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The Visual Double Star Program at the USNO

Since the original 1846 charge to the first Superintendent, Matthew Fontaine Maury, the U.S. Naval Observatory has been engaged in the observation of double stars. The reasons for their importance fall under two broad categories:


The majority of stars in the sky are part of double or multiple star systems. The only way to determine stellar mass, the most fundamental property of a star, is through analysis of binary star systems. While stars similar to the Sun are known well, the most common stars, Red Dwarfs, and those that have the greatest impact on Galactic Evolution, the Massive OB stars, are not well determined.

While double or multiple stars are broadly characterized as more abundant than single stars, how different subsets, either based on stellar type or environment, may be enhanced or not can have significant implications for the evolution of the Galaxy. Unknown binaries could be responsible for a significant amount of the "missing matter'' of the Universe.

The coeval nature of binary stars makes them insolated sets which can be studied together. While the individual stars may be different, they are of at least approximately the same age and have the same chemical composition.

Binary stars are not only the predominent stellar evolutionary track, but they are a boon to astronomers for the plethora of data that can be determined from them.


Astrophysical questions relate only to pairs which are physically associated with each other: the true binary stars. However, for navigational purposes two stars which appear to be near each other in the sky but which not physically related are also a concern.

Determining the effective "center of light" of a close pair of stars may depend on many factors: type of detector used, colors of the stars, angular distance between them and any motion one star might have with respect to the other. For this reason, double and multiple stars have the navigational nom de guerre: Vermin of the Sky.

Simply avoiding double stars is not an option, as they are the predominent type of celestial object and new pairs are discovered every year. Furthermore, the brightest stars, which would presumably be best for navigation, are preferentially members of double or multiple star systems.

USNO Double Star Products

The Washington Double Star Catalog (WDS): Since the mid 1960s, when the Index Catalogue of Double Stars was transferred from Lick Observatory to the US Naval Observatory, the resultant catalog, redesignated the ``Washington Double Star'' (WDS) Catalog, has been the official double star catalog of Commission 26 (Double and Multiple Stars) of the International Astronomical Union (IAU). Maintained online in summary form, this catalog lists the first and last observation parameters for every known pair in the sky. On demand we provide every cataloged measure: 752,772 measures of the 103,824 measured pairs (as of 17 November 2008), varying from 1 to 1679 measures per system (mean = 7.25, median = 3). Using the resources of the USNO Library, extensive catalog work has been done to include all available measures from 1690 to the present day. Notes to individual systems are also provided. We also, on demand, can prepare custom observing lists suited to the observing capabilities and desires of observers worldwide.

The Sixth Catalog of Orbits of Visual Binary Stars (ORB6) lists what has been characterized as the best available orbit of a pair. Published orbits exist for 1955 of the WDS pairs and determining which one of possibly several published solutions is best has been done by an objective numerical grading scheme (described in the catalog text). ORB6 also includes ephemerides giving predicted positions for the next several years, orbital plots of all WDS data on the calculated orbit, as well as a separate list of "calibration-quality" orbits which are adequate for most scale and orientation determinations.

The Catalog of Rectilinear Elements (LIN1) is the most recent addition to the suite of double star catalogs maintained at the USNO. Using some of the same methods used to generate ORB6, this catalog lists rectilinear elements for 1176 pairs whose motion is fit adequately by a straight line. While it is possible that some of these may be physical doubles with a very long period and perhaps high eccentricity, most are likely optical pairs whose differential proper motion can be linearly characterized. As double star observations have been made for a longer time and often with greater frequency than classic astrometry techniques, many of these pairs have differential proper motions determined to greater precision and accuracy than can be done by other, more conventional, methods.

The Fourth Catalog of Interferometric Measurements of Binary Stars (INT4) enumerates all measurements of double stars made by high resolution techniques. These include interferometry as well as adaptive optics, satellite observation, lunar occultation as well as other methods enumerated in the catalog notes. Complete photometric information, when available, is also provided, as are interferometric "single star'' detections. These stars have been examined for close pairs and no similar brightness pair has been found to the listed limit. These single stars are then appropriate for scale determination (with other equipment) or point source characterization.

The Third Photometric Magnitude Difference Catalog (DM3) lists all measures of magnitude difference made by precise techniques. The magnitude difference and the color in which it is determined, can aid in the determination of a companion's spectral type. If one or both of the components is variable, this can aid in the characterization of the variability.

In addition to this suite of double star catalogs, work is in progress on the Washington Multiplicity Catalog (WMC). Approved at a Multi-Commission Meeting at IAU General Assembly XXIV (Manchester, 2000) and re-affirmed at a Special Session at IAU General Assembly XXV (Sydney, 2003), the WMC is a designation scheme which will hierarchically arrange all double stars detected by multiple techniques: astrometry, spectroscopy, photometry and other methods.

While not a double star catalog, the USNO also maintains the Double Star Library (DSL), an eclectic collection of links, announcements, and other data of relevance to double star astronomers. Serving as the official webpage of Commission 26, the DSL provides Commission history as well as current and archival issues of the Commission 26 Information Circular, its official tri-annual publication.





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