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