Name That Star!

How many stars can you call by name? Unless you are a devotee of the night sky, the number will likely not require more than one hand to count. Betelgeuse perhaps, simply because its pronunciation brings to mind a movie starring Michael Keaton. Polaris if you remember that it is the North Star. Alpha Centauri if you know that it is our nearest stellar neighbor beyond the sun. Any more?

orion labeled

Have you wondered about how these stars are named? You might guess that some names are ancient, bestowed upon the brightest stars visible throughout human history. But what about something like SAO 101729, or HD 2341? Did you know that Betelgeuse is also known as Alpha Orionis, HD 39801, SAO 113271 AND GSC 129:1873? Let’s back up a little and see if we can make some sense out of this scrambled mess.

The brightest stars were known and named by many ancient civilizations, but the names most familiar to us today are largely from medieval Islamic astronomers and are Arabic in origin. Aldebaran, Rigel, Betelgeuse, Altair, Caph: all of these derive from Arabic, even if they are sometimes mistranslations or mistransliterations. Some names are obviously Greek or Latin: Arcturus, Bellatrix and Polaris betray their naming origins to anyone even vaguely familiar with classical languages.

The first attempts to turn this into something systematic came with Johann Bayer’s Uranometria star catalog of 1603. This grouped stars by constellation, and named the stars from the brightest (alpha) in decreasing order of brightness, with the Greek letter followed by the genitive case of the constellation. (I know that last clause will send shivers of pleasure up the spines of my English major readers.) The brightest star in the constellation of Centaurus is therefore Alpha Centauri, the next brightest is Beta Centauri, etc. The problem with this is that there are only 24 letters in the Greek alphabet, and most constellations have more than 24 stars, even without the aid of a telescope.

Uranometria_titlepage

John Flamsteed (England’s first Astronomer Royal) to the rescue! Replace the Greek letter with a number, and you are no longer bound by the limits of an alphabet. But, just to keep this interesting, Flamsteed listed stars by their position rather than their brightness. So while Arcturus is easily the brightest star in the constellation of Boötes (pronounced “boo-OH-tees”, not ‘booties”) and therefore earns the designation of Alpha Boötis, its position in the sky means the Flamsteed designation is 16 Boötis. Confused yet?

With the invention of the telescope, the impossibility of naming every single star we can see became apparent. Catalogs only attempted to name all the stars brighter than some minimum value; there are always more stars visible with larger telescopes. Very bright stars are first magnitude, and the dimmest stars visible to a naked-eye observer under a dark sky are sixth magnitude. We’ll limit ourselves here to just the more common designations still is use today, and use Betelgeuse as our example for multiple designations of a single star.

Henry Draper (HD): Covering the entire sky down to ninth or tenth magnitude, this was compiled in the early 1920s by astronomers at the Harvard College Observatory. It numbers stars in order of increasing right ascension (the celestial equivalent of terrestrial longitude; see more here). Those right ascension values were standardized to the 1900 epoch since stars do gradually move relative to each other; the catalog was named for an astronomer whose widow funded the work. Betelgeuse, with a right ascension of 5 hours 55 minutes, is designated as HD 39801.

Smithsonian Astrophysical Observatory (SAO): Published in 1966, this overlapped considerably with the Henry Draper catalog but only contained stars for which the proper motion (apparent motion relative to the more distant background) was known. The sky is divided into 18 ten-degree bands of declination (celestial equivalent of terrestrial latitude), then numbered in increasing order of right ascension within each band. Betelgeuse is SAO 113271.

Guide Star Catalog (GSC): This was first published in 1989 (with subsequent revisions) for use with the Hubble Space Telescope. The intent was to guide the Hubble in accurate pointing by marking the positions of stars down to magnitude 15. The latest version is still being used for that purpose and contains nearly a billion stars. The numbering system is too esoteric to be worth describing unless you plan to apply for observing time on the Hubble. Betelgeuse is GSC 129:1873.

The U.S. Naval Observatory is the source of today’s most accurate and detailed star catalogs. The current standard-bearer is UCAC-3 (the third U.S. Naval Observatory CCD Astrograph Catalog), with compiled information on around 100 million stars. A list of catalogs (somewhat outdated; it lists the four-year-old UCAC-3 as “forthcoming”) is here.

As a practical matter, even the most dedicated backyard astronomer seldom knows and can point out more than a hundred or so of the brightest or most interesting stars. A good way to begin to learn the night sky is to learn the names and locations of the one or two brightest stars visible in each constellation visible from your location. Right now (mid-May), some of the brighter stars visible from Lynchburg around 11 p.m. EDT are Castor and Pollux in the constellation of Gemini in the west; Regulus in Leo; the stars of the Big Dipper (Ursa Major): the “pointer stars” of Dubhe and Merak, the double stars Alcor and Mizar occupying the middle position in the dipper’s tail; Arcturus in Boötes nearly overhead; Spica in Virgo a little below it in the south; Vega (Lyra) and Deneb (Cygnus) rising in the northeast. Start with those, add a few each month, and you are well on your way to learning your way around sky.

May Sky

A postscript: if you are ever tempted to pay money to name a star, one word of advice.  DON’T!!  Here is why not.

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Posted in Sky Phenomena, Stars

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