About the Subject

M63 was first discovered in 1779 by astronomer Pierre Méchain and was cataloged by Charles Messier as the sixty-third entry in his deep sky catalog of “things that aren't comets.” It was one of the first galaxies in the sky whose spiral structure was observed by Lord Rosse using his six foot diameter telescope. It is considered a flocculent galaxy because of the discontinuous nature of its spiral arms. It is located in the constellation Canes Venatici at a distance of roughly thirty million light years. It is part of the M51 group of galaxies. 

At an estimated diameter of 98,000 light years, M63 is roughly the same size as the Milky Way galaxy and has a mass of roughly 140 billion Suns, quite a bit less than the Milky Way. Looking at this image, you will notice a faint halo surrounding the disk of M63. You will also see a faint outer "loop" of light above and to the left of the galaxy. Both the halo and the loop are thought to have been caused by the recent assimilation of dwarf galaxies into M63. As the dwarf galaxies pass near and through M63, they are torn apart by tidal forces, converting them from distinct clumps of stars to "streams" spread out across tens of thousands of light years. The loop is thought to have been formed when M63 absorbed a dwarf galaxy some two and a half billion years ago.

At magnitude 9, M63 is a relatively easy target for most amateur astrophotographers. The halo and tidal streams, however, have extremely low surface brightness, and so are harder to capture. The loop above and to the left of M63 has a surface brightness around 28.5 magnitudes per square arc second. For comparison, a good, rural dark sky has a surface brightness of around 21.7 magnitudes per square arc second, so the stream is about 400 times fainter than the blackness of dark sky. As a result of this very low surface brightness, the tidal stream around M63 was first detected in 1979, and not confirmed until 2011 by a professional/amateur collaboration.