September 2024 - Double Star of the Month
Just two and a bit degrees north of the Saturn Nebula (NGC 7008) is the long period binary STF 2751 (21 02 09.00 +56 40 11.1). This pair of white stars should be divided easily in 10-cm and may be split in a good 7.5-cm aperture.
Since 1828 when an early measure by the discoverer F G W Struve gave 343 degrees and 1".5 the stars have now moved on to 356 degrees and 1".6. Gaia DR3 indicates that the stars are at the same distance from us at least within the formal quoted errors. They lie 1,100 light-years away and are moving across the sky at close to 0".1 per year.
Three degrees due south of the first magnitude Achernar in Piscis Austrinus is delta PsA (22 55 56.89 -32 32 22.9) which was found to be double by Herbert Howe in Cincinnati in 1876 and is number 91 in his catalogue, although the Washington Double Star catalog (WDS) also call this system BU 772 which reflects an independent, but later, discovery by S. W. Burnham.
The WDS gives magnitudes of 4.3 and 9.3 whilst the Gaia DR3 catalogue records G magnitudes (similar to visual) of 3.9 and 9.8. As the current separation is 4".7 this is a pair which needs a night of steady seeing and, given its very low declination from the UK, probably 15-cm aperture. The stars appear to form a physical system and are 171 light-years away. Both beta PsA and gamma PsA are pairs with bright primaries and considerably fainter comites (see this column for Oct 2016 and Oct 2012 respectively).
Bob Argyle - Double Star Section Director
