The Lost Pleiad

As Aratos of Soli (~3rd century b.c.) described the mystery,

Seven in number are placed in record, but to our sight only six appear. It is not known for certain when the missing star disappeared. Nevertheless they each have their respective names. Alcyone, Merope, Celaeno, Electra, Sterope, Taygeta, and the queenly Maia. (cited in Payne-Gaposchkin and Haramundanis 1954/1970, p. 18)

The Aratus list has been used by medieval scholars to assign names to the individual stars of the Pleiades group that are still used today; it is far from certain, however, that they made the same identifications as were done in antiquity. The present naked-eye stars of the Pleiades are depicted in Figure 5.18.

In order of present-era brightness, they are as follows:

(1) Alcyone (otherwise called h Tau or HR1165, V =

Alcyone

Figure 5.18. A telescopic photo of the Pleiades, from the RAO archives, courtesy of James T. Himer of Calgary. Labeling provided by E.F. Milone.

(9) Asterope (or Sterope = HR1151, 5.76) (10) Sterope II (22 Tau = HR1152, 6.43)

Therefore, Aratos's list is not in order of brightness if the ancient names correspond to the modern names. Atlas (the father of the seven sisters) and Pleione are slightly to the east of the others and were clearly not in the original asterism. Aratos's list appears to be the order of the western group counted CCW, but Celaeno (16 Tau = HR1140, V = 5.46) and Electra are reversed, if current assignments are correct. If they are not, then following Aratos's list, Celaeno could be somewhere between, and possibly south of, 23 and 17 Tau, while 16 Tau, between (but not in line with) 17 and 19 Tau, could be a candidate for Sterope. There are other interpretations. E.C. Pickering [1846-1919], then director of the Harvard College Observatory, suggested that Pleione, a rapidly rotating B subgiant with a P Cygni spectrum, was the missing Pleiad,36 having faded since earlier times. The idea is not implausible that shell stars, or as they are usually called, Be stars (the "e" subscript signifies emission features in the spectra), are variable in large amplitude on very long timescales. McNamara (1987) found Atlas, Pleione, and Merope to be variable. Curiously, in Greek myth, Pleione

36 Payne-Gaposchkin and Harmundanis (1970, p. 498) suggested the possibility also, because Pleione was already known to be a low-amplitude variable.

was the wife of Atlas and the mother of the seven Pleiades and thus hardly qualified to be the missing Pleiad. Hertzog (1984, 1986) discovered a Hipparchos observation of the variable star V344 Car, indicating a previously unknown period of variability of greater than ~100 years. Hertzog (1987) argues, that Celaeno (16 Tau) is the lost Pleiad; he cites Asian depictions from the 7th to 13th centuries a.d., which show a seventh star as one of three south of the 19-20-h Tau line. This suggestion follows Needham (1959, p. 3, Figs. 93, 104, 106, 107). The three stars are presumably 16, 17, and 23 Tau (Celaeno, Electra, and Merope, respectively).

It is also interesting to note that the non-Pleiad, currently visible, Be star, o Tau, is missing from the Almagest list of bright stars and from the oldest Chinese records.

The bright star Electra has been a candidate solely on the basis of legend. As the mother of Dardanos, the founder of Troy, Ovid (in the Fasti) wrote of her that she hid her light in sorrow at the city's destruction; Hyginus [d. 10 a.d.] suggested that she left her place to wander off as a comet or to be a companion to Mizar (namely, Alcor = 80 UMa). Hyginus's suggestions are interesting. The first is reminiscent of the passage of Halley's Comet near the Pleiades during its 1985-1986 apparition, and one wonders if the suggestion came about as the result of a similar phenomenon in Hyginus's time. The latter is curious, because 80 UMa is not especially bright (V = 4.01), certainly fainter than Electra is at present. Among the Bugis mariners of Indonesia, the "lost Pleiad" is Antares (a Tau), but here the myth has navigational utility (see §11.3). Sterope I and Sterope II have also been suggested as the missing Pleiad, mainly on the basis of a 5th-magnitude assessment of the combined light of the pair by the Persian astronomer al Sufi [903-986] and through the circumstance that Ovid referred to the Pleiades as a whole as Steropes sidus (cf. Allen 1899/1963, p. 407). On the other hand, the Roman poet Valerius Flaccus is said to have used the name Pleione to refer to the Pleiades as a whole (Allen 1899/1963, p. 408); so these may be but instances of poetic license.

The star cluster is partially screened by nebulosity, which can be seen in time exposures, scattering, and obscuring some of its stars. The collective appearance of the many faint stars of a star cluster to the naked eye can convey the impression of fuzziness even in the absence of nebulosity. This may account for the comment by Ovid (in his Latin translation of Aratus's poem, the Phainomena):

Six only are visible, but the seventh is beneath the cloud.

Supporting this view is a tale from the Nez Perce of the North American Plateau about the Seven Sisters, among whom the next to youngest sister was known as "Eyes-inDifferent-Colors" (Miller 1997, p. 117). She loved a mortal, but at his death, she hid herself so that we now see only six of the sisters. (Note that if she represented the second faintest of the seven in order of current brightness, and Atlas and Pleione are excluded, Celaeno is suggested.)

However, even in antiquity, Hipparchos criticized Aratus for suggesting that

Yet six alone are viewed by mortal eyes.

Hipparchos's comment is that

One star escaped his attention, for when the eye is attentively fixed on this constellation, on a serene and moonless night, seven stars are visible. (Allen 1899/1963, p. 408).

This comment, and the circumstance that the 7th Pleiad in our list of ranking according to brightness (including this time Atlas and Pleione) is of magnitude 5.09, suggests a visibility and perhaps acuity problem for some observers, but intrinsic long-term variation by one or more stars cannot be ruled out. The matter is further confused when one considers that to a sharp eye, the current brightnesses of stars may permit 12 or more to be seen. Allen (1899/1963, p. 410) cites claims of 9 to as many as 16 (Johannes Kepler reported that his student, Michel Mostlin, could see 14), and one of us (DHK) claims to have been able to see 11 on dark, clear nights in his youth. Presumably, the relative contrast plays a role in judging the visibility of the 7th star because most modern observers can see only six Pleiads. It is possible that light pollution at observing sites was worse than we might wish to imagine (see comments in §3.1 about effects of cooking fires even outside urban centers), and that six or maybe seven stars were as many as could be discerned under most circumstances. This would suggest that an additional star was (usually) somewhat more prominent in Aratus's day than are the fainter alternatives today, and perhaps fainter than Hipparchos found in his day.

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