The New General Catalogue of Nebulae and Clusters of Stars (abbreviated as NGC) is a catalogue of deep-sky objects compiled by John Louis Emil Dreyer in 1888. The NGC contains 7,840 objects, known as the NGC objects. It is one of the largest comprehensive catalogues, as it includes all types of deep space objects, including galaxies, star clusters, emission nebulae and absorption nebulae.
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NGC 6558
NGC 6558 is a globular cluster, located about 24,000 light years away in the constellation Sagittarius. Its apparent magnitude is about 11 and its apparent diameter is about 10 arcminutes. The globular cluster was discovered in 1784 by the astronomer William Herschel with his 18.7-inch telescope and the discovery was later catalogued in the New General Catalogue. It is located 1.5 degrees south-southeast of Gamma2 Sagittarii.
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Wikipedia Page: NGC 6558
NGC 6558 at In-The-Sky website