The letters indicate the catalog listing of the galaxies. Galaxies are listed in several different catalogs. The most common catalog is NGC, which stands for New General Catalog. Other catalogs include M (Messier), ESO (European Southern Observatory), IRAS (Infrared Astronomical Satellite), Mrk (Markarian), and UGC (Uppsala General Catalog).
The numbers following the letters, such as Mrk 917 or NGC1433, indicate a galaxy’s entry in the catalog and are often related to the galaxy’s relative position in the sky.
Sometimes a galaxy appears in more than one catalog and can have more than one name.
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