Double Star of the Month - November 2009
In this 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.
Iota Tri (02 12 22.28 +30 18 11.1), also STF 227, is another of the elder Herschel's discoveries and was included in his second class as number 34. There is a similar if considerably fainter pair called H II 35 following about half a degree distant which became STF 232, both being found on 1781 Oct 8.Interestingly, iota Tri does appear in Piazzi's 1814 catalogue (but not as a double) whilst Baily in his Catalogue of Stars for the British Association in 1845, gives it only as 6 Tri. Whatever it is called, iota is an attractive pair of yellow and blue stars according to Webb (Smyth - topaz-yellow and green) which forms a very slow binary system which has changed by only 11 degrees in over 200 years with the stars closing very slowly. The position for 2008 is 68 degs, 3".7. Both stars are spectroscopic binaries with periods of 14.73 and 2.24 days for the brighter and fainter visual stars respectively. A is also called TZ Tri, an RS CVn binary which has been resolved using the Palomar Test bed Interferometer. This reveals that the angular size of the semi-major axis of the orbit is but 2 milliarcseconds.
Tau Scl (01 36 08.50 -29 54 26.5) lies in a large rather empty area of sky north of Phoenix with only the galaxy NGC 613 some one degree north following for immediate company. This `close yellow pair', as Hartung calls it, is nevertheless worth seeking out. Separated by 3".9 in 1835 when John Herschel listed it as 3447 in his catalogue the pair has closed ever since and is currently just past minimum separation (2010, 186 degrees, 0".81), according to a recent orbit which gives the period as 1503 years. The magnitudes of the stars are 6.0 and 7.4, and the Hipparcos satellite puts them at a distance of 227 light years.
Bob Argyle - Double Star Section Director