Washington Double Star Catalog
FAQ : Frequently Asked Questions
Glossary of terms and acronyms:
- ADS = the "New General Catalogue of Double Stars within
121 degrees of the North Pole" (a.k.a., "Aitken Double Star
Catalog"; Aitken, 1932). When Burnham retired, he gave all his
double star catalog information to W. Hussey of Lick
Observatory, with the intent that he publish an updated version
of the BDS. Hussey died before this could be accomplished,
however, and the task was assumed by Robert Grant Aitken, also
of Lick. Lick later obtained the files of the SDS and merged all
these data to create the IDS.
- BDS = the "General Catalogue of Double Stars within 121
degrees of the North Pole" (a.k.a., "Burnham Double Star
Catalog"; Burnham 1906). Based on S.W. Burnham's observing notes
collected over three decades, this was the first attempt to
collect all published double star measurements.
- DM3 = the "USNO Third Photometric Magnitude Difference
Catalog". All relevant measures folded into WDS and INT4.
Earlier published version were DM2 (2006.5) and DM (2001.0).
- DSL = the "Double Star Library". This is the official
webpage of IAU Commission 26 (Double and Multiple Stars). No
longer maintained. Transitioned to the webpage for IAU
Commission G1 (Binary and Multiple Stars). This is maintained
here as well.
- IAU = the International Astronomical Union. This is a
professional organization for astronomers around the world. Most
of our work is centered around IAU Commission 26: Double and
Multiple Stars and now G1: Binary and Multiple Stars.
- IDS = Lick Observatory's "Index Catalogue of Visual
Double Stars, 1961.0" (Jeffers & van den Bos, 1963). This
catalog by Hamilton Jeffers and Willem van den Bos combined data
from the ADS and SDS into the first all-sky compilation of
double star data. Due to its size, individual measures were
maintained on computer punch cards, and only the first and last
observations were published. These boxes of punch cards were
brought to the USNO by Charles Worley soon afterward to form the
basis of the WDS.
- INT4 = the "Fourth Catalog of Interferometric
Measurements of Binary Stars". Currently maintained by the USNO,
although future versions will only include non-binary star
detection limits. An earlier version published at the USNO was
INT3 (2001.0). INT2 and INT1 were published by Georgia State
University's Center for High Angular Resolution Astronomy
(CHARA) in 1988 and 1984, respectively.
- LIN1 = the "Catalog of Rectilinear Elements". Currently
maintained by the USNO.
- ORB6 = the "Sixth Catalog of Orbits of Visual Binary
Stars". ORB5 was published in 2001. Earlier versions, ORB4
(Worley & Heintz 1983), ORB3 ( Finsen & Worley 1970), ORB2
(Finsen 1938), and ORB1 (Finsen 1934) were printed publications.
- SDS = the "Southern Double Star Catalogue, -19 to -90
degrees" (Innes, 1927). This catalog by R.T.E. Innes was the
southern equivalent to the BDS and ADS, and was later
incorporated into the IDS.
- WDS = the "Washington Double Star Catalog". Currently
maintained by the USNO. Major releases were made in 1984, 1996,
2001, and 2006.5, with additional incremental releases over the
- WDSS = the "Washington Double Star Supplemental" Catalog
is a database intended for large, faint duplicity surveys. It is
expected to grow significantly as future data releases from
Gaia, PanStarrs, etc. lead to ever-larger-scale duplicity
Frequently Asked Questions
(most frequent questions posted at top)
- What is the difference between complex multiple systems and
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
- 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
- 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.
- Are there actually doubles that still need to be
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
- What sort of parameters are needed for a double star
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.
- 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
- 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
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"
- Who created and maintains all the USNO double star
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.
- Is it possible to get a copy of the most recent double star
Yes. Fill out the form and one will be mailed to you.
- 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.
- 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".
- 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
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.
- 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.
- 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 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.
- 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
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"
- 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.
- 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
- 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
If not, it is the brightest component (considering bolometric
If we don't know the magnitudes in many bands it is the brightest
component as assigned by observers (most commonly in the visual
If the magnitude difference is zero or unknown the primary is
arbitrarily assigned such that the angle of position is less than
In some cases other techniques, such as spectroscopy, can assist in
assigning the primary.
- Do you have any information on spectroscopic
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
- 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.
- I found an error in the WDS. How can I get it
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.
- Most the the doubles in the WDS have good right ascensions
and declinations, but a few are only listed with very imprecise
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
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
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."
- 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
- What is the orbit "grade" which is assigned in the orbit
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
- 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.
- What are the weights of individual measurers used for orbit
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
Evaluating an observer is a sociologically complex consideration. We
do not release these parameters. But, for the record, ours are not
- 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.
- Why aren't all the measures used to compile the WDS published
Size is one consideration.
Integrity of the product as it is not completely corrected is
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
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.