Why Are There Seven Sisters?

Why are there Seven Sisters? Ray P. Norris1,2 & Barnaby R. M. Norris3,4,5 1 Western Sydney University, Locked Bag 1797, Penrith South, NSW 1797, Australia 2 CSIRO Astronomy & Space Science, PO Box 76, Epping, NSW 1710, Australia 3 Sydney Institute for Astronomy, School of Physics, Physics Road, University of Sydney, NSW 2006, Australia 4 Sydney Astrophotonic Instrumentation Laboratories, Physics Road, University of Sydney, NSW 2006, Australia 5 AAO-USyd, School of Physics, University of Sydney, NSW 2006, Australia Abstract of six stars arranged symmetrically around a seventh, and is There are two puzzles surrounding the therefore probably symbolic rather than a literal picture of Pleiades, or Seven Sisters. First, why are the Pleiades. the mythological stories surrounding them, In Greek mythology, the Seven Sisters are named after typically involving seven young girls be- the Pleiades, who were the daughters of Atlas and Pleione. ing chased by a man associated with the Their father, Atlas, was forced to hold up the sky, and was constellation Orion, so similar in vastly sep- therefore unable to protect his daughters. But to save them arated cultures, such as the Australian Abo- from being raped by Orion the hunter, Zeus transformed them riginal cultures and Greek mythology? Sec- into stars. Orion was the son of Poseidon, the King of the sea, ond, why do most cultures call them “Seven and a Cretan princess. Orion first appears in ancient Greek Sisters" even though most people with good calendars (e.g. Planeaux , 2006), but by the late eighth to eyesight see only six stars? Here we show that both these puzzles may be explained by early seventh centuries BC, he is said to be making unwanted a combination of the great antiquity of the advances on the Pleiades (Hesiod, Works and Days, 618-623). stories combined with the proper motion of Curiously, similar stories about the Pleiades and Orion are the stars, and that these stories may predate told in Aboriginal Australia. For example, most Aboriginal the departure of most modern humans out cultures associate Orion with a hunter, or a young man, or of Africa around 100,000 BC. a group of young men, or a male ceremony, and many have stories in which the men in Orion are trying to chase and rape the girls of the Pleiades (e.g. Massola, 1968; Mountford Keywords: Aboriginal astronomy – ethnoastron- , 1939, 1976). The similarity between the Aboriginal and omy – history of astronomy Greek stories of the Pleiades and Orion includes three specific elements: both identify the Pleiades as a group of young girls, both identify Orion as male, and both say that Orion is 1 INTRODUCTION attempting to have sex with the girls in the Pleiades. The Pleiades, or Seven Sisters, is an open stellar cluster of These strong similarities suggest a common origin, which hot, blue, young stars, which were formed about 115 to 125 appears to predate European contact with Aboriginal Aus- million years ago (Stauffer et al., 1998; Ushomirsky et al., tralia. 1998), and they are still surrounded by a blue reflection nebula. This comparison is particularly interesting because there The cluster is called the “Seven Sisters" in many cultures, has been almost no cultural contact between the European with a remarkable similarity in the stories surrounding it. (i.e. Greek) and Aboriginal Australian cultures from about The importance of the Pleiades in many cultures has been 100,000 BC, when the ancestors of both cultures migrated listed by several authors (e.g. Allen, 1899; Avilin, 1998; Burn- out of Africa, until 1788 when the British invaded Australia. ham, 1978; Dempsey, 2009; Krupp, 1994; Kyselka, 1993; Nevertheless, there is a remarkable similarity between the Sparavigna, 2008), from the first written record by the Chi- stories in both cultures. Norris & Norris (2009) first suggested nese in 2357 BC through to the present day. In most cultures, that one explanation for this similarity is that the roots of the the Pleiades are seen as seven young women, or ‘daughters’ Seven Sisters story could date back to 100,000 BC, thus (Krupp, 1994). providing a common ancestry for this story in all modern The oldest representation of the Pleiades is thought to be human cultures. on the Nebra disk, found in Germany, and constructed around This paper examines this “Out of Africa” hypothesis. 1600BC (Ehser et al., 2011), but that representation consists A related puzzle concerns the number of stars in the 1 2 Norris & Norris Table 1 The visible stars of the Pleiades, taken from the Hipparcos B stars are often variable (e.g. Waelkens & Rufener, 1985) catalog (van Leeuwen, 2009) Those corresponding to the Pleiades of and the variability of the Pleiades has been well-studied (e.g. Greek mythology are marked with an asterisk. Asterope is a binary White et al., 2017). Several have been observed to be variable, with a separation of 2.5 arcmin, so the two stars are indistinguishable and Pleione is known to be an irregular variable, varying to most human eyes. The combined brightness of the two stars is by as much as 0.5 magnitude in the last century (Burnham, Vmag = 5.66 1978). However, such studies are only sensitive to short-term name RA dec Vmag PM(RA) PM(delta) spectral variability (on a timescale of days to years). On a timescale J2000 J2000 mas/yr mas/yr type Celaeno* 03 44 48.20 +24 17 22.5 5.45 20.73 -44.00 B7IV of tens of millions of years, comparable to the lifetime of the Electra* 03 44 52.52 +24 06 48.4 3.72 21.55 -44.92 B6III 18 Tau 03 45 09.73 +24 50 21.7 5.66 19.03 -46.64 B8V star, the star is expected to gradually increase in luminosity Taygeta* 03 45 12.48 +24 28 02.6 4.30 19.35 -41.63 B6V Maia* 03 45 49.59 +24 22 04.3 3.87 21.09 -45.03 B8III because of its expansion as it moves across the main sequence Asterope* 03 45 54.46 +24 33 16.6 5.76 19.44 -45.36 B8V Merope* 03 46 19.56 +23 56 54.5 4.14 21.17 -42.67 B6IV (Langer, 2012), but the change over a period of 100,000 Alcyone* 03 47 29.06 +24 06 18.9 2.85 19.35 -43.11 B7III Atlas 03 49 09.73 +24 03 12.7 3.62 17.77 -44.7 0 B8III years is probably still too small to be visible to the human Pleione 03 49 11.20 +24 08 12.6 5.05 18.71 -46.74 B7p eye. Additional variability may also be caused by motion of the obscuring dust that veils the Pleiades. Because of Pleiades. Although, in principle, ten stars in the Pleiades are these unknown factors, we have no information on how the sufficiently bright (mv < 6) to be seen with the naked eye, brightness of these stars varies on timescales of thousands of most people with good eyesight, in a dark sky, see only six years. stars (Kyselka, 1993). This is not a new phenomenon: even The proper motion of these stars has been measured ac- in the third century BC, the Greek poet Aratus of Soli gave curately by Hipparcos (van Leeuwen, 2009). Gravitational the names of the Seven Sisters (Halcyone, Merope, Celaeno, forces from the mass of the cluster, or from tidal fraction, Electra, Sterope, Taygete, and Maia) but then reported that are negligible over human timescales, and so we can linearly “only six are visible to the eyes” (Krupp, 1991). Thus, while extrapolate their motion back to prehistoric times, as shown many cultures regard the cluster as having seven stars, they in Figure 1. acknowledge that only six are normally visible, and then have a story to explain why the seventh is invisible. These “lost Pleiad” stories are found in European, African, 3 THE SEVEN SISTERS AND ORION IN Asian, Indonesian, Native American and Aboriginal Aus- ABORIGINAL AUSTRALIA tralian cultures (Burnham, 1978; Gibson, 2017). In Greek 3.1 Aboriginal Astronomy mythology, one of the sisters, Merope, was ashamed of falling in love with a mortal and therefore faded from sight (Sparavi- Astronomy is a central part of many Aboriginal cultures. An gna, 2008). In Australian Aboriginal mythology, one (or oc- extensive review, citing all known publications in this field, casionally two) of the sisters has died, is hiding, is too young, is given by Norris (2016). Mountford (1976) reported that or has been abducted, so only six (or five) are visible (Fuller some Aboriginal people knew the name of every star as faint et al., 2014; Kyselka, 1993). Krupp (1991) gives a story from as fourth magnitude, and knew myths associated with most the Onondaga Iroquois in which one of the stars sang as they of those stars. Even now, some elders can name most stars ascended to the sky and thus became fainter. In Islam, the in the sky visible to the naked eye, and have an intuitive seventh star fell to earth and became the Great Mosque (Am- understanding of how the sky rotates over their heads from marell & Tsing, 2015). It is hard to escape the conclusion that east to west during the night, and how it shifts over the course once upon a time there really were seven easily visible stars, of a year (Norris, 2016). Maegraith (1932) says that ‘The one of which is no longer visible. Hertzog (1987) describes most interesting fact about Aboriginal astronomy is that all this phenomenon as “the combined testimony of numerous the adult males of the tribe are fully conversant with all that societies, spanning continents and millennia, for a seventh is known, while no young man of the tribe knows much easily visible ..

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