NGC 3521, also known as the Bubble Galaxy, is a flocculent intermediate spiral galaxy located in the constellation Leo. Its velocity with respect to the cosmic microwave background is 1,167 ± 26 km/s, which corresponds to a Hubble distance of 56.1 ± 4.1 Mly (17.21 ± 1.26 Mpc). However, 26 non-redshift measurements give a much closer distance of 37.17 ± 1.83 Mly (11.395 ± 0.56 Mpc). It was discovered by German-British astronomer William Herschel on 22 February 1784.
NGC 3521 has a morphological classification of SAB(rs)bc, which indicates that it is a spiral galaxy with a trace of a bar structure (SAB), a weak inner ring (rs), and moderate to loosely wound arm structure (bc). The bar structure is difficult to discern, both because it has a low ellipticity and the galaxy is at a high inclination of 72.7° to the line of sight. The relatively bright bulge is nearly 3/4 the size of the bar, which may indicate the former is quite massive. The nucleus of this galaxy is classified as an HII LINER, as there is an H II region at the core and the nucleus forms a low-ionization nuclear emission-line region.
NGC 3521 is structurally similar to the Milky Way; additionally, its supermassive black hole has a similar mass to that of the Milky Way, at about 7 million solar masses.
