Keenan's System NGC 5216 and 5218

A fascinating pair of interacting galaxies in the constellation Ursa Major, Keenan's System lies about 17.3 million light-years from Earth. NGC 5216 (right) and NGC 5218 (left) have a debris trail between them that is 22,000 light years in length. P. C. Keenan noted this double galaxy in 1935, but his notes seem to have been overlooked, and the pair  later ‘rediscovered’ by observers at the Lick and Palomar observatories. Both galaxies have a ‘countertide’ - a fingerlike extension pointing in a different direction to the debris trail, a typical structure of interacting double galaxy systems. Much thanks to my good friend Martin Winder for allowing me to process his wonderful data acquired from Linz, Austria. Our photograph was selected as NASA's APOD (Astronomy Picture of the Day) for July 31, 2008!

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