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 1167 ñ 26km/s, which corresponds to a Hubble distance of . However, 26 non-redshift measurements give a much closer distance of . 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 72.7ð 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.
One supernova has been observed in NGC 3521: SN2024aecx (TypeIc, mag. 14.543) was discovered by ATLAS on 16 December 2024. Astronomers originally classified it as TypeIIb, but spectroscopy suggests this supernova is very similar to SN 1994I.