Washington Double Star Catalog
FAQ : Frequently Asked Questions


Glossary of terms and acronyms:

Frequently Asked Questions

(most frequent questions posted at top)

  1. What is the difference between complex multiple systems and sparse clusters?

    The difference between the two can be fuzzy and if you are aware of references discussing this topic directly we would appreciate being pointed to them. Lacking other criteria we consider what has already been considered to set the limits. The largest number of components (N = 7) where physicality seems reliable for all is WDS11551+4629. Those with components more than this are considered as clusters or moving groups and not multiple stars when the mean separation is more than a certain value. The most widely separated pair where the companion is physical is Alpha Cen AC (=14396-6050LDS 494) with a separation of around 15,000 au. Therefore, systems with N > 7 and mean separation more than 20,000 au are considered clusters/groups/associations and not multiple systems. Having a distance to the system through trigonometric, orbital or spectroscopic parallax is a requirement. Historic systems already in the WDS or any of its predecessors determined to be too wide or multiples which are too dense are not removed.

  2. The WDS website notes that some of the USNO catalogs are "updated nightly". What does that mean?

    The WDS, WDSS, and ORB6 are routinely updated by astronomers at the USNO. While the internal version can change multiple times a day, currently, the latest versions of the catalogs are uploaded to the website approximately once a month. Other updates, such as to LIN2 or orbital ephemerides, occur approximately annually.

  3. I am interested in making some double star observations but don't know what doubles are appropriate for my telescope or need observing. Can you help me?

    Yes, custom observing lists can be generated to your specifications by following the directions linked under 'Requests and Acknowledgments'. Typical questions to make the list are provided, but you can specify exactly what you want or ask the sorts of questions that can guide us in helping you generate an observing list.

  4. Are there actually doubles that still need to be observed?

    Quite a few. Naturally, the ones that most need observation are those which are hardest to observe, and those that are easiest to observe do not typically need more. The number of pairs needing observation that are accessible to you depends on your capabilities.

  5. What sort of parameters are needed for a double star observation?

    Typically date, position angle and separation. If the magnitude difference is estimated, providing that is helpful, too. For publication the aperture of the telescope, method of data collection, etc., would be needed.

  6. What about calibration?

    Ah, yes. An uncalibrated measure is worthless. Independent methods for determining your calibration parameters, such as looking at a single star with a slit-mask and performing Young's experiment, is preferred. However, should you be unable to do this orbits with grades of "1" or "2" can be used from ORB6. Many of the presumably differential proper motion linear solution pairs should also be good for this.

  7. I have made some double star observations. How do I get them into the WDS?

    Data are added to the WDS from refereed journals, both professional and amateur, as time permits.

    To ensure the data get into the WDS, feel free to send us a flat ascii file which includes the tabular information from the publication.

    The fastest way to get the data added is to send us a request for the "ready to fold into the catalog" format and submit the data in that format.

  8. Who created and maintains all the USNO double star catalogs?

    The oldest USNO double star catalog, the WDS, was created when Charles Worley brought the IDS from Lick Observatory in the early 1960s. It is a direct descendant of the first comprehensive double star catalog, the BDS (BDS --> ADS, ADS plus SDS --> IDS, IDS --> WDS), giving the WDS a lineage tracing back over 100 years). For over thirty years it was maintained by Charles with help mainly from Geoff Douglass. Over this time, he painstakingly combed through the enormous resources of the USNO Library, adding measures by hand. In addition to periodic releases of intermediate versions, Charles made two major releases in 1984 and 1996. As he compiled the WDS he also collected accurate magnitude difference measures in his own internal "Delta-M Catalog." Shortly after his arrival here, Charles began collaborating with W.S. Finsen in producing the "Third Catalog of Binary Orbits." He later made the "Fourth Catalog" with W.D. Heintz. The Interferometric Catalog was first compiled at Georgia State University in the early 1980s and maintained there for over 15 years. When two of the authors came to the USNO the Catalog came with them. In 2001 new versions of all four catalogs were released on the first USNO double star CD. In 2006.5 the second double star CD was released with these four plus the new linear elements catalog for likely optical pairs. This catalog also included a history of USNO double star work. Currently the USNO catalogs are maintained by two astronomers at the US Naval Observatory. In addition to cataloging double stars we have observing and other responsibilities as assigned --- the actual work spent cataloging is probably from one to 1.5 FTE on average, depending on circumstances.

  9. Is it possible to get a copy of the most recent double star CD?

    The USNO no longer provides double star CDs as the latest version is over a decade out of date and all the double star catalogs are accessible from our website.

  10. Why is there also an interferometric catalog? Aren't all these data included in the WDS?

    Originally the interferometric catalog contained a subset of WDS data as well as one-dimensional results not found in the WDS (such as lunar occultation measurements) and single star information from large surveys for duplicity. Since it is no longer maintained, all observations have been copied to the WDS, with the exception of the single stars/never-resolved pairs.

  11. Why is there a separate catalog of magnitude differences? Aren't all those data in the WDS and/or INT4?

    The vast majority of published photometric measurements are included in the WDS, and those obtained by interferometric means are also in INT4. However, there are some published measures which don't fit in either of these catalogs, such as photometry measures published without dates, pairs with no astrometry, etc. The Magnitude Difference Catalog is the repository for these "odds and ends".

  12. Why do astronomers care about double stars?

    The majority of stars in the sky are members 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 well known, 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 an isolated set 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 predominant stellar evolutionary track, but they are a boon to astronomers for the plethora of data that can be determined from them.

  13. Why does the U.S. Navy care about double stars?

    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 are also a concern even if they are not physically related. Navigation requires precise star positions, but it is much more difficult to determine the "center of light" location of an elongated double star image than it is an isolated single star. Determination of this center of light may depend on many factors: type of detector, colors of the stars, angular distance between them, and any motion one might have with respect to the other. For this reason, double and multiple stars have earned the navigational nom de guerre: Vermin of the Sky.

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

  14. I am interested in a particular double star, but the WDS only lists the first and last observations. How do I obtain all the data?

    Contact us via the Data Request form and we will provide all data, notes, and references for the double star. If it has an orbit we will also provide elements, ephemerides and an orbit plot. These typically are returned within 24 hours.

  15. The date for the "first" measurement of a pair doesn't always seem to correspond to the discovery date - why? There are pairs in the WDS with theta and/or rho information listed, yet the number of measurements is given as zero - why? There are pairs with negative values for the first or last date or theta or rho - why?

    Typically the first and last measures refer only to "full" measures (measures which include date + theta + rho) and the number of measures (nmeas) counts only those full measures. If the discovery observation included only a "partial" measure (a measure lacking one or more of those three quantities), that measure would be skipped over, in favor of the first full measure. A small number of pairs have only partial measures, however. If nmeas = 0, the first and last partial measures are given instead; any missing value is given as "-1"

  16. What are "discoverer designations"?

    Historically, each discoverer of a pair would provide a list of his "new" discoveries in his publications. When a pair was resolved and published for the first time it was added to the catalog with that designation. In more recent years pairs discovered, but not resolved, for the first time (by, for example, spectroscopy) were credited to someone sometimes many years later.

    Generally speaking, the discovery designation can also tell you something about the difficulty of seeing the pair. For example, STF pairs (first seen by F.G.W. Struve) are easier to split than BU pairs (first seen by S.W. Burham).

    Also, the "discovery designation" helps personalize the star and perhaps provides a little historical context to its discovery.

  17. Some double-star names include components such as AB or AC, while others have Aa,Ab or Ba,Bb and still others have no components listed at all. Why?

    When a component designation is given the relative position is of the secondary relative to the primary. For example, for an AB pair at 180 degrees and 3", in a polar coordinate system the A component is at the origin and the B component is at a position angle of 180 degrees (due south) at a separation of 3". Pairs such as AC or BC are measured in a similar manner.

    By default a simple binary is listed without components; the primary is understood to be A and the secondary to be B.

    Another common arrangement, such as AB,C indicates that C is measured relative to the center of light (or photocenter) of the AB system. This usually occurs when the AB pair is too close for one observer to resolve with their equipment.

    If, say, the A component of a pair is later resolved into two stars by a larger telescope and/or more sensitive technique (such as speckle interferometry), the two components are designated Aa and Ab. In a few rare cases, an Aa or Ab component also been resolved (by a technique such as long-baseline interferometry). The two stars comprising Aa are then designated Aa1 and Aa2.

    More complex hierarchical arrangements follow a strict set of rules and details are available.

  18. What defines the primary star of a pair?

    It depends on how much information we have. If we have a full characterization of the system, it is the most massive component.

    If not, it is the brightest component (considering bolometric magnitude).

    If we don't know the magnitudes in many bands it is the brightest component as assigned by observers (most commonly in the visual band).

    If the magnitude difference is zero or unknown the primary is arbitrarily assigned such that the angle of position is less than 180 degrees.

    In some cases other techniques, such as spectroscopy, can assist in assigning the primary.

  19. Do you have any information on spectroscopic binaries?

    Currently, none of the USNO double star catalogs contain a comprehensive list of spectroscopic binaries. The notes file to the WDS contains some information, but again is not comprehensive. The best source for spectroscopic binaries is the "9th Catalogue of Spectroscopic Binary Orbits".

  20. I know of a paper of double star measurements and orbits, but it is not in your catalogs or even in your list of references. How can I get it included?

    Most of the papers added to our catalogs are found through literature searches, and we may well have just missed it. Contact us with, at minimum, the reference to the paper or a flat ascii file with the data if you have it. Speed of addition is based on how much information is provided.

  21. I found an error in the WDS. How can I get it corrected?

    Just tell us! Directions to contact us with questions or to report errors are available in the 'Requests and Acknowledgements' section.

  22. Most of the doubles in the WDS have good right ascensions and declinations, but a few are only listed with very imprecise coordinates. Why?

    Historically, double star positions were given only to the nearest minute of arc and double star catalogs did not include proper motion information. The result of this is that if a pair was not followed on a regular basis it could become "lost". It could also have been a chance alignment that is no longer there, or there might be a transcription error or some other sort of mistake during publication. In any event, these are pairs we have thus far been unable to match to known objects. These "lost" pairs form a substantial subset of the "Neglected Doubles" lists we have published.

    Sometimes it takes only a look through the telescope to see where it is and "rediscover" the pair. Using the 26-inch telescope in Washington, we have confirmed a considerable number of John Herschel pairs that were last seen in 1820 but subsequently lost due to poor coordinates.

    When a pair is judged to be false for one or more of the above reasons, we add an "X" code for the pair in the WDS. Removing the pair entirely might at first glance seem preferable. However, a future cataloger might come across the original publication one day and mistakenly put the bogus pair back in the catalog; adding the "X" code is like giving it the "Black Spot of Binaries."

  23. If there are bad doubles are there also bad measures?

    Yes, there are. When we have thoroughly analyzed a measure and found it to be erroneous or wildly aberrant we similarly flag it. It remains in the WDS but is henceforth not considered when, for example, counting measures of systems. It is a marker that lets someone else know that the measure has been added, evaluated, and found wanting.

  24. What is the orbit "grade" which is assigned in the orbit catalog?

    Both the Third and Fourth orbit catalogs assigned a quality grade to an orbit. This subjective grading was based on many factors (phase coverage, number of measures, orbit residuals, etc.) as judged by probably the most experienced double star astronomers and catalogers at the time. Since their expertise and experience could not be replicated, a painstaking method was developed for the Fifth Catalog to replicate their grading based on many key parameters: thus objectifying the previous subjective grade. See the Orbit Catalog for details.

  25. How is this grade assigned?

    When an orbit is added, all data are plotted with this new orbit and then evaluated. If the grade of a new solution is deemed significantly better than that of a previously published orbit it becomes the new default orbit for that binary. Simply adding one measure and re-computing an orbit is rarely justification for your orbit being the new "best" orbit, however!

    In evaluating an orbit the "weight" of each measure in that orbit is considered; these weights takes into account the observing method, size of the telescope, separation, magnitude, magnitude difference of the binary, N (number of measures averaged into a mean position), and the technique used to make the measure.

  26. Who has observed the most double stars?

    It depends on how you count it. Counted by the number of individual measures, the three top individuals are W.H. van den Bos, W.D. Heintz, and C.E. Worley. A full listing of the top twenty-five groups and individuals counted many different ways is available on our website.

  27. Why aren't all the measures used to compile the WDS published online?

    Size is one consideration.

    Integrity of the product as it is not completely corrected is another concern.

    Finally, an earlier version of the full database was once taken without permission, then repackaged and presented as a new catalog with no attribution. Given the enormous number of man years spent in the USNO Library and at the computer in maintaining this database, this will not be allowed to happen again.

    All measures of specific systems are always available via the Data Request form.

    Bottom line: You can have some of the data on all of the systems or all of the data on some of the systems, but you cannot have all of the data on all of the systems.