WASAT (Delta Geminorum). With a name that does not ring much of a bell among
astronomers, and shining only at the bright end of fourth
magnitude, neither does the star itself. Seventh brightest in
the constellation Gemini,
Bayer still gave it the Delta designation, the star
beaten out by Pollux (Beta), Castor (Alpha), Alhena (Gamma),
Epsilon,Eta, and Mu. Yet it is
not without glory. The name is a mess, "Wasat" meaning "middle" in
Arabic, but the middle of WHAT is not clear, whether the middle of
Gemini, of the sky, or of the neighboring constellation Orion
(which the Arabs referred to as the "Central One"), the star name
improperly applied to our Delta. Wasat serves as a "marker" star
like Polaris and Mintaka (which reveal the celestial pole
and equator). Wasat leads our eye to the ECLIPTIC, the apparent
path the Sun seems to take around us as we orbit. Only two-tenths
of a degree south of the ecliptic, it is but half the angle of the
first magnitude ecliptic marker Regulus.
A line drawn from Wasat through Regulus nicely defines the solar
path. Halfway between lies the "Beehive" star cluster in Cancer.
Better, Wasat marks the famed and last planet Pluto, as Clyde
Tombaugh discovered the body close to the star in 1930. Taking 248
years to orbit the Sun, in the past 66 years Pluto has moved only
into Ophiuchus 16 degrees north of Antares in Scorpius. The whole observed
70-year path can be taken in one glance after Antares rises. Since
the ecliptic passes only 5 degrees north of Antares, we also see
that Pluto is now far (over 10 degrees) above the plane of the
Solar System, testimony to its strangely tilted orbit, Wasat
closely marking the place where Pluto crosses the ecliptic on its
way north. The star is also somewhat more solar than most of its
naked-eye brethren, a class F "subgiant" with a temperature of 6700
Kelvin, only 1000 Kelvin hotter than the Sun, its distance of 59
light years telling that it shines with just 10 times the solar
luminosity. Subgiants are stars that are in the first stages of
expanding to gianthood after the central nuclear fire goes out, so
Wasat is now beginning to expire. It will soon be a "gap" star
like Caph (Beta Cassiopeiae). Tucked next
to it is a cooler ordinary class K companion, easily seen through
a small telescope, that is 1200 Kelvin cooler than the Sun. Over
100 times the Earth-Sun distance from Wasat proper, the companion
takes 1200 years to orbit. Wasat itself may be a close inseparable
double, though no one is sure. Keep your eye on this star: Pluto
will pass it again in just 179 years.