NGC 6553

NGC 6553 is a globular cluster in the constellation Sagittarius. NGC 6553 has an apparent magnitude of about 8th magnitude with an apparent diameter of 8.2 arcminutes.^[2] Its Shapley–Sawyer Concentration Class is XI,^[2] meaning the star concentration is very loose even at the center; it has stars of magnitude 20 and dimmer. It is located just over a degree southeast of Messier 8, the Lagoon Nebula.

NGC 6553
The globular cluster NGC 6553 imaged by the Hubble Space Telescope
Observation data (J2000 epoch)
Class	V
Constellation	Sagittarius
Right ascension	18h 09m 15.68s[1]
Declination	−25° 54′ 27.9″[1]
Distance	19.6 kly (6.01 kpc)[1]
Apparent magnitude (V)	8.06[2]
Physical characteristics
Radius	4.1' x 4.1'[2]
Metallicity	${\displaystyle {\begin{smallmatrix}\left[{\ce {Fe}}/{\ce {H}}\right]\end{smallmatrix}}}$ = -0.18[3] dex
Other designations	GCl 88, C 1806-259[1]
See also: Globular cluster, List of globular clusters

Unlike common globular clusters, NGC 6553 is relatively metal-rich, and there is evidence of at least two periods of star formation.^[4] Due to a complex star-forming record, the stars in the cluster are differing in composition, most notably in concentrations of sodium and aluminium.^[5]

NGC 6553 is very similar in composition to NGC 6528, suggesting an origin in a similar environment.^[6]