Segue 2

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Segue 2 Dwarf Galaxy[1]
Observation data (J2000 epoch)
Right ascension 02h 19m 16s[1]
Declination+20° 10′ 31″[1]
Distance114 kly (35 kpc)[2]
Apparent magnitude (V)15.2 ± 0.2[2]
Mass5.5×105[2] M
Mass/Light ratio650[2] M/L
Apparent size (V)6.8+0.4
Other designations
Segue 2[2]

Segue 2 is a dwarf spheroidal galaxy situated in the constellation Aries and discovered in 2009 in the data obtained by Sloan Digital Sky Survey.[2] The galaxy is located at the distance of about 35 kpc (35,000 parsecs (110,000 ly)) from the Sun and moves towards the Sun with the speed of 40 km/s.[2] It is classified as a dwarf spheroidal galaxy (dSph) meaning that it has an approximately round shape with the half-light radius of about 34 pc.[2]

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[note 1] of the Milky Way[4]—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.[2] However, the mass of the galaxy—about 550,000 solar masses—is substantial, corresponding to the mass to light ratio of about 650.[2]

The stellar population of Segue 2 consists mainly of old stars formed more than 12 billion years ago.[2] The metallicity of these old stars is also very low at [Fe/H] < −2, which means that they contain at least 100 times less heavy elements than the Sun.[2] 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.[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 or its star cluster.[2]

In June 2013 The Astrophysical Journal reported that Segue 2 was bound together with dark matter.[5][6][7]

Circa 1,000 stars are supposed to exist within the galaxy.[8]


  1. ^ Only Segue 1 and Willman 1 are fainter.[3]


  1. ^ a b c "NAME Segue 2". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2016-08-24.
  2. ^ a b c d e f g h i j k l m n o p Belokurov, V.; Walker, M.G.; Evans, N.W.; et al. (2009). "Segue 2: A Prototype of the Population of Satellites of Satellites". Mon. Not. R. Astron. Soc. 397 (4): 1748–1755. arXiv:0903.0818. Bibcode:2009MNRAS.397.1748B. doi:10.1111/j.1365-2966.2009.15106.x.
  3. ^ Martin, Nicolas F.; de Jong, Jelte T. A.; Rix, Hans-Walter (2008). "A Comprehensive Maximum Likelihood Analysis of the Structural Properties of Faint Milky Way Satellites". The Astrophysical Journal. 684 (2): 1075–1092. arXiv:0805.2945. Bibcode:2008ApJ...684.1075M. doi:10.1086/590336.
  4. ^ "The Smallest Galaxies In the Universe Have the Most Dark Matter". Forbes. 2 December 2015. Retrieved 8 April 2016.
  5. ^ Kirby, Evan; Michael Boylan-Kolchin; Judith G. Cohen; Marla Geha; James S. Bullock; Manoj Kaplinghat (June 10, 2013). "Segue 2: The Least Massive Galaxy" (PDF). The Astrophysical Journal. arXiv:1304.6080. Bibcode:2013ApJ...770...16K. doi:10.1088/0004-637X/770/1/16.
  6. ^ Megan Gannon (2013-06-10). "Dwarf Galaxy Segue 2 Called Smallest Ever Discovered". Huffington Post. Retrieved 2013-06-23. The dwarf galaxy known as Segue 2 is bound together by a tiny clump of dark matter. Scientists who measured it using Hawaii's Keck Observatory say the finding adds support to theories about the formation of the universe.
  7. ^ UCI Scientists Size Up Universe’s Most Lightweight Dwarf Galaxy with Keck Observatory Steve Steve, W. M. Keck Observatory updated June 7, 2013
  8. ^ Bryan Nelson (11 June 2013). "Just how big is the smallest galaxy in the universe?". Mother Nature Network. Retrieved 8 April 2016.
Preceded by
Willman 1
Least massive galaxy known
2013 — 
Succeeded by