M51 Whirlpool Galaxy and NGC5195; Canes Venatici 
Astro Systeme Austria 12" f/3.5 astrograph 
Total Exposure: 7.5 hours (LRGB  120:90:120:120 minutes;  unbinned) 
March-April 2008;  Inkom, ID 
Comments: M51 was the first spiral structure ever observed by Lord Rosse in 1850 at a time when the true nature of galaxies was unknown. Its undeniable beauty explains its recognition as a true showpiece of the night sky. Probably less appreciated is that M51 represents a dramatic demonstration of an interacting pair of galaxies. As truly immense structures, galaxies are relatively large compared to the average distance between them. Most galaxies are separated by a distance of only 20 times their diameter compared to stars which are separated by 10 million times their diameter. As a result most galaxies are likely to encounter other galaxies at least a few times over their immense lifetimes. The encounters run the gamut from "major" mergers involving the collision of two comparable mass galaxies to more "minor" mergers involving the consumption of a small companion or satellite galaxy by the more dominant galaxy (Gendler). A slightly cropped version is here