How are black holes named?
There is no unified naming system for black holes. The supermassive black holes in the cores of galaxies are named for the galaxies. A name like M31 indicates that the galaxy was catalogued by Charles Messier in the 1700s. A name that begins with "NGC" is listed in the New General Catalog, which was compiled in 1888 and expanded in later years. A few black holes are catalogued by their constellation and the order in which they were discovered; Cygnus X-1 was the first X-ray object discovered in Cygnus, for example. And many black holes are identified by the instrument or survey that discovered them and their position in the sky. XTE J1118+480 was discovered by the Rossi X-Ray Timing Explorer (XTE) satellite, and is at the celestial coordinates 1118+480. GRO J1655-40 was discovered by the Compton Gamma-Ray Observatory, and so on.
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