February 2017 - Double Star of the Month

STF 958 Lyncis (06 48 12.23 +55 42 16.0) is a neat, bright pair in SW Lynx near the border with Auriga.

Start by locating the bright trio of 12, 15 and 14 Lyn, all visual binaries of increasing difficulty, and move 2 degrees south of the beautiful triple 12. This brings you to 13 Lyn, an orange giant of magnitude 5.3. Move a further 1.5 degrees south and you will alight on STF 958.

One of William Herschel's finds, it is doubtless a binary, and even although the movement since 1782 amounts a slight decrease only in the apparent separation, the WDS 6th Orbit catalogue contains two orbits computed for it. With magnitudes of 6.3 and 6.3 it is not clear which star is the brightest and even SIMBAD appears uncertain. The SB9 catalogue assigns spectroscopic duplicity to the A star and the period is given as 4.26 days. The WDS notes that the other component has variable proper motion, thus indicating that it is a physical quadruple system.

Herschel called STF 958 A pretty double star and assigned colours of pale rose to both components. Struve, on the other hand, found both stars yellow, as did Webb in the 1850s, whilst Sissy Haas notes that both stars are khaki-white. A 11.2 magnitude star can be found 176" away in position angle 268 degrees.

19 Pup (08 11 16.32 -12 55 37.3) is in a fairly sparse area of sky but it forms an approximate isoceles triangle with Sirius and Procyon.

It is the bright star on the south edge of the galactic cluster NGC 2539 and not surprisingly it comes with a number of faint distant companions in the WDS catalogue.

The primary is a G8 giant of magnitude 4.8 and small telescopes will easily show two distant companions, a magnitude 8.9 at 58" and a 9.3 a further 12" out from the primary.

The WDS lists two more which are fainter but Burnham in 1899 found a very faint star at a distance of about 2" which became BU 1064 AB. It was also measured by Aitken but no further sightings seem to have been made since then. Van den Bos looked twice, in 1936 and 1939 without success. Steve Coe noted 19 Pup and recorded that it was a triple star with the primary yellow and the two brightest comites being white.

Bob Argyle - Double Star Section Director