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| | Binary and Multiple Stars |
 | | 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. |
| | The component stars in multiple systems orbit each other, and move around their center of mass, because of their mutual gravitational interaction, an effect which can be noted by observation of changes of their relative positions and radial velocities, and are all at about the same distance from us. |
| www.seds.org /messier/bina.html (437 words) |
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