Castor: alpha (α) Geminorum
March 2011 :
Are you looking for something new and different to add to your late winter/early spring star party repertoire – a cosmic showpiece guaranteed to elicit a gasp of surprise and wonder from anyone who peers into your telescope? I suggest the double star alpha (?) Geminorum, better know as Castor. One glance at these sparkling magnitude 2.0 and 2.9 diamonds and it’s easy to understand why William Herschel’s son, John, considered Castor the finest double star in the northern sky.
Castor’s duplicity was discovered by the English astronomers Bradley and Pound in 1718, although evidence exists that it may have been observed by Cassini forty years earlier. Based on his own observations and those of other astronomers in previous decades, William Herschel announced in 1803 that Castor was, in fact, a true binary system and not a chance alignment of widely separated stars. Castor A (the brighter component) and Castor B (the companion) have yet to undergo a complete orbit since their discovery, but calculations point towards an orbital period of 467 years.
At the time of its discovery, Castor was an easily-resolved pair separated by a comfortable 4 arc-seconds. Throughout the 19th century and early decades of the 20th, it remained a readily accessible small-scope target. But as the pair approached periastron (the point in a binary orbit when the stars are closest together), they became harder and harder to split. By the mid-1960s, Castor had dropped off the backyard astronomer’s radar. Since then, however, Castor A and B have rapidly separated. Today, the two are nearly 5 arc-seconds apart – once again an easy split for the smallest of telescopes.
The sight of this binary pair is so spell-binding that the observer may fail to notice a 9th magnitude companion 73 arc-seconds away. Though distant both in angular and physical separation (100 billion miles), Castor C is nonetheless gravitationally bound to the main pair, orbiting them in a period that probably exceeds 10,000 years.
There’s more! Spectroscopic studies reveal that each of Castor’s components is a tight binary pair. Castor A and B are both comprised of almost identical A-type main sequence stars with orbital periods of 9.2 days and 2.9 days, respectively. C is also a binary twin set, this time made up of low-mass red dwarfs locked in a 19.5-hour orbit. What the unaided eye sees as a single star is actually a triplet of twins!
Besides its value as a “wow” object, there’s another good reason for adding Castor to your star party repertoire. Bright double stars aren’t as adversely affected by haze or light-polluted skies as are deep-sky objects like nebulae and galaxies. Can’t get out to your dark-sky observing site in the country? No problem! Castor is waiting for you.
Your comments on this column are welcome. E-mail me at gchaple@hotmail.com.
