Segue 3

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Segue 3
Observation data (J2000 epoch)
Right ascension 21h 21m 31s[1]
Declination19° 07′ 02″[1]
Distance55.1 ± 2.3 kly (16.9 ± 0.7 kpc[2])
Apparent magnitude (V)14.9[1]
Apparent dimensions (V)52 ± 10″ (half-light diameter)[2]
Physical characteristics
Radius6.8 ± 1.3 ly (2.1 ± 0.4 pc)[2]
Other designationsSegue 3
See also: Globular cluster, List of globular clusters

Segue 3 is a faint star cluster of the Milky Way galaxy discovered in 2010 in the data obtained by Sloan Digital Sky Survey.[1] It is located in the Pegasus constellation at the distance of about 17 kpc from the Sun and moves away from it with the velocity of 167.1 ± 1.5 km/s.[2]

Segue 3 is extremely faint—its visible absolute magnitude is estimated at −1.2[1] or even at about 0.0 ± 0.8,[2] which means that the cluster is only 100 to 250 times brighter than the Sun. Its small radius—of about 2.1 pc—is typical for the galactic globular clusters.[2] The cluster has a slightly flattened shape and shows some evidence of the tidal disruption.[2]

The metallicity of Segue's 3 stars is [Fe/H] ≈ −1.7, which means that they contain 70 times less heavy elements than the Sun. These stars are more than 12 billion year old.[2] Segue 3 appears to be one of the faintest globular clusters of the Milky Way.


  1. ^ a b c d e Belokurov, V.; Walker, M. G.; Evans, N. W.; Gilmore, G.; Irwin, M. J.; Just, D.; Koposov, S.; Mateo, M.; Olszewski, E.; Watkins, L.; Wyrzykowski, L. (2010). "Big Fish, Little Fish: Two New Ultra-Faint Satellites of the Milky Way". The Astrophysical Journal. 712: L103. arXiv:1002.0504. Bibcode:2010ApJ...712L.103B. doi:10.1088/2041-8205/712/1/L103.
  2. ^ a b c d e f g h Fadely, R.; Willman, B.; Geha, M.; Walsh, S.; Muñoz, R. R.; Jerjen, H.; Vargas, L. C.; Da Costa, G. S. (2011). "Segue 3: An old, extremely low luminosity star cluster in the Milky Way's halo". The Astronomical Journal. 142 (3): 88. arXiv:1107.3151. Bibcode:2011AJ....142...88F. doi:10.1088/0004-6256/142/3/88.