Alpheratz

Alpheratz is Alpha Andromedae, apparent magnitude 2.06, at about 97 light years. It is a blue-white subgiant, and chemically a mercury-manganese star, its atmosphere showing anomalous over-abundances of those and other heavy elements. It marks the northeastern corner of the Great Square of Pegasus and was historically shared between the two constellations, once catalogued as Delta Pegasi before the boundaries were fixed and it was assigned wholly to Andromeda. The Arabic name, surrat al-faras, means the navel of the horse, of Pegasus, not of Andromeda, a reminder that the star belonged to the horse before it belonged to the chained woman.

Alpheratz's Arabic name is a genuine case of a popular etymology gone wrong: it is often glossed loosely as simply "the horse", but surrat al-faras parses precisely as surrat, navel, plus al-faras, the horse, a compound descriptive phrase rather than a single proper name, of exactly the kind the older Arabic star catalogues favour over invented names. Ptolemy catalogued the star in the Almagest as of the second magnitude, close to its true modern value of 2.06, one of the cases where the ancient and modern brightness classes agree well; the star's slow drift in catalogued position across two millennia is precession, not any real change in the star itself.

The planetary nature usually given to Alpheratz is Jupiter with Venus, and the delineation, honour, riches, a keen mind, independence, is not Ptolemy's but comes from the later compiling tradition, in English overwhelmingly from Vivian Robson's Fixed Stars and Constellations in Astrology of 1923. Ptolemy in the Tetrabiblos treats the stars of Andromeda together and does not single this one out with the specificity later writers imply. As with most bright-star delineations in circulation, the honest position is that the ancient layer is thin and the confident modern reading is Victorian in its wording. Alpheratz is not among Agrippa's fifteen Behenian stars.

Alpheratz is a useful check on how thin the Ptolemaic layer actually is beneath a Victorian-sounding delineation: Ptolemy's Andromeda passage in the Tetrabiblos gives the whole figure a Venus-and-Mercury-leaning character in general terms and stops there, so the specific Jupiter-with-Venus pairing later writers state for this particular star is an inference laid onto that general remark rather than a sentence Ptolemy actually wrote about Alpheratz by name. The habit of converting a constellation-wide Ptolemaic remark into a star-specific delineation, then stating the result with the confidence of a direct citation, recurs through the whole Robson-descended tradition and is worth watching for generally.