TAYGETA (19 Tauri). Few sights charm the astronomical heart more
than the sight of the Pleiades, or
Seven Sisters, star cluster in Taurus.
Six of them are normally visible to the naked eye. At fourth
magnitude (4.30), Taygeta (Flamsteed's 19) is ranked sixth
in brightness, and thereby the faintest to be seen (though some
with especially sharp vision can indeed find 7 or even 8 stars).
Taygeta, in mythology one of the daughters of Atlas and Pleione
(both represented by stars), is a hot (13,400) class B (B6)
subgiant, the latter suggesting that the star is beginning to
evolve away from being a hydrogen-fusing dwarf. The star's total
luminosity (after allowing for a bit of dimming by interstellar
dust, some of which is involved with the star itself) of 750 Suns is somewhat deceptive. Spectra reveal
that Taygeta is a close double with a period of 3.60 years. Since
the star is close to the ecliptic, it is occasionally covered, or
occulted, by the Moon, which gives both separation and magnitude
difference. The brighter component is thereby found to have a
luminosity around 600 times that of the Sun, the companion about
150, making the latter roughly class B9. The two have respective
masses around 4.5 and 3.2 solar. The orbital period then gives an
average separation of 4.6 Astronomical Units (AU), which is
consistent with that found from the lunar occultation. The primary
star has an equatorial rotation velocity of 133 kilometers per
second. From the radius of 4.5 times that of the Sun, the rotation
period must be less than 1.7 days. Much farther out, just over two
minutes of arc away, lies an eighth magnitude white "companion"
that is probably a class F0 dwarf. If it is indeed bound to the
inner close double (which seems unlikely given the dense population
of stars that could tear it away), its orbital radius is at least
9000 AU and its period at least 300,000 years. The inner double,
however, is clearly real.