A Selection of Double Stars in Gemini
January 2008 :
It’s appropriate that the constellation Gemini, the Twins, should be home to a numerous collection of double stars. Here are ten stellar – pardon the pun - examples (data from the Washington Double Star Catalog (WDS).
mu Geminorum mags 2.9 and 9.4, sep 121.7”, P.A. 141° (1899). The M-type primary of this wide optical pair displays a distinct ruddy color.
nu Geminorum mags 4.1 and 8.0, sep 111.6”, P.A. 331° (2002)
15 Geminorum magnitudes 6.7 and 8.2, separation 25.2”, Position Angle 203o (2004). Both pairs are part of an attractive low-power field that includes 16 Gem. 15 Gem’s K-type primary sports a golden yellow hue. A fine sight!
20 Geminorum mags 6.3 and 6.9, sep 19.7”, P.A. 211° (2002). An easily resolved pair located roughly midway between nu and gamma Gem. Both white.
epsilon Geminorum mags 3.1 and 9.7, sep 110.6”, P.A. 95° (2002). On the evening of April 7, 1976, this star drew plenty of attention when it was occulted by the planet Mars. Even without Mars to spice things up, epsilon Gem is a nice, wide double.
38 Geminorum mags 4.8 and 7.8, sep 7.3”, P.A. 146° (2004). An exquisite little pair! Use a magnification of 100X to separate the faint companion from the yellowish primary. An interesting note: the WDS gives a P.A. of 326o, while earlier sources cite a P.A. that differs by 180o! A quick check with my 3-inch reflector shows that the WDS is in error.
zeta Geminorum mags 3.6v, 7.7, sep 100.7”, P.A. 349° (2000). The primary is a Cepheid variable star (magnitude range 3.6-4.2) with a 10-day period. This wide optical pair is easily split at 20-30X.
delta Geminorum mags 3.6 and 8.2, sep 5.8”, P.A. 227° (2003). This slow-moving binary has thus far eluded my 3-inch reflector. The separation is within this scope’s reach, but the magnitude difference means that the secondary gets lost in the glare of the main star. This winter I’ll be pursuing delta Gem with a bigger scope (a 127mm Mak-Cas) and a magnification of at least 150X.
alpha Geminorum (Castor) mags 1.9 and 3.0, sep 4.2”, P.A. 62° (2004). The close pair is binary (P = 510 years). A showpiece, arguably the finest double star visible in the northern sky. The separation has steadily increased since a minimum of 1.8” in the mid-1960s. Currently, these two white stars are easily resolved in a 60mm refractor at 100X. A 9.8 magnitude companion, located 71.0” from the main pair, is a physical member of the system. In fact, each of these stars is a spectroscopic binary, making Castor a sextuple star!
kappa Geminorum mags 3.7 and 8.2, sep 7.2”, P.A. 241° (2002). Like delta Gem, this close, unequal pair has escaped my 3-inch reflector. A 4 or 5 inch scope on an evening of steady seeing should capture kappa Gem. Again, high power and steady seeing conditions are a must!
