Binary and Multiple Stars
Click here to view a binary star from Messier's catalog
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 stars discovered and separated with a telescope were found
in 1617: Benedetto Castelli (1578-1643) found Mizar in and descried a double
star in Monoceros observed in January, 1617, while
Galileo observed Theta1 Orionis, the
bright stellar object in the Orion Nebula M42,
as a triple star, in February, 1617.
Castelli also found Beta Scorpii double in 1827.
These early discoveries, however, were not immediately recognized, so that
Mizar was independently found double by Riccioli in 1651,
and Theta Orionis multiple by Huygens in 1656.
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
View "other objects" in Messier's Catalog
(those which are not galaxies, nebulae, or clusters)
Last Modification: April 7, 2014