KAPPA TAU (Kappa Tauri). Here is a two for one special, two stars
together known as Kappa Tauri, a seeming naked-eye double a bit
north of Aldebaran in Taurus. Kappa-1, at fourth magnitude (4.22) the
brighter, is very slightly to the west of fifth magnitude (5.28)
Kappa-2 and 5.6 minutes of arc to the north. There is no question
that the two are related, as they lie within the Hyades cluster, which (at a distance of 151 light
years) spreads rather well beyond the main vee-shaped head of the
celestial bull, a generally under-appreciated fact. But are the
Kappas really a pair? They are remarkably similar, both white
class A stars, Kappa-1 an A7 subgiant-dwarf (meaning that it is
about to give up core hydrogen fusion), Kappa-2 an A7 hydrogen-
fusing dwarf. Moreover, both are classed as multiperiodic Delta-Scuti type variables, which change
brightness by a few percent over periods of hours (though this
pairs' variations are not well studied). Kappa-1 has a measured
temperature of 8290 Kelvin, a luminosity of 34 times that of the Sun, a radius 2.9 solar, and an estimated mass
2.2 solar. Oddly, though the stars are of the same class, Kappa-2
is cooler, only 7600 Kelvin. That and the calculated luminosity of
13 solar give a radius 2.1 solar and a mass 1.8 solar. They may
not, however be single stars, which leaves these numbers somewhat
problematic. Kappa-1 seems to have a spectroscopic companion,
while lunar occultations (in which the Moon covers the stars)
suggest close companions to both. There may be as many as 6 stars
in the system (assuming that Kappa-1 and Kappa-2 are a real pair).
The class A stars seem to dominate, however, and the parameters are
probably not far from wrong. But back to the original question.
The two are moving more or less through space together. Assuming
they are at the same distance (which they may not be), they are
16,000 Astronomical Units apart, which implies a period of 900,000
light years and an orbital speed of only 0.6 kilometers per second.
Measures of the motion, however reveal a relative speed of 4
kilometers per second, much too high for them to be a real pair,
which given the gravitational tugs from other stars within the
half-billion-year-old cluster makes sense. Even factoring in
observational errors, the relative speed cannot be brought down far
enough. Mere proximity is not enough to believe a seeming pair to
be an actual double. Both of the Hyades family, they seem to be
cousins rather than siblings.