Double Star of the Month in Pavo
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Double Star of the Month - August 2011
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.
Delta Cygni (19 44 58.4 +45 07 51), like Alpha Pav, is a B subgiant, if a little later in the spectral class. Its duplicity was discovered by the elder Herschel when the separation was around 2".3. Over the next half century or so, the mag 6.3 companion moved closer to A (mag 2.9) thus making the pair more difficult for early micrometrists. Indeed Webb reports that in the mid C19 the separation was such that the B star sat on the first diffraction ring of A making it difficult to spot and some reported that it was much easier to see in the twilight sky before sunset. Reports came in of brightness variation of up to 2 magnitudes in the companion but it may well be that this was due to the large difference in magnitude and close separation. At present the distance has increased slightly since Herschel's time and the companion has traced out about 150 degrees. An orbit of 780 years period currently occupies the USNO 6th orbit catalogue and predicts 219°, 2".70 for 2012.0. Its not an easy pair for the small aperture and can occasionally evade the 20-cm user if the seeing is not at least reasonably steady. Recently, Jim Daley in the US has added four faint field stars, arranged as two 3" pairs on either side of delta and distant 42" and 148" from it.
Alpha Pavonis (20 25 38.9 -56 44 06) sits in an empty part of the southern sky but at mag 1.9 it is unmistakeable. From Feldhausen in South Africa, John Herschel examined the star and noted a distant wide pair - both stars of which are much fainter than alpha. It entered the catalogue as HJ 5193 and although there is probably no physical connection between any of the stars the system, caught the attention of the writer last year whilst using the 26-inch refractor in Johannesburg. The primary is a brilliant white, star B is reddish (John Herschel thought it `very red' and its closer companion C appeared blue - an unusual and rather patriotic combination of hues. The distance AB is 245" whilst C is 17" from B, and the magnitudes of BC are given as 9.14 and 9.17 in the WDS. It would be interesting to know if these colours are apparent in smaller apertures.
Bob Argyle - Double Star Section Director
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Double Star of the Month - August 2008
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.
95 Herculis (18 01 30.40 +21 35 44.5) is a double star much beloved of the Victorian observers, due to the suspicion that the colours of the components changed over a period of years. Hartung records pale and deep yellow, a conclusion agreed with by Frew and Malin in their revision of his book. Chambers, in his revised version of Smyth's `Celestial Cycle' notes that a friend of Smyth, a Mr. Higgens of Bedford, claimed that the intensity of the green and red colours of A and B varied from time to time and that the green star recovered its hue first. `On this statement being submitted to Sir G. B. Airy he did not view it with favour'.
It seems likely that this pair is binary - the significant proper motion in dec of A (0.039 arc seconds per year) would have carried it 9 arc seconds away from B over 230 years, whilst the separation has reduced from 9".0 in 1777 to 6".3 in 2007 with a small decrease in position angle. The revised Hipparcos parallax puts the A5 giant primary at 123 parsecs. This is one of the finest pairs in the northern sky for small telescopes.
The Australian amateur Walter Gale has his name on three stars in the WDS catalogue. Gle 3 was described in August 2007 notes and Gle 2 is xi Pavonis, (18 23 13.62 -61 29 38.1) a bright yellow giant K4 star. The revised Hipparcos parallax puts the star at 143 parsecs and the relative position of the companion has changed from 140°, 4".0 in 1894 to 156°, 3".4 in 1988. Hartung gives the colour of the companion, some 3.7 magnitudes fainter than the V=4.4 primary, as white.
Bob Argyle - Double Star Section Director
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Double Star of the Month - August 2007
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.
beta Cygni (19 30 40.29 +27 57 34.9) One of the most famous and beautiful pairs in the sky has been a favourite for small telescopes for many years but the use of modern imaging techniques has confirmed that this is a multiple system, possibly quintuple. The bright pair was certainly noted by Flamsteed in June 1691. Spectroscopy showed the primary to have a composite spectrum, a K giant combined with a late B star, whilst the wide visual companion is a B8 dwarf. This difference in spectral type explains the marked contrast in colours between the stars. Exact shades depend upon the individual but the Victorians called them topaz and sapphire whilst modern descriptions tend towards yellow and blue. Whichever applies, the pair is a magnificent sight in binoculars. The primary component was resolved in 1976 by Harold McAlister using speckle interferometry, and subsequently seen visually by Charles Worley with the 26-inch refractor at Washington. Aa is a difficult pair, since the visual magnitude difference is about 2. A recent orbit by Marco Scardia and colleagues gives a period of 213 years and the current separation is 0".37. This pair has been resolved from the UK by Christopher Taylor with a 12.5-inch Calver reflector.
In 1980 another component, closer in than a was reported by Bonneau and Foy and confirmed about a decade later but has not been seen since. There is little doubt that A and B form a very long period binary system. The Hipparcos parallaxes agree within the errors of both, placing the stars about 118 parsecs away, whilst the proper motions are similar. In 2007 February, a paper published by Roberts et al reports the presence of a faint companion to B some 4 magnitudes fainter in the I band which may be a G dwarf.
Gale 3 (19 17 12.22 - 61 39 39.7) is a bright, relatively close naked-eye star in the constellation of Pavo. It consists of two white stars of spectral types A5 and A8. The period of the pair is 156.7 years and in mid-2007 the position angle is 340 degrees and the separation 0".51 making it a good test for a 10-inch telescope. The separation increases slowly to 0".56 over the next 50 years and then closes down to 0".15 one hundred years from now.
This is one of five pairs that Walter F. Gale (1865-1945) found with an 8.5-inch With reflector from New South Wales. Gale noted the pairs in 1894 and this list appears in Astronomische Nachrichten (AN 143, 293, 1897). However, R. T. A. Innes also found two of the stars independently but somewhat later in 1894, and acknowledges Gale's contribution in his paper - `detected by Mr. Gale on his 8.5-inch, previously to my seeing them '. Since Innes published his list first (in MN 55, 312, 1895) the pairs are given the catalogue letter I rather than GLE. Ironically, Innes was using a 6.25-inch Cooke refractor of 1851 which he had borrowed from Gale!
Bob Argyle - Double Star Section Director
