April 2017 - Double Star of the Month
Iota Leo (11 23 55.37 +10 31 46.9) is a naked-eye star which sits below the hind legs of Leo. It is a relatively easy double star to resolve given 15-cm of aperture and it seems to have been missed by William Herschel during his double star surveys around 1780.
The current orbit, which has a period of 186 years predicts a separation of 1".05 for 1780 but the pair is a very unequal one: the visual magnitudes are 4.06 and 6.71 making it somewhat easier than zeta Her which Herschel did discover. At the present time, the stars are separated by 2".1 and they are now almost back in the configuration in which they would have appeared to F. G. W. Struve in 1827.
From the UK iota Leo is relatively low and I have never found them particularly easy to measure. The stars will continue to widen until 2060 when they are 2".7 apart and then closing to 0".63 in 2128.
Admiral Smyth found the colours to be pale yellow and light blue. T. W. Webb noted white and tawny in 1870, whilst Hartung found yellow and whitish.
Pz 3 Velorum (10 31 57.33 -45 04 00.1) consists of two luminous B-type giants which lie over 600 light years away. Pz 3 lies at the end of a 5 degree arc of third, fourth and fifth magnitude stars starting with bright binary star mu Vel.
The arc also contains t Vel (HJ 4330 5.2, 8.6, 163º, 40") the primary of which is a recently discovered close pair, separation 0".4) whilst about 6 arcminutes east is HJ 4332 (mags 7.1, 9.8, 162º, 28").
Both components of Pz 3 were observed by Hipparcos and the resulting parallaxes show agreement although the scatter in each case is large.
With such a distant system, relative motion, if any, is very small and the current position angle and separation are 219º and 13".7 are little different from the first measures in 1826.
Gould notes that the components of the double star itself are white.
Bob Argyle - Double Star Section Director