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.