ACHIRD (Eta Cassiopeiae). At almost fourth magnitude (3.44) and
faint to the eye, and not a part of the traditional outline of Cassiopeia, Achird is often neglected. It
is, however, among the glories of the celestial Queen's retinue.
The proper name "Achird" seems to have been applied in recent times
and has no clear meaning, one unsupported source suggesting
"girdle." Better really to know it by its Greek letter name of
Eta Cas. If you want to see a sunlike star, look no farther.
Achird is a yellow-white class G hydrogen-fusing dwarf (variously
G0 to G3, we adopt the latter) with a surface temperature of 5730
Kelvin, just a bit cooler than the Sun.
Achird is also among the closer stars, its distance a mere 19.4
light years, from which it shines with a luminosity of only 1.28
times solar, its radius a mere 15 percent greater than the Sun's.
What makes the star really stand out, however, is its eighth-
magnitude (just barely, 7.51) companion, an orange class K (K7)
dwarf. Star colors are subtle. But put two stars of even somewhat
different color together, especially if there is a brightness
difference, and the contrast can become quite intense. Eta Cas A
(the brighter) and B thus put on a fine show through even a small
telescope, the pair an easily-resolvable 11 seconds of arc apart.
The fainter, with a temperature of 4100 Kelvin, shines with a
luminosity only 0.07 solar, the radius half that of the Sun's. Two
centuries of observation reveal an orbital period of 480 years,
which combined with the an average separation of 70 astronomical
units (1.75 times farther than Pluto is from the Sun) show stars
with masses of 1.07 (the brighter) and 0.42 solar. Except for the
much-longer period, Eta Cas is something of a northern-hemisphere
version of Alpha Centauri. If the
brighter G-type star were to have an Earth-like planet, the dimmer
K star would shine in the planet's sky with an orangy light of 5
full moons. Stars with planets, however,
seem to have metal contents similar to or even greater than that of
the Sun. Achird-A (and presumably, since the stars were no doubt
born from the same interstellar cloud, Achird-B) is however metal-
poor, its iron (and other) content relative to dominant hydrogen
only half that found in the Sun, so a planet rather sadly seems
unlikely. Thanks to J. M. Aguiar, who suggested this star.