The icon shows the 4-star system M73.
Moreover, there frequently occur chance alignments of optical double or multiple stars, the "member" stars of which all lie at different, independent distances. These can be distinguished from physical binaries by observation, as the "component" stars, at their different distances, move independent from each other and show different and mutually uneffected velocities (radial velocities and proper motions).
Although Messier's catalog was intended to contain only nebulous objects which may be taken for comets, and which we today have found to be clusters, nebulae, or galaxies, and not binary or multiple stars which hardly fall in this category, two have found their way into the Messier catalog: M40 and M73. These entries both were more positional notations, in the case of M40 for a mistake of Hevelius who had reported a nonexistent nebula, and in the case of M73 because Messier had the impression that its four stars look nebulous at first glance, and measured its position together with that of M72. These two objects are, at least very probably, optical doubles or multiples, i.e. chance alignments of independent stars at different distances, as explained above.
Physical binaries are found as members of many open and globular clusters, including Messier's clusters.
Galileo didn't believe in physical binaries, but proposed to observe optical doubles in order to find relative parallaxes, i.e. small apparent annual position changes caused by the parallax of the nearer star. It was Reverend John Michell in 1767 who concluded from probability considerations that some double stars should be binaries, and William Herschel who had obtained observational results of orbital motion for a number of physical binaries in 1802.
Last Modification: February 24, 2019