Washington Double Star Catalog
FAQ : Frequently Asked Questions


Glossary of terms and acronyms:

Frequently Asked Questions

  1. The Double Star Library notes that some of the USNO catalogs are "updated nightly". What does that mean?

    At present there are two astronomers at the USNO who make changes to the WDS and associated catalogs. At approximately 2am local time the WDS is re-compiled from the existing data files and put on line for users to access.

    Changes are made in a sporadic fashion. Some days, such as over the weekend, there may be no changes. Other days the changes may be significant.

  2. 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. The Observing List Request form is designed for people to make requests for observing lists. Typical questions to make the list are provided, but the field is free form, and you can specify exactly what you want or ask the sorts of questions that can guide us in helping you generate an observing list.

  3. 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.

  4. 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.

  5. 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 we provide a set of calibration-quality orbits. Many of the presumably differential proper motion linear solution pairs should also be good for this.

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

    The easiest way to get data in the WDS is to have them published in a refereed journal. These will then be added as time permits.

    A faster way to get them into the WDS is to also send us a flat ascii file which includes the tabular information from the publication.

    The fastest way is to get in touch with us via the comment form and let us provide you with the "ready to fold into the catalog" format.

  7. 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.

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

    Yes. Fill out the form and one will be mailed to you.

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

    No, not all of them. The interferometric catalog contains a subset of WDS data, but also contains one-dimensional results not found in the WDS (such as lunar occultation measurements), as well as single star information from large surveys for duplicity.

  10. 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".

  11. 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.

  12. 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.

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

    The Data Request form will return to you all data, notes, and references we have for double stars. If it has an orbit we will also provide elements, ephemerides and an orbit plot. These typically are returned within 24 hours.

  14. 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"

  15. 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. The current plan of the more comprehensive WMC is to not use the discovery designation if alternate designators of greater usage are available.

    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, so can make it a little more interesting.

  16. 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 or 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 his equipment.

    If, say, the A component of a pair is later resolved into two star 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. See the Washington Multiplicity Catalog.

  17. 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 magntiude).

    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.

  18. Do you have any information on spectroscopic binaries?

    While the WMC will contain information about spectroscopic binaries when it is fully populated, none of the all-sky USNO double star catalogs contains a comprehensive list of spectroscopic binaries. The notes file to the WDS contains some information, as does the orbit catalog, but neither is at all comprehensive. The best source for spectroscopic binaries is the "9th Catalogue of Spectroscopic Binary Orbits".

  19. 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?

    See question 6 above. At the very least provide the reference in the comment form. Most of the papers added to our catalogs (8,000+ references to date!) are found through literature searches, and we may well have just missed it. Speed of addition is based on how much information is provided.

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

    Just tell us! The online comment form is designed for people to let us know of errors or ask us questions. Like the observing list form, it is free form so prattle on to your heart's content.

  21. Most the 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."

  22. 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.

  23. 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.

  24. 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 expertise of the observer who made the measure.

  25. What are the weights of individual measurers used for orbit grading?

    Not all observers are the same. Some get a low weight because they were always working at the limits of their telescope. Some get a low weight because their observations or calibration were not as good.

    Evaluating an observer is a sociologically complex consideration. We do not release these parameters. But, for the record, ours are not the best.

  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 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.

  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.