225
(Ammarell & Tsing, 2015). It is hard to escape the conclusion that once upon a time
there really were seven easily visible stars, one of which is no longer visible. Hertzog
(1987) describes this phenomenon as “the combined testimony of numerous societies, spanning continents and millennia, for a seventh easily visible … Pleiad which
subsequently dimmed”.
2 The Astronomy of the Pleiades
The Pleiades is one of the nearest open clusters to the Sun, at a distance of about 135
pc (Melis, Reid, Mioduszewski, Stauffer, & Bower, 2014), and one of the youngest,
with an age of ∼115–125 million years) (Stauffer et al., 1998; Ushomirsky et al.,
1998). The Pleiades contains stars spanning a wide range of masses, but the brightest visible stars are all B stars. The dynamics of the cluster as a whole are wellstudied (e.g. Converse & Stahler, 2010) but subsequent discussion in this paper is
limited to the ten stars that are, in principle, visible to the human eye, with mv < 6,
listed in Table 1.
Table 1 The visible stars of the Pleiades, taken from the Hipparcos catalog (van Leeuwen, 2009)
Name
RA
(J2000)
Dec
(J2000)
Brightness
(m v )
PM (RA)
(mas/yr)
PM (Dec.)
(mas/yr)
Spectral
type
Celaeno* 03 44
48.20
+24 17
22.5
5.45
20.73
−44.00
B7IV
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
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
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
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
−44.70
B8III
Pleione
03 49
11.20
+24 08
12.6
5.05
18.71
−46.74
B7p
Those corresponding to the Pleiades of Greek mythology are marked with an asterisk. Asterope is
a binary with a separation of 2.5 arcmin, so the two stars are indistinguishable to most human eyes.
The combined brightness of the two stars is m V = 5.66. The columns marked PM give the proper
motion in Right Ascension and Declination, in milliarcsec per year
Why Are There Seven Sisters?
(Ammarell & Tsing, 2015). It is hard to escape the conclusion that once upon a time
there really were seven easily visible stars, one of which is no longer visible. Hertzog
(1987) describes this phenomenon as “the combined testimony of numerous societies, spanning continents and millennia, for a seventh easily visible … Pleiad which
subsequently dimmed”.
2 The Astronomy of the Pleiades
The Pleiades is one of the nearest open clusters to the Sun, at a distance of about 135
pc (Melis, Reid, Mioduszewski, Stauffer, & Bower, 2014), and one of the youngest,
with an age of ∼115–125 million years) (Stauffer et al., 1998; Ushomirsky et al.,
1998). The Pleiades contains stars spanning a wide range of masses, but the brightest visible stars are all B stars. The dynamics of the cluster as a whole are wellstudied (e.g. Converse & Stahler, 2010) but subsequent discussion in this paper is
limited to the ten stars that are, in principle, visible to the human eye, with mv < 6,
listed in Table 1.
Table 1 The visible stars of the Pleiades, taken from the Hipparcos catalog (van Leeuwen, 2009)
Name
RA
(J2000)
Dec
(J2000)
Brightness
(m v )
PM (RA)
(mas/yr)
PM (Dec.)
(mas/yr)
Spectral
type
Celaeno* 03 44
48.20
+24 17
22.5
5.45
20.73
−44.00
B7IV
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
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
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
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
−44.70
B8III
Pleione
03 49
11.20
+24 08
12.6
5.05
18.71
−46.74
B7p
Those corresponding to the Pleiades of Greek mythology are marked with an asterisk. Asterope is
a binary with a separation of 2.5 arcmin, so the two stars are indistinguishable to most human eyes.
The combined brightness of the two stars is m V = 5.66. The columns marked PM give the proper
motion in Right Ascension and Declination, in milliarcsec per year
Why Are There Seven Sisters?