Everyone who starts out in astronomy faces a bewildering variety of numbers and letters denoting the great works of creation. Sometimes the nomenclature almost seems designed to confuse. Anyone can look up and recognize a star as Vega — so why does it also need the names BD +38°3238, Alpha Lyrae, 3 Lyrae, HR 7001, GC 25466, HD 172167, SAO 67174, ADS 11510, and dozens of others?
At least beginners aren't alone in their confusion. The First Dictionary of the Nomenclature of Celestial Objects, 1983, described well over 1,000 different naming systems then in use, mostly for faint objects studied by professionals. Its editors despaired of the list ever being made orderly, reasonable, or complete. Celestial nomenclature is too freakish for that, too full of schemes from times long past.
Fortunately, a well-rounded amateur needs to know only a tiny fraction of these naming systems. In this article we'll cover those most often encountered for stars, with their meanings and histories. Another article covers the nomenclature of deep-sky objects.
Where the Heck is Zujj Al Nushshabah?
Since ancient times stars, like people, have had their own proper names, such as Vega or Deneb. But today proper names are widely used only for the brightest few dozen stars — and it's a good thing. Star names are poetic and embody old constellation lore (usually in garbled Arabic), but confusion runs wild. "Deneb" to most people interested in astronomy means the brightest star in Cygnus. But the same name has also been bestowed, at some time, on at least five other stars. It simply means "tail," a body part that a lot of constellations possess.
Moreover, there are simply too many proper names to ever remember. The Bright Star Catalogue, 5th edition, lists more than 800 of them. Every astronomer knows what you mean by Sirius or Polaris, but not one in a hundred could identify Pishpai (Mu Geminorum), Alsciaukat (31 Lyncis), Dhur (Delta Leonis), or Zujj al Nushshabah (Gamma Sagittarii).
More tractable is the Greek-letter system introduced by the German astronomer Johann Bayer in 1603. In his beautiful star atlas, Uranometria, Bayer identified many stars in each constellation with lower-case Greek letters. He often named a constellation's brightest star Alpha, then sorted the rest into brightness classes and assigned letters within each class in order from the head to the feet of the traditional constellation figure.
Bayer's letters caught on immediately. They are used with the Latin genitive of the constellation name, so the leading star in Centaurus is Alpha Centauri ("Alpha of Centaurus"). Back when most educated people knew Latin and Greek this phrasing flowed off the tongue naturally, but today it's many skywatchers' first exposure to the Greek alphabet and Latin declensions. Sooner or later everyone who deals with stars has to sit down and learn the Greek letters (listed below) and the genitives of the 88 constellation names (listed in the back of most astronomy handbooks).
All bright stars were numbered whether they had a Greek letter or not, which is why Alpha Lyrae is also 3 Lyrae. In all, 2,682 stars received Flamsteed numbers. The highest Flamsteed number within any constellation is held by 140 Tauri.
There are occasional confusions. When the constellation borders were formalized in 1930, a number of Flamsteed stars found themselves stranded in exile. Thus the star 30 Monocerotis is today considered to be in Hydra, and 49 Serpentis is in Hercules. Such confusing designations are best swept under the rug, never to be used.
Nobody got around to numbering stars farther south than could be seen from England. So in far-southern constellations one often encounters upper- and lower-case Roman letters, such as g Carinae and L2 Puppis. Roman letters were applied all over the sky by various star mappers from Bayer on, but in the northern sky they have largely passed out of use.
By the 19th century all these naming efforts were falling far short of the mushrooming need. Telescopes were revealing stars by the hundreds of thousands, every one of them an individual crying out for its own identity.
In 1859 the German astronomer F. W. A. Argelander at Bonn Observatory began measuring star positions with a 3-inch refractor to compile a gigantic list, the Bonner Durchmusterung (Bonn Survey). The BD eventually included 324,188 stars as faint as about magnitude 9.5. Argelander and his successors divided the sky into thin, 1°-wide declination bands wrapping around all 24 hours of right ascension. Stars within each band were numbered in order of right ascension; constellations were ignored. Thus Vega's designation BD +38°3238 means it was the 3,238th star (counting from 0 hours right ascension) in the zone between declination +38° and +39°.
The original BD covered just over half the sky, from the north pole to a declination of –2°. A later southward extension, the SBD, marched down to declination –23° to garner another 133,659 stars. The Cordoba Durchmusterung (CD or CoD) completed the job, picking up 613,953 more on its way to the south celestial pole. All in all, Durchmusterung, or "DM," names were bestowed on a grand total of 1,071,800 stars.
The BD, with its detailed star charts and its reliable, well-checked list of positions, remained an essential everyday tool of working astronomers for nearly a century. Durchmusterung designations are still sometimes encountered. The magnitudes of stars in these catalogs, however, are notoriously unreliable by modern standards. Most were merely quick eyeball estimates.
Variable stars have a naming system all their own. This too was instigated by the energetic Argelander. He denoted the first variable star found in a constellation by the capital letter R with the genitive of the constellation name (since the previous letter, Q, was the farthest Bayer had gone in Roman star-lettering). The next variable would be named S, and so on to Z. After Z came RR, RS, and so on to RZ, then SS to SZ, on up to ZZ. If a variable already had a Greek letter, Argelander left it alone.
But new variable stars kept getting discovered! After ZZ, astronomers decided to go to AA, AB, and on to AZ (omitting J since in some languages it could be confused with I), then BB to BZ, on up to QZ.
Even these 334 designations proved insufficient for the variables in some crowded constellations. Rather than start an even more awkward three-letter system, astronomers ruled that further variables in a constellation would simply be designated V335, V336, and so on forever. It was a wise move. By 2003 the highest numbered variable was V5112 Sagittarii.
The next great, widely used star list to appear after the BD was the Henry Draper Catalogue of stellar spectra, which Annie J. Cannon compiled in the 1910s at Harvard College Observatory. It includes 225,300 stars numbered in simple order of right ascension. More were added later in the Henry Draper Extension; these bear HDE numbers. Any star with an HD or HDE designation is guaranteed to have had its spectrum measured.
Meanwhile another catalog had been issued at Harvard: the Revised Harvard Photometry of 1908, which sought to provide accurate magnitudes for the brightest 9,110 stars to about magnitude 6.5. Stars in this catalog bear HR numbers. Even now the HR list remains the basis of the modern Yale Bright Star Catalogue, which remains widely used for its detailed information about naked-eye stars.
Another star-numbering system used today is the SAO designation. This refers to the Smithsonian Astrophysical Observatory Star Catalog (1966), which also was produced (with companion star charts) on Harvard's campus. This catalog gives very accurate positions for 258,997 stars down to about 9th magnitude, though coverage is spotty for the fainter ones. The SAO stars are numbered by right ascension within 10°-wide declination bands. They cover the entire celestial sphere. SAO numbers supplanted the once widely used GC designations, from the General Catalogue of 33,342 Stars by Benjamin Boss (1937).
One of the largest modern star lists is the Hubble Space Telescope Guide Star Catalog. The GSC lists positions generally good to nearly 1 arcsecond and magnitudes accurate to a few tenths for 18,819,291 objects. The GSC's brightest entries are 9th magnitude (brighter stars couldn't be used by Hubble's guiding cameras); the faintest are typically about 13th or 14th magnitude, sometimes 15th. Of this total, 15,169,873 are listed as being stars; most of the remaining 3.6 million objects are small, faint galaxies. Most have never been examined by human eyes; machines measured their properties off of photographic plates. A typical individual in this list is GSC 1234 1132, a 13.3-magnitude luminary in Taurus. The first four digits specify one of 9,537 small regions of the sky; the last four give the object's serial number within this region.
More recently, the Hipparcos and Tycho Catalogues have largely supplanted the GSC for the brightest 1 million stars. TYC and especially HIP stars had their positions, magnitudes, distances, and motions measured to high accuracy by the European Space Agency's epoch-making Hipparcos satellite in the 1990s.
In the last decade much vaster star catalogs have been coming online — such as the Sloan Digital Sky Survey (SDSS), the infrared Two-Micron All Sky Survey (2MASS), and the U.S. Naval Observatory's gigantic and standard-setting UCAC. And of course, no end is in sight.
P.S.: Name-It-Yourself Stars Are a Hoax!
People sometimes call Sky & Telescope to ask about one of the several competing companies advertising that they will name a star after you or a loved one for about $50. You get a pretty certificate and some papers. Is this for real, we are asked?
No. The certificate is a "novelty item" only. With just as much validity, you can step outside on a clear night, choose any star you like, and name it for anyone you want. For free.
We know lots of amateur astronomers who have done this for their spouses or children. To one of Sky & Telescope's editors, Iota Ursae Majoris is "Lucy's Star," and Zeta Hydrae is "Andrew's Star." Why not?
Why pay some commercial outfit to mediate your personal life? Even a fancy certificate, if it appeals to you, can be printed with shareware for a lot less than $50. One of the companies
advertises that it keeps the names in a Swiss bank vault, as if that means something. If that appeals to you, you can put a piece of paper with a star name in your own bank's safe-deposit box. But why bother?
Sometimes planetariums "sell" stars on their domes to help raise needed funds. They are careful to tell donors that the certificate they get denotes a contribution to a worthy institution, not the purchase of a real star name. If you insist on paying someone else to pretend to name a star, this is a more worthwhile way to do it.