FSR 1758

FSR 1758
The Sequoia Cluster (FSR 1758) in near-infrared Credit: Legacy Surveys / D. Lang (Perimeter Institute) & Meli thev
Observation data (J2000 epoch)
Constellation	Scorpius
Right ascension	17h 31m 12s
Declination	−39° 48′ 30″
Distance	11.5 kpc (38 kly)
Apparent magnitude (V)	<7
Apparent dimensions (V)	1.56 ± 0.44 °
Physical characteristics
Absolute magnitude	<–8.6
Mass	~107 M☉
Radius	10 ± 1 pc
Tidal radius	150 ± 45 pc
Metallicity	${\begin{smallmatrix}\left[{\ce {Fe}}/{\ce {H}}\right]\end{smallmatrix}}$ = −1.5 dex
Notable features	Possible nucleus of a dwarf galaxy

FSR 1758 (also known as Sequoia Cluster) is a large and bright but heavily obscured globular cluster belonging to the Milky Way galaxy. It is located at a distance of about 11.5 kpc from the Sun and about 3.7 kpc from the center of the galaxy. As FSR 1758 lies behind the galactic bulge, it is heavily obscured by the foreground stars and dust. It was first noticed in 2007 in 2MASS data and believed to be an open cluster, until data from the Gaia mission revealed in 2018 that it is a globular cluster.

The size and brightness of FSR 1758 may be comparable to or exceed that of the Omega Centauri cluster, which is widely believed to be the nucleus of a dwarf galaxy that merged into Milky Way in the past. Therefore, FSR 1758 may be the nucleus of dwarf galaxy tentatively named Scorpius Dwarf galaxy. It may also be similar to another globular cluster, Messier 54, which is known to be the nucleus of Sagittarius Dwarf Spheroidal Galaxy.

After Barbá et al. used the term Sequoia to describe the size of FSR 1758, Myeong et al. used the term Sequoia in a slightly different way. They believe that FSR 1758 was one of five globular clusters that populated a dwarf galaxy that Myeong et al. re-name as the Sequoia dwarf galaxy. This dwarf was accreted into the Milky Way in the Sequoia Event. The members of Sequoia have a retrograde galactic orbit. This term has been adopted by several other groups.