Double Star of the Month in Ursa Major

• March 2018 - Double Star of the Month

  Tucked away is an obscure part of Ursa Major is the red dwarf binary STF 1321 (09 14 22.79 +52 41 11.8). Containing stars of magnitudes 7.8 and 7.9 it sits about half a degree west of the centroid of a triangle of stars formed by 15, 18 and θ UMa.

  Although discovered by Struve almost 200 years ago the orbital motion has amounted to only 50 degrees or so and the projected period is 975 years. Both stars are a distinct yellow colour and currently separated by 16".8 in position angle 99 degrees.

  This is one of the nearest stellar systems to the Sun. Hipparcos measured both stars and came up with a distance of 19 ± 0.6 light years for the A component but most recently an interim measurement from Gaia gives a distance for the B star of 20.52 ± 0.05 light years.

  The Hipparcos parallax error is exceptionally large and implies there may be underlying structure. Both stars have been suspected of being spectroscopic binaries but this was disproved, at least at the 0.1 km per second level by Morbey and Griffin. Further searches for faint companions have so far revealed nothing. It will be interesting to see what Gaia finds for the A star - the fact that the parallax has not yet been published may be revealing in itself.

  The pair move across the sky at more than 2.5 arc-seconds per year and is fast approaching two faint companions found by Ball (mag. 11.9) and Espin (mag. 14.5).

  BSO 18 in Vela (08 42 25.41 -53 06 50.5) contains two stars bright enough to be in the HR catalogue, the primary HR 3467 of magnitude 4.8 and HR 3466 of magnitude 5.6, 76 arc-seconds distance in position angle 311 degrees, so this is a fine pair of white stars for the binocular user.

  It is easy to find as it is 25 arc-minutes south following the bright star o Velorum (mag. 3.6) which is itself embedded in the galactic cluster IC 2391, so the whole area is a spectacular telescopic view.

  B in turn has a magnitude 9.9 star (D) at 266 degrees and 60 arc-seconds. The bright components of BSO 18 share the proper motion of the cluster and are both at a distance of about 500 light years.

  In 1929 W. H. van den Bos found a close companion to the B component some 2.5 magnitudes fainter and 0.5 arc-seconds away. Since then the distance has increased only slightly and the position angle has increased by 40 degrees to 153 degrees. This pair (B 1625 BC) was last measured in 1991 and poses a challenge for 30cm.

  Bob Argyle - Double Star Section Director

• April 2015 - Double Star of the Month

  STF1555 (11 36 17.94 +27 46 52.7) is in Ursa Major in a fairly sparse part of the constellation down by the Bear's front foot. It is perhaps most eassily found by moving 5 degrees south-east from the bright binary xi UMa (STF1523). This was an easy pair for the small aperture at discovery at 1".4 but during the next one hundred years the two stars approached each other until minimum separation of about 0".1 was reached in the 1930s and since then they have been separating. The nature of this system is a little unclear but observations over the next decade or so will show whether it is an optical pair (as classified in the WDS, or a highly inclined binary system, as suggested by Docobo in 2007 when he derived an orbit of 916 years for it. The orbit predicts star B beginning to turn back towards A with the separation slowly decreasing again. In spring 2015 B can be found at 150°, 0".67. The stars are magnitudes 6.4 and 6.8, and a third component of magnitude 11.2 which is listed as HJ 503, can be seen at 158° 22".5, both values are slowly increasing.

  RMK 14 (12 14 02.71 -45 43 26.1) can also be found by reference to a nearby bright binary star. It forms an isoceles triangle of side about 5 degrees with gamma and delta Centauri to the south. Unfortunately, as of early 2015, gamma is near closest separation and needs a large aperture to resolve but RMK 14 (D Cen) is a beautiful pair which is worth searching out. The primary is a K3 giant of visual magnitude 5.8 and is also known to be a spectroscopic binary. The companion can be found at 243° and 2".7 having closed up from 4" at discovery. This is a distant pair - Hipparcos lists the parallax as 5.71 mas which, as it happens, translates to 571 light years. The colours seem to be well determined. E. J. Hartung gives orange and white whilst more recently, Richard Jaworski finds yellowish-orange and white. Sissy Haas notes it as a showcase pair.

  Bob Argyle - Double Star Section Director

• May 2014 - Double Star of the Month

  The northern target for this month's column is one of the most popular of all double stars - zeta UMa (13 23 55.42 +54 55 31.5). Known equally well as Mizar, this beautiful pair of early A stars has the longest history of all telescopic pairs. First seen by Castelli in 1617, the two stars, whose magnitudes are 2.2 and 3.4 and which are separated by 14".5 were also the first double star to be photographed by Bond in 1857, and Mizar A was the first star to be shown to be a spectroscopic binary in 1889. Mizar A was also high on the list of pairs observed by Michelson with the interferometer on the 100-inch telescope at Mount Wilson in the 1920s, and was duly resolved. Some 706" distant is Alcor (mag 4.0) which forms a naked-eye pair with the Mizar system. It seems likely that the three stars are moving through space together in common with other members of the UMa moving group. In addition to Mizar A, Mizar B is also a single-lined spectroscopic binary and Alcor was also suspected a number of times of being a close binary but no proof has yet been put forward. There is, however, a faint, low-mass star only 1".1 distant from Alcor, discovered in 2009 which does seem physically connected, making the whole group a sextuple system.

  Another easy pair of B stars can be found in 3 Cen (13 51 49.58 -32 59 38.6). Discovered by William Herschel it was also picked up by James Dunlop in Australia and appears in his catalogue as Dun 148. With 32.5-cm Gould finds the colours are pale yellow and white. The stars have closed slightly since discovery and are now separated by 7".8 in 104° as measured by the author in 2013. The parallax of A, determined by Hipparcos, puts the star at a distance of 343 light years and the system is part of the Sco/Cen moving group, as is the neighbouring 4 Cen which is barely 1° north following. Another Herschel discovery, in 2013 these stars were 14".8 apart in PA 185°. Hartung notes the colours as pale yellow and ashy. Hartung notes both stars are spectroscopic binaries but the WDS only mentions the duplicity of the brighter component which has a period of 6.93 days.

  Bob Argyle - Double Star Section Director

• April 2014 - Double Star of the Month

  STT 235 (11 32 20.76 +61 04 57.9) is in UMa close to the bowl of the Big Dipper and about 5 degrees slightly south preceding alpha UMa. The pair has a period of 72.7 years and is presently opening, reaching maximum separation of 1" in 2027. At the time of writing the stars are separated by 0".88 so this is a good opportunity to resolve this pair. The components have visual magnitudes of 5.7 and 7.6 so pick a night when the seeing is good and use at least 20-cm, although 15-cm, if the optics are particularly fine, would probably show the object as double. The star appears in the Hipparcos catalogue as HIC 56290 and it has an annual proper motion in declination of about 0".1 towards the south. The mag. 11.3 star some 195" away would seem to be travelling through space with a similar motion, and was noticed by Helmut Abt. STT 235 has a parallax of 35.73 mas putting it at a distance of 91 light years.

  Far down in the southern sky, epsilon Cha (11 59 37.58 -78 13 18.5) is the brightest member at the centre of a small cluster of stars some 111 pc distant. Its nearby co-moving companion, HD 104237 (mag 6.6) is also called DX Cha and is the nearest Herbig Ae star. This is a stellar quintet with most of the companions being very young stars. In 1836, John Herschel divided eps Cha itself into two components 1".6 apart, and the pair is known as HJ 4486. The WDS gives magnitudes of 5.3 and 6.0 but orbital motion has taken the fainter star to within about 0".4 of A.

  The author made a measure of this pair from Johannesburg in 2008 and obtained 210° and 0".37 very similar to the last measure in the WDS dated 1997. A substantial aperture will be required to see this pair and it would be interesting to have a confirmatory sighting.

  Unlike many of the stars in the cluster and a wider association which are spectral class M, eps Cha is a late B star.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - May 2010

  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.

  78 UMa (13 00 43.59 +56 21 58.8) was found to be double by S. W. Burnham in 1889 using the then new 36-inch refractor at Lick Observatory. The pair can be found just following epsilon UMa. Although the stars are very unequal in brightness (the WDS gives 5.02 and 7.88 and Baize and Petit list the system in their catalogue of double stars with variable components), it was apparent from the proper motion of A and the relative motion of B that the pair formed a binary system. A recent orbit gives a period of 106.4 years but at no point in its apparent orbit can the pair be called `easy'. On good nights with the 8-inch OG at Cambridge the companion was measured several times in the last two decades but since then the separation has reduced and in mid-2010 the companion can be found at 99 degs, 1".18. The motion is relatively slow however with minimum separation of 0".48 not occurring until 2026. Sissy Haas notes an observation with 275-mm which gives colours of gold-white and bluish-turquoise.

  Another double star with a variable component is the wide southern system alpha Circini (14 42 30.69 -64 58 28.5). Catalogued by Dunlop (Delta 166) this beautiful pair is one of the finest systems in the southern hemisphere. The primary is a member of the rapidly oscillating peculiar A stars which has a basic period of 6.8 minutes and an amplitude of a few millimagnitudes. Other frequencies of oscillation have also been found. The visual magnitude of A is 3.18. B (mag. 8.47) is a K0 dwarf now separated some 15 arc seconds from A and the two stars form a very long period binary system since the proper motions of both are substantial and identical. Hipparcos places the stars at a distance of 54 light years. Since 1826, the companion has widened from a distance of 10" and the position angle has decreased by 38 degrees to about 225 degrees now. Several observers make the colours yellow and red including Hartung some 50 years ago and more recently Richard Jaworski, both from Australia.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - March 2010

  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 subjects of this month's column have several things in common. Both are binaries with unequal components, both contain F stars and in each case both contain a variable component according to the 1989 paper by Baize and Petit.

  sigma2 UMa = STF1306 (091023.53 +670803.3) lies in NW UMa not far from the galaxies M81 and M82. The apparent orbit of this 1140 year system shows that at closest approach the stars were about 1"1 apart in PA 153 degrees as happened in 1912. With the magnitudes of 4.87 and 8.85 this would have been a difficult object. Since then the pair has continued to widen and the current position is 350 degrees and 4.15 arc sec. Widest separation occurs around 2520 when the pair are 11.3 arc sec apart. Suspicion of variability of B was voiced by Webb who noted that both Sadler and Dembowski had recorded this, the latter giving the range of B as 8.0 to 10.0. Smyth gives colours of flushed white and sapphire blue whilst Webb noted greenish and orange. The distance to this system is 66 light years.

  psi Velorum = Copeland 1 (093041.97 -402800.2) lies on the Vela/ Antlia border. A close and occasionally very difficult binary of short (33.95 years) period it escaped the attention of John Herschel (it was only 0".3 in 1835/6) and was discovered by Ralph Copeland, later Astronomer Royal for Scotland, in the early1880s whilst separated by 1". One of the brightest systems in the sky, with components of magnitudes 3.91 and 5.12, psi is now widening and offers a

  chance, in the next few years, for those with small to medium apertures to see it divided. The ephemeris is as follows:

  • 2010.0 101.3 0.85
  • 2012.0 110.1 1.00
  • 2014.0 117.0 1.08
  • 2016.0 123.2 1.11
  • 2018.0 129.4 1.08

  The stars are both subgiants of spectral type F0 and F3 respectively and the distance to this system is 61 light years.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - April 2008

  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 binary stars being highlighted this month have several common traits - they are both bright and somewhat unequal in magnitude and can be easily seen in small telescopes. But whereas xi UMa has been measured almost 1700 times beta Muscae, although no less attractive an object, has but 78 measures in the WDS reflecting the concentration of effort on binaries in the northern hemisphere.

  xi UMa (11 18 11.24 +31 31 50.8) is one of the best-known systems in the northern sky. Found by William Herschel in 1780, it became the first pair to submit to the science of orbital analysis by Savary in 1828. Later on, first A and then B were found to be spectroscopic binaries with periods of 1.83 years and 3.98 days respectively and about 20 years ago, speckle observers noted indications of a 5th component, attached to B. This is clearly a very difficult object as it has not been seen since 1994. The multiplicity of xi has clearly caused problems with Hipparcos as the system is missing from the Hipparcos catalogue. To the small telescope the stars appear yellowish orange and the 59.9 year orbit is now currently taking the stars further apart. In 2008, B can be found 1.63 arc seconds distant from A in PA 223°.

  beta Muscae = R 207 (12 46 16.87 -68 06 29.1) was discovered by Russell in Sydney in 1880 when the position angle was 317° and the separation 0.54 arc seconds. Since then it has been closing again and in 2008 can be found at 48° and 1.27 arc seconds according to the 383 year orbit calculated by R. R. de Freitas Mourao in 1964. The stars are both white with the primary star being an early B-type dwarf. Hipparcos puts the distance at 340 light years whilst the WDS gives magnitudes of 3.52, 3.98, both some 0.4 magnitudes fainter than the V magnitudes given in Hipparcos.

  Bob Argyle - Double Star Section Director