Binary and Multiple Stars

[M Binary] Click here to view a binary star from Messier's catalog

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The icon shows the 4-star system M73.

Binary and multiple stars are common in the universe. Stellar formation results in multiple systems at least as often as in single stars like our Sun, as observations suggest. 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.

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.

Historically, double stars were first noted by Ptolemy who described Eta Sagittarii as such an object. Also, some other double stars had been known since ancient times, e.g. the pair Mizar-Alcor. The first double star discovered and separated with a telescope was Mizar (by Riccioli, 1651), followed by the multiple Theta Orionis (Huygens, 1656) which is the Trapezium Cluster in the Orion Nebula. The first catalog of double stars, a list of 80, was compiled by Christian Meyer of Mannheim in 1778, followed by William Herschel's catalogs of 1782 with 269 pairs and 1785 of about 700 pairs.

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.


  • More on double stars - including Christian Mayer's catalog
  • Binary and Multiple Star Catalogs List
  • View "other objects" in Messier's Catalog (those which are not galaxies, nebulae, or clusters)
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    Last Modification: January 1, 2002