Double Star of the Month - August 2013
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.
Those who have read last month's version of this column will have noted that the northern object was 26 Dra, one of Burnham's very unequal pairs. This month's selection is another from that stable, BU 648 (18 58 01.47 +32 54 05.8) can be found in Lyra, in the same low power field as gamma, and 20 minutes or so north-west of the bright star. Burnham assigned magnitudes of 6 and 9.5 to this discovery which was only separated by 0".6 at the time. The WDS gives magnitudes of 5.3 and 8.0 and it can certainly be seen with 20-cm on a good night. At the time of writing, the separation is 1".23 and the position angle 247° at mid-2013. This star has also recently been in the news because it is accompanied by a planetary object whose mass depends on which of the two stars it is orbiting. This is not known at present but if, for instance, it accompanies star B, then the mass is estimated at 1.5 Jupiters. This discovery is unusual because it was made astrometrically, rather than by radial velocity measurement, using the PHASES (Palomar High-precision Astrometric Search for Exoplanet Systems) part of the Palomar Testbed Interferometer instrument. There are 4 distant comites ranging between magnitudes 11.0 and 12.6.
H N 126 (19 04 21.53 -21 31 53.7) belongs to William Herschel's last double star catalogue which was published in 1823, a year after his death. In this case the N stands for 'New'. This 'small yellow binary' says E. J. Hartung, can be found in a 'field profusely sown with stars' and it is very close to omicron Sagittarii. Burnham's Celestial Handbook also attributes the label HU 261 to the system but it has now reverted back to its original discoverer. This is a pretty pair of long period and the two components are moving slowly retrograde. At this time the position is 186°, 1".25. The writer has not observed this pair from the UK but it should be well seen in 20-cm provided the seeing and transparency are sufficiently good at this low declination.
Bob Argyle - Double Star Section Director