NAMES OF THE STARS
|By Alan MacRobert|
|Adapted from Sky & Telescope|
IF A ROSE by any other name would smell as sweet, would BD +38°3238 by any other name sing more sweetly to lovers down from the summer sky? 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, describes well over 1,000 different naming systems currently in use, mostly for faint objects studied by professionals. Its editors despair 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 that just grew.
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 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. Which one do you really mean?
Moreover, the list quickly becomes too big. The Yale Bright Star Catalogue, 4th edition, 1982, gives some 845 star names, more than most people would ever want to memorize. 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 published that year, 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. This simply means "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 and the genitives of the 88 constellation names (listed in the back of most astronomy handbooks).
There are swarms of stars per constellation but only 24 Greek letters. Sometimes one letter is used repeatedly with superscripts to cover several adjacent stars, such as Pi¹ through Pi6 Orionis, the ragged row forming Orion's Shield. But as more and more stars needed names because of better sky surveys, astronomers adopted numbers.
Around 1712 England's Astronomer Royal, John Flamsteed, began numbering stars in each constellation from west to east in order of right ascension -- a big help when looking for a star on a map. For instance, 80 Virginis is east of 79 Virginis and west of 81 Virginis (at least in the coordinate system Flamsteed used, equinox 1725, which still matches today's celestial east and west pretty well).
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 number within any constellation is 140 Tauri.
All nice and logical -- but when it comes to celestial nomenclature, there's a fly in every ointment. When the constellation borders were formalized in 1930, many Flamsteed stars found themselves stranded in exile. Thus 30 Monocerotis is today considered to be in Hydra, and 49 Serpentis is in Hercules. Such names are best discreetly 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. These 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 to about magnitude 9.5. Argelander and his successors divided the sky into thin bands 1° of declination wide wrapping around 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 0h right ascension in the zone between declination +38° and +39°.
The original BD covered just over half the sky, from the north pole to declination -2°. A later southward extension, the SBD, continued the system down to declination -23° to garner another 133,659 stars. The Cordoba Durchmusterung (CD or CoD) completed the job to the south celestial pole with 613,953 more, so that visual durchmusterung, or "DM," names were bestowed on a grand total of 1,071,800 stars.
The BD, with its detailed star charts to 9th or 10th magnitude and its reliable, well-checked list of positions, remained an essential everyday tool of working astronomers for nearly a century. Durchmusterung designations are still often encountered. The star magnitudes 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 highest 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 1990 the highest numbered variable was V4153 Sagittarii.
The next great, widely used star list to appear after the BD was the Henry Draper Catalogue of stellar spectra, compiled by Annie J. Cannon at Harvard from 1911 to 1915 and published from 1918 to 1924. 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, widely used for its detailed information about naked-eye stars.
Perhaps the most common star-numbering system today is the SAO designation. It 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 to about 9th magnitude, though coverage is spotty for the fainter ones. The SAO stars are numbered by right ascension within 10° bands of declination from the north to the south pole.
SAO numbers have almost completely supplanted the once widely used GC designations, taken from the General Catalogue of 33,342 Stars by Benjamin Boss (1937).
The latest and greatest star list is the Hubble Space Telescope Guide Star Catalog. It is too big ever to print; instead it's distributed on two CD-ROMs. The GSC lists positions generally good to nearly 1 arcsecond and magnitudes accurate to a few tenths for 18,819,291 objects from 9th to usually about 13th or 14th magnitude, sometimes as faint as 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; they were measured automatically from 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.
Many more lists have been compiled for special purposes. A star with an ADS number is in the Aitken double star catalog (1932); IDS refers to the Index Catalogue of Visual Double Stars (Lick Observatory, 1963). These comprehensive lists are more rational than the 150-odd types of older double-star designations you are likely to encounter, generally named for astronomers who published lists, however short, of their own discoveries. Nevertheless the older names are so much a part of double-star usage that there will be no getting rid of them.
Lunar occultation observers often refer to stars by ZC number, referring to the Zodiacal Catalogue of stars that the Moon can cover. And so on, and so on.
P.S.: Name-It-Yourself Stars Are a Hoax!
Often Sky & Telescope gets inquiries about companies that advertise on the radio or on the Web that they will name a star for 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 a number of amateur astronomers who have done this for their spouses or children. To one of the Sky & Telescope 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 made with shareware for a lot less than $50. One star-naming company 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.
Alan MacRobert is an associate editor of Sky & Telescope magazine and an avid backyard astronomer.
Sky Publishing Corporation
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