Double Star of the Month in Lyra

• July 2025 - Double Star of the Month

  Zeta Lyrae (18 44 46.36 +37 36 18.4) is the third star in the tight, naked-eye triangle that includes Vega and the Double-Double (epsilon). It appears in many double star catalogues, including those of Herschel, the Struves and South and Herschel but according to the Washington Double Star catalog (WDS) it was found by Francesco Bianchini (1662—1729) with the date 1737 which presumably refers to a book published posthumously called Astronomicae et geographicae observationes selectas... written in Latin, but available in full on the internet.

  

  The brightest component, zeta¹ is magnitude 4.34, and is a spectroscopic binary, whilst the fainter component, zeta², is 5.62. The stars lie 157.9±0.2 light-years away according to Gaia DR3. An observation with the Cambridge 8-inch in 2010 gave 149 and 45".4 but there is very little relative motion between the two stars.

  Zeta Lyrae is resolvable in binoculars and both stars appear white. There are four faint field stars ranging from mags 13.3 to 15.7 which lie between 22" and 78", two of which were found by Burnham with the 18.5-inch at Dearborn and the faintest was detected in the Lick 36-inch.

  WNO 6 (18 28 57.36 -26 34 55.5) lies 1.5 degrees south of the 2.8 magnitude lambda Sagittarii, the orange-hued K1 giant star at the top of the 'Teapot' lid of Sagittarius. It is a fine pair for the small aperture with stars of magnitudes 6.7 and 8.0 separated by 42" with a position angle of 182 degrees. Gaia DR3 places them at significantly different distances, the A component has a distance of 639 light-years away but it has a large formal error of 24 light-years. B is 472 light-years away.

  

  It makes a tight triangle with two other stars the most north-westerly of which is BU 133, a rather difficult pair requiring at least 25-cm as the stars have passed 0".6 and are still closing, having been 1".8 apart when discovered by Burnham using his 6-inch Clark refractor. The magnitudes of 6.6 and 8.5 add to the difficulty of resolution.

  Bob Argyle - Double Star Section Director

• July 2023 - Double Star of the Month

  STF 2367 (18 41 16.36 +30 17 40.9) is a challenge to the observer with 30-cm available. It is always a difficult object throughout its 92 year orbital cycle, partly because the apparent orbit is inclined and very eccentric and so at times the separation of the stars dips below 0".02 and the angular motion amounts to 1 degree every 6 days. At apastron the stars reach a maximum separation of 0".45 which is where they are now, (actually apastron is reached in 2027), so this is a very good opportunity to resolve this difficult pair.

  

  The stars have magnitudes of 7.7 and 8.0 but another star of magnitude 8.8 is 14" distant and there are three much fainter components at 22" (mag. 12), 42" (15.1) and 152" (11). The system lies in Lyra in a region to the south-west of beta and gamma Lyrae but with no obvious pointers to aid location. There is a curve of stars nearby which can be found from gamma Lyrae.

  H 6 50 (18 49 40.96 -05 54 46.2) lies in Scutum close to the Wild Duck Cluster (M11) which is just 25 arc minutes to the south-east. The Cambridge Double Star Atlas (2nd ed.) also shows the pair STF 2391 (18 48 39.49 -06 00 15.5) nestling just 15 arc minutes to the west-south-west.

  

  H 6 50 is a very wide and somewhat unequal pair. The magnitudes are 6.2 and 8.2 and the stars are 112" apart in 2016 at PA 171 degress. A much fainter star, magnitude 12.5 was added by Burnham in 1879. It is due north and 25 arc seconds distant. A rather puzzling note in the Washington Double Star catalog (WDS) Notes identifies H 6 50 AB with STF 2391 but it is difficult to see how this can be. STF 2391 has, according to the WDS, magnitudes of 6.5 and 9.6 at 332 degrees and 38".

  Bob Argyle - Double Star Section Director

• 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

• Double Star of the Month - August 2012

  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.

  The two pairs selected this month are almost copies of one another. They have the same separation, and position angle, similar magnitudes and even the colours are close.

  Eta Lyrae (19 13 45.49 +39 08 45.5) appears as H 4 2 in the great astronomer's first list of double stars published in 1782, but it was first listed by Christian Mayer in 1779. The primary is a luminous object more than 1,000 light years away. The companion appears to be an F star and with the primary being an early B subgiant, one might have expected colours to be white and yellow when in fact Hartung records yellow and ashy. The WDS gives magnitudes of 4.38 and 8.58 with current distance and PA being 28".5 and 79°. A fainter and more distant star (mag 11.4) can be seen at a distance of 161". Along with theta, also a wide, unequal pair, it follows Vega by about 7 degrees.

  Beta1 Sgr (19 22 38.29 -44 27 32.1) is not only not the second brightest star in Sagittarius but it is actually about 12th in the list. The naked eye can make out both beta 1 and its close neighbour beta 2 (mag 4.27) some 20 arc mins away. Dunlop listed beta 1 as a telescopic double in 1826 and and since that time the position angle has increased slowly to 76° and the separation decreased slightly to 28".7. Hartung gives the colours as pale yellow and ashy white - the spectral types are actually B8V and F0V perhaps reversing what one might expect from the colours. However, Ross Gould using a 175-mm refractor disagrees with this judgement and gives the colours as white and yellowish. The stars are magnitudes 3.98 and 7.21 and again this is an optical system. Beta1 lies in the far south-western part of Sagittarius, close to the border with Corona Australis and Telescopium.

  Bob Argyle - Double Star Section Director