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
If you'd like to try out the Clear Skies Observing Guides (CSOG), you can download observing guide for the current Double Stars of the Month without the need to register. CSOG are not associated with the Webb Deep-Sky Society but the work of Victor van Wulfen.
