around 1,000 stars are supposed to exist within the galaxy.

References

{{DEFAULTSORT:Segue 2 Dwarf spheroidal galaxies Aries (constellation) Local Group Milky Way Subgroup ? 4713565