Double Star of the Month Archive 2017

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.

March 2017 - Double Star of the Month

STF 1282 in Lynx (08 50 44.28 +35 04 15.4) is a neat pair which can be found 7 degrees north of iota Cnc and a little east. The components are magnitude 7.6 and 7.8 and in 2015 I found the relative position to be 282 degrees and 3".41.

There has been little motion between the two stars since the pair was found by F. G. W. Struve. Also called S 582. Burnham records yellowish white and very white.

A faint and distant star, mag 12.4 at 49", has a common proper motion with AB and is therefore likely to be a physical member of the system. It was found by the French observer G. Soulie. It, in turn, has a companion of magnitude 14.6 some 19" away which was found by Robo-AO, the first robotic adaptive optics system which is mounted a 60-inch telescope on Palomar Mountain.

upsilon Carinae (09 47 06.12 -65 04 19.2) is a 3rd magnitude (actually V = 2.97) star which is brilliant white and is accompanied by a mag 6.0 companion which is also white.

The separation between the two has barely changed since the early 19th century when the pairing was first noted by Rumker in Australia. It is almost certain that this is a binary system and both stars are massive, luminous and hot.

The primary is an A8Ib supergiant with an absolute magnitude of -5 giving it a luminosity of about 9000 suns whilst the companion is somewhat earlier in spectral type (B3 or B4) and is about 600 times brighter than the Sun.

Not surprisingly Sissy Haas includes it as a showcase pair and small telescopes should suffice in dividing the stars although the significant brightness difference might militate against using too small an aperture - the separation is currently 5".

A paper written in 1986 speculates that the period of the pair might be close to 19,500 years, and gives the distance as 400 pc. In 2007 the Hipparcos satellite found 440 pc but with a formal error of 54 pc. E. J. Hartung notes the pair HJ 4252 (9.3, 9.5, 303 degrees, 12") about 5 arc mins south following.

Bob Argyle - Double Star Section Director

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

January 2017 - Double Star of the Month

The two pairs selected for this month are ostensibly relatively, bright and wide pairs; easy for the small aperture, or even binoculars. In fact, both are more complex.

The northern object is S 503 in Orion (05 56 03.43 +13 55 29.7). The brightest stars are mags 6.7 and 8.4 and they are currently 76" apart in position angle 322 degrees. The WDS however lists 6 companions, most are field stars but at least one is travelling through space with S 503.

Component C (mag 11.6 at 310° and 55".3) and D (mag 8.2 at 334° and 299") both also found by South are optical and should be visible in 10-cm aperture. Willem Luyten found a 13 magnitude star at 205° and 47" (E) which has the same space motion as A and which the WDS notes is a white dwarf.

In late 2015, Marcel Fay, who has been publishing his CCD astrometry with a 283mm reflector in El Observador de Estrellas Dobles found two further stars of around magnitude 13 (F and G) at distances of 32" and 9" from A. The status of these is as yet unclear but further measures will show whether they are co-moving with A.

In 2011 Shaya and Olling published a list of widely separated pairs of stars with common proper motions. This list includes pairs such as Capella and 50 Per, and gamma and tau Persei which have almost 100% probability of being physically related.

Number 185 on this list is a wide pair of apparently unrelated stars in Columba: SHY 185 (06 36 54.07 -36 05 18.4). Star A is a G1 dwarf of visual mag 6.4. Some 288" away in PA 129 is a mag 7.3 star (B) of spectral type G0V. Both stars are close binaries of short period.

A is RST 4816 which has a period of 14 years and is currently at 99° and 0".1 according to the orbit, whilst B was found by W. S. Finsen (FIN 19) and is predicted to be 339° and 0".3 at present. The Hipparcos parallaxes for A and B agree well within the quoted errors and the proper motions are similar.

There are two other double stars to be found close by. One is UC 1454 (7.2 and 12, 181°, 33"), on the southern edge of SHY 185 and about 40 arc minutes south of the group is the fine pair BU 755 whose stars of mags 5.9 and 6.9 are now 1".5 apart and a faint third component which is mag 11.5 can be see 21" away (HJ 3875).

Bob Argyle - Double Star Section Director