1 GEM (1 Geminorum). In the late 17th century, John Flamsteed measured precise
telescopic positions for some 2500 stars, which were later numbered
from west to east within their parent constellations. Here is Number One in Gemini, the most westerly of his stars
within its boundaries. Sometimes considered to be part of the
classic outline of the figure, the curve of stars at the
northwestern end, fourth magnitude (4.16) 1 Gem takes a special
place as the "Solstice Star," as it lies within just a degree to
the east of the Summer Solstice, marking
that position much as Polaris marks the
North Celestial Pole. No other naked
eye star is closer. Too bad it has no special name, "Solstaris" or
some such. But "Number One" is not too bad. Precession, the 26,000 wobble of the
Earth's axis, however, is doing its work, and is slowly moving the
Solstice to the west and away from "One." The two passed closest
in 1932, when 1 Gem was only about a sixth of a degree to the south
of the Solstice. (Precession has also technically moved the
Solstice just barely into Taurus,
though it is still closer to the classical outline of Gemini.) But
that's not all. The star also provides a nice guide to the bright
open cluster Messier 35, which lies just
to the northeast of it. Nominally a class G (G7) giant, 1 Geminorum has much more
to recommend it. A careful look shows it to be a very close double made of a fourth
magnitude (4.7) G6 giant coupled with a fifth magnitude (5.1) G8
giant that orbit each other with a period of 13.35 years at an
average separation of 0.2 seconds of arc (impossible to split with
the eye) or 9.15 Astronomical Units. A modest eccentricity takes
them from as far as 12.5 AU to as close as 5.5 AU, which happens at
the end of 2008. Kepler's laws then give a total mass to the system of 4.30
times that of the Sun. But that's not all. The brighter, 1 Gem A,
was discovered by lunar occultation to be itself double with the G6
giant (Gem 1 Aa) coming in at magnitude 4.7 and coupled to a star
(Gem 1 Ab) that from its seventh magnitude (6.9) brightness should
be a class F (F6) dwarf. The spectrograph then shows 1 Gem B also
to be double in a 9.60-day orbit with a very close companion about
which nothing else is known, rendering 1 Gem a quadruple star.
Assuming 1 Gem Bb (the companion) is inconsequential, the two are
a mere 0.06 AU apart. Looking at the three stars in order
(ignoring 1 Gem Bb), 1 Gem Aa, Ab, and Ba, the temperatures are
5120 (apparently an actual measure), 6500, 4900 K (estimates), the
luminosities are 24, 2.9, and 21 times that of the Sun; radii are then 6.2, 1.35, and 6.4 times
solar, and the masses 1.7, 1.25, and 1.7 times solar, which add up
to 4.65 solar masses, satisfyingly close to that derived from the
binary orbit. The giants are probably quiet helium fusers, but if
they are just beginning to brighten with dead helium cores, they
would come in at 2 solar masses. All are consitent with ages of
around two billion years. From 1 Gem B, the Aa-Ab pair would be
easily separable, by almost the width of our full Moon. A
thirteenth magnitude "component," 1 Gem C about 100 seconds of arc
away, is merely a line-of-sight coincidence. Pairing of giants is
not common, the most famous case being Capella in Auriga. The star beckons observers for closer
examination.
Written by Jim Kaler 3/7/08. Return to STARS.