Double Star of the Month - February 2007
In this new series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.
The constellation of Gemini is well up in the northern sky during the middle of the month with the famous pair of stars Castor and Pollux at the eastern end of the group. The two stars are well contrasted with Castor showing pure white, as befits its membership of the class of A stars. Pollux, on the other hand, is orange, in reality a cool giant star and is actually the brighter of the two visually, prompting suggestions that one of the stars has changed in output in recent times.
For the small telescope user, Castor (7 34 35.9 +31 53 18) is of real interest. The star is a brilliant binary with two white components of magnitudes 2.0 and 2.9 currently separated by 4.4 arc seconds in position angle 59 degrees, making it easily visible in a 60-cm telescope. It may have been first resolved by Cassini in 1678 but it was certainly noted by Bradley in 1718. Since then the position angle has decreased by almost 300 degrees, with closest approach around 1965 when the separation was 1.8 arc seconds. Some 72 arc seconds to the south-east is a star of magnitude 9, known as Castor C. This star revolves around Castor AB in a period of many thousands of years. The remarkable fact about the Castor system is that all three visible stars are spectroscopic binaries, making the Castor system a rare example of a sextuple star.
Sirius (06 45 08.9 -16 42 58, mags -1.5, 8.5). The brightest star in the sky is also one of the nearest, located 8.7 light years away. The details of the discovery of the white dwarf companion are well-established. Bessel first noted that the proper motion of Sirius was not linear but the predicted companion was not seen until January 1862 when Alvan Clark was testing the 18.5-inch objective for Dearborn Observatory. Uniquely, Peters calculated an orbit for the Sirius system 11 years before the star was first seen. His value for the period, 50.01 years is very close to the currently accepted value.
There is much speculation about the smallest aperture required to see the Pup. It depends crucially on several factors - the separation of B from A, the quality of the atmosphere and the quality of the telescope optics. When B is near periastron it cannot be seen in any telescope. Between 1890 and 1897 when the separation was less than 4 arc seconds, there were no sightings recorded.
A recent observation of Sirius B was reported by Ralph Aguirre of the Sacramento Valleys Active Astronomers in March 2006. At a separation of 7.3 arc seconds B was seen with a 130-mm Takahashi refractor at x140 but he found it was better seen at x220, a point which earlier observers seem to agree about. This year the writer plans to use a hexagonal diaphragm on the 8-inch refractor at Cambridge in an attempt to get his first glance of this elusive object.
Bob Argyle - Double Star Section Director