The University Record, November 5, 1996

Greek, Arabic and modern star names can confuse stargazers

Hanging nearly overhead in November's mid-evening sky is the W-shaped constellation we know as Cassiopeia, Queen of Ethiopia. One of the oldest of the star patterns, it was recognized by the ancient Greeks. Yet some of its stars carry Arabian names given by Arabian peoples. This same mixture of Greek and Arabic names is common in many constellations, according to astronomer Richard Teske.

"To add to the confusion, modern astronomers apply their own rules for naming stars, with the result that each of Cassiopeia's stars has many aliases," Teske says.

Observers who face north while looking up at Cassiopeia will see the star called "Caph," meaning the palm of a hand, on the left end of Cassiopeia's upside-down "W." "The star's name comes from a star picture envisioned by the Arabic peoples that is very different from the Greek conception of the constellation," Teske explains. "Despite this, its Arabic name was inserted into the Greek conception of Cassiopeia around 400 years ago."

Modern rules for naming stars add to the grief of ordinary observers. For example, Caph also is called Beta Cassiopeiae. "The Greek letter beta, second in the Greek alphabet, tells that Caph is the second brightest star in Cassiopeia. The first Greek letter, alpha, is reserved for the brightest star and the third one, gamma, for the third brightest, and so on," Teske added.

Alpha Cassiopeiae is located next to Caph, at the left bottom of the "W." It is also called Schedar, Arabic for "breast." Gamma, located at the middle of the "W," has no Arabic name.

Many other names have been given to stars by astronomers who inventory them for research purposes, according to Teske. Each time a star is put on a listing, it gains a new name. One of the best known lists was created at Harvard Observatory almost a century ago to classify 224,300 of the brighter stars according to a characteristic related to their temperatures.

"This task took one person four years to complete," Teske says. "Caph appears as the 432nd star on the roster. Consequently, astronomers sometimes speak of it as `HD 432.' Because it is a prominent star in the sky, Caph has been enrolled into many star lists, so it sometimes goes under peculiar aliases like `+58,0003,' `HR21' or `SAO 21609.' "

According to Teske, the largest directory of celestial objects is the Space Telescope Guide Star Catalog, which contains 20 million entries giving exact sky positions for stars, galaxies and other features. It was compiled by the Space Telescope Science Institute for use with the Hubble Space Telescope.

"Hubble has very precise and sensitive light sensors for locating stars. To point it accurately, the telescope's controllers select a few of the 20 million guide stars and designate which of them must appear in which of the telescope's sensors. Once the telescope locks onto the guide stars, it remains pointed until the time exposure is complete," Teske says.

The Guide Star Catalog is so large it is stored and read electronically. It is available online to the public at http://www-gsss.stsci.edu/casbhome.html, and also can be purchased on CD-ROM.