ALPHA: Name CASTΟR, “The Horseman“. The 23rd brightest star in the sky; magnitude 1.59; spectrum A1 V and A5. Opposition date is about January 12.
Castor is located at a distance of 45 light years and has a total luminosity of about 36 times that of our Sun.
This star is probably the finest double in the north sky for fair- sized glass.
It was probably resolved first by G.D. Cassini as early 1678, but not re-discovered until J. Bradley began his series of measurements in March 1718. In the years between 1719 and 1759, a change of about 300 was noted in the PA (Position Angle).
In 1803, Sir William Herschel announced that the components form a system in which the two stars are gravitationally connected and revolve about each other in space. According to R.H. Allen, however, this possibility had been suggested by the Reverend John Michell as early as 1767.
Castor, in any case, was the first true physical binary to be recognized, and the first object beyond our own Solar System in which the force of gravitation was shown to be operating, as it does in the planetary system.
The two stars visible in the amateur telescope, Castor A and B, are magnitudes 2.0 and 2.8; their period of revolution is about 4 centuries and their mean separation approximately 90 AU or about 8.4 billion miles, a little more than the distance across the entire Solar System. The pair was widest (6.5΄΄) in 1880; they have since been slowly closing their separation toward a minimum of about 1965. The exact period and orbital elements are not precisely determined. In most current texts, the period of 380 years, derived by K.A. Strand some 30 years ago, is still quoted; more recent calculations by P. Muller (1956) and W. Rabe (1958) are compared in the following table.
P. Muller ______511 years___ 7.370΄΄ ______0.36 _____________1950.6
W. Rabe ______420 years ___6.295΄΄______ 0.33_____________ 1965.3
The interesting fact about Castor is that each of three visible stars is itself a spectroscopic binary; the entire system of six components forming one of the most remarkable examples of a multiple star in the heavens.
Robert Burnham, Jr.
Burnham’s Celestial Handbook, Volume Two.