Messier 63, also known as NGC 5055 or the Sunflower Galaxy, is a spiral galaxy in the northern constellation of Canes Venatici with approximately 400 billion stars.
The shape or morphology of this galaxy has a classification of SAbc, indicating a spiral form with no central bar feature (SA) and moderate to loosely wound arms (bc).
M63 is a weakly active galaxy with a LINER nucleus – short for ‘low-ionization nuclear emission-line region’. This displays as an unresolved source at the galactic nucleus that is cloaked in a diffuse emission. The latter is extended along a position angle of 110° relative to the north celestial pole, and both soft X-rays and hydrogen (H-alpha) emission can be observed coming from along nearly the same direction. The existence of a supermassive black hole (SMBH) at the nucleus is uncertain; if it does exist, then the mass is estimated as (8.5±1.9)×108 M☉, or around 850 million times the mass of the Sun.
Radio observations at the 21-cm hydrogen line show the gaseous disk of M63 extends outward to a radius of 130,000 light-years (40 kiloparsecs), well past the bright optical disk. This gas shows a symmetrical form that is warped in a pronounced manner, starting at a radius of 33,000 light-years (10 kiloparsecs). The form suggests a dark matter halo that is offset with respect to the inner region. The reason for the warp is unclear, but the position angle points toward the smaller companion galaxy, UGC 8313.
The distance to M63, based upon the luminosity-distance measurement is 29,300,000 light-years (8.99 megaparsecs). The radial velocity relative to the Local Group yields an estimate of 15,200,000 light-years (4.65 megaparsecs). Estimates based on the Tully–Fisher relation range over 16,000,000–34,000,000 light-years (5.0–10.3 megaparsecs). M63 is part of the M51 Group, a group of galaxies that also includes M51 (the ‘Whirlpool Galaxy’).