Gaia Shows That Messier 40 is Definitely Not a Binary Star

Michael R. Merrifield, Meghan E. Gray, Brady Haran

Published 2016-12-02Version 1

M40 has always been something of an oddity in the Messier Catalogue, since it is just a pair of stars rather than an extended object. Doubts have also been expressed as to whether it is even a physical binary. Here, we note that the Hipparcos/Gaia parallaxes for these two stars remove all doubt: Messier 40 comprises a pair of entirely unrelated stars.