M63 a été découverte par l'astronome français Pierre Méchain en 1779. Charles Messier a confirmé cette observation le même jour et il l'a inscrite à son catalogue comme M63[4]. On lui donne parfois le nom de galaxie du Tournesol. Sa structure en spirale fut l'une des premières spirales reconnues comme telles par Lord Rosse qui la cite comme l'une de 14 nébuleuses spirales découvertes à cette époque (1850)[7].

Cette galaxie est trop rapprochée du Groupe local et on ne peut utiliser son décalage vers le rouge pour déterminer sa distance. Mais, à ce jour, 27 mesures non basées sur le décalage vers le rouge (redshift) donnent une distance de 7,711 ± 1,582 Mpc (∼25,2 millions d'al)[3].

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Messier 63 or M63, also known as NGC 5055 or the seldom-used Sunflower Galaxy, is a spiral galaxy in the northern constellation of Canes Venatici with approximately 400 billion stars.=10.5px M63 was first discovered by the French astronomer Pierre Méchain, then later verified by his colleague Charles Messier on 14 June 1779.The galaxy became listed as object 63 in the Messier Catalogue. In the mid-19th century, Anglo-Irish astronomer Lord Rosse identified spiral structures within the galaxy, making this one of the first galaxies in which such structure was identified.The shape or morphology of this galaxy has a classification of SAbc,=10.5px indicating a spiral form with no central bar feature (SA) and moderate to loosely wound arms (bc). There is a general lack of large-scale continuous spiral structure in visible light, so it is considered a flocculent galaxy. However, when observed in the near infrared, a symmetric, two-arm structure is seen. Each arm wraps 150° around the galaxy and extends out to 13,000 light-years (4,000 parsecs) from the nucleus.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.=10.5px 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☉,=10.5px or around 850 million times the mass of the Sun.

Credit: Wikipedia