Segue 2 is a dwarf spheroidal galaxy situated in the

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 origins of the e ...

Aries and discovered in 2009 in the data obtained by

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 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 System. ...

is located at the distance of about from the Sun 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 is emitted. This assumes the galaxy has either intrinsic circular symmetry, spherical symmetry or is at least circularly symmetric as vie ...

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. 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—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 clusters. 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 ...

es—is substantial, corresponding to a mass-to-light ratio 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 (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

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 a ...

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. Currently, there is no star formation in Segue 2. Segue 2 is located near the edge of

Sagittarius Stream In astronomy, the Sagittarius Stream is a long, complex structure made of stars that wrap around the Milky Way galaxy in an orbit that nearly crosses the galactic poles. It consists of Tidal stripping, tidally stripped stars from the Sagittarius Dw ...

and at the same distance. It may once have been a satellite of Sagittarius Dwarf Elliptical Galaxy or its star cluster. In June 2013 '' The Astrophysical Journal'' reported that Segue 2 was bound together with dark matter. 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