M81 is one of the most prominent grand-design spiral galaxies in our sky.  It, and its close neighbor M82, form a galaxy pair that has long been the delight of amateur astronomers.  M82, also known as the Cigar Galaxy, passed in close proximity to M81. It is unclear how long ago this occurred with estimates ranging frorm several tens of millions to as much as 600 million years ago  This encounter gravitationally distorted both galaxies.  M82, being the smaller of the galaxies, was disrupted to a greater extent than was M81.  The encounter resulted in a flurry of new star formation in M82 that is still continuing at the present time.  M82 is one of the most active galaxies visible to astronomers and is the subject of extensive research.  M81 and M82 lie in the constellation Ursa Major at distance of approximately 12 million light years.

¹http://www.seds.org/messier/m/m081.html