On a particularly dark night, out of reach of city lights somewhere a good pair of binoculars are all you need to see the distant light of the Whirlpool galaxy, some 23 million light years away from us. Just find the easternmost star of the Big Dipper and look to the southeast: on most sky maps (or cellphone apps) it is designated as M51.

On this day, October 13, in 1773 the French astronomer Charles Messier, tracking a comet in the night sky, came upon what he described as a “very faint nebula, without stars.”

The Whirpool Galaxy evidently has a companion, a smaller galaxy that was found by Messier’s friend Pierre Méchain four years later, and added to Messier’s map. Somewhere around 500 to 600 million years ago, according to current theory, the companion galaxy passed through the main disk of M51 and caused it to acquire its spiral shape.