Herschel described his discovery as "faint, pretty large, irregular round, brighter middle". Further observations were made by both his son, John Herschel, who simply noted "big" on his first and "very faint" on his second observation, as well as R. J. Mitchell, who noted "pretty big, spiral galaxy, disc enveloped in faint outlying neby and looks like an unresolved cluster."[6] NGC 521 was later catalogued by John Louis Emil Dreyer in the New General Catalogue, where the galaxy was described as "faint, pretty large, round, gradually brighter middle".[5]
The galaxy's large apparent size can be attributed to the fact that it is face-on. Despite its size, it only has an apparent visual magnitude of 11.7. It can be classified as spiral galaxy of type SBbc using the Hubble Sequence.[2] The object's distance of roughly 220 million light-years from the Solar System can be estimated using its redshift and Hubble's law.[4]
^ abcAn object's distance from Earth can be determined using Hubble's law: v=Ho is Hubble's constant (70±5 (km/s)/Mpc). The relative uncertainty Δd/d divided by the distance is equal to the sum of the relative uncertainties of the velocity and v=Ho
^Marsden, Brian G. (22 August 1966). "Circular No. 1966". Central Bureau for Astronomical Telegrams. Smithsonian Astrophysical Observatory. Retrieved 2 December 2024.
^"SN1966G". Transient Name Server. IAU. Retrieved 1 December 2024.
^Szeidl, B.; Lovas, M. (1982). "Supernova in NGC 521". International Astronomical Union Circular (3724): 1. Bibcode:1982IAUC.3724....1S.
^"SN1982O". Transient Name Server. IAU. Retrieved 1 December 2024.
^Baek, M.; Li, W.; Puckett, T.; Sostero, G.; Garzia, S. (2006). "Supernovae 2006D, 2006E, 2006F, 2006G, 2006H". International Astronomical Union Circular (8660): 1. Bibcode:2006IAUC.8660....1B.
^"SN2006G". Transient Name Server. IAU. Retrieved 1 December 2024.