Double Star of the Month in Scorpius

• September 2019 - Double Star of the Month

  STF 2744 is located in northern Aquarius (21 03 03.09 +01 31 55.9) near the border with Equuleus.

  It seems to have missed the attention of William Herschel which is surprising considering that the current orbit of 1532 years gives a separation of fully 2" for 1780 and the stars are of magnitude 6.3 and 7.0. That fact and the general run of observations plotted in the United States Naval Observatory (USNO) 6th Orbit Catalogue tend to suggest an orbit of smaller angular size and orbital period. In any case observations of position angle are almost ten degrees away from the predicted position.

  In 2014 I found it at 113 degrees and 1".4 and motion is slow so it should still be within the resolution range of 10-cm. Greater aperture would be needed to spot two faint field stars - one of magnitude 12.9 at 99 degrees and 98" and another of 14.3 at 300 degrees and 74". The A component does not appear in the Gaia DR2 catalogue, whilst the distance of B is given as 233 ± 5 light-years.

  Lambda Sco, also known as Shaula, is the brilliant white star in the tail of the Scorpion (17 33 36.52 -37 06 13.8).

  It first appeared in a double star catalogue when James Dunlop noted a magnitude 9.2 star which is currently at 330 degrees, 94" (2016). Dunlop recorded the distance as 60" but this is clearly an error. In 1897 T.J.J. See found a magnitude 14.9 star 42" distant from the magnitude 1.6 primary which has, unsurprisingly, only one observation in the Washington Double Star Catalog (WDS) since discovery.

  Slipher found the bright star to be a spectroscopic binary (SB) in 1903, which has subsequently been shown to have a 6 day period, whilst the SUSI interferometer array in Australia showed that this system rotates around another star in a period of 2.9 years. The brighter member of the SB is an early B star whilst its companion is either a massive white dwarf or a TT Tau star. The SUSI companion is another early B dwarf. Like lambda, the Dunlop companion is a brilliant white star.

  Bob Argyle - Double Star Section Director

• June 2018 - Double Star of the Month

  Easily found four degrees due west of alpha CrB, STF 1932 (15 18 20.19 +26 50 24.7) is a visual binary whose period is 203 years, so the current aspect of the stars closely resembles that at the time of discovery.

  The separation around the cycle ranges from 0".6 to 1".6 and at the moment the stars are as widely separated as they get (2018.5, 266 degrees, 1".62). The magnitudes are nearly equal (7.3 and 7.4) and both stars are F-class giving a yellowish aspect to the observed colours.

  Whilst in the area look at eta CrB (STF 1937), four degrees NNE which is now a test for 25-cm. The position angle is currently increasing 20 degrees per year and by the middle of 2018 will be at 246 degrees and 0".42.

  Beta Scorpii (16 05 26.23 -19 48 19.4) skirts the southern horizon during the short northern summer nights. It was first seen as double by Benedetto Castelli in 1627 and was later catalogued by William Herschel as H 3 7. With the two bright components of magnitude 2.6 and 4.5 separated by 13".7 and 20 degrees the pair is not difficult even low down.

  A century after Herschel, S. W. Burnham noticed a close and very faint companion to A about an arc second away and of magnitude 10. More modern measures show that this system has closed in considerably and that the estimated period is 610 years.

  Slipher found that A was a spectroscopic binary and a lunar occultation observation of A in 1976 indicated another component at a distance of 0".1, but no further observations of this pair have been forthcoming. McAlister found that C was also a close binary with a period of 39 years and a separation of about 0".1. Take into account the distant magnitude 7.5 at 519" and 30 degrees from A and this is a physical sextuple star.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - July 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.

  S W Burnham discovered 26 Dra (17 34 59.58 +61 52 28.4) using the 18.5-inch refractor at Dearborn Observatory in 1879. It was soon apparent that this was a binary system because the large proper motion of the primary star (almost 0".6 per year) was clearly shared by the faint and close companion. Along with many of the other similar pairs he discovered, Burnham substantially underestimated the brightness of the companion, and gave the magnitude of the stars as 5.5 and 10.1 is his General Catalogue of 1906. The WDS gives 5.28 and 8.54 and I saw the comes perfectly well with the 8-inch Cooke refractor at Cambridge in summer 1999. At that time the separation was 1".6 but the pair is now closing quickly and it will take a larger aperture to see them in 2013 when the separation is 0".65. The large proper motion is a consequence of the proximity of this star system. The Hipparcos catalogue gives a revised parallax of 70.47 mas which corresponds to 46.3 light years with a quoted error of 0.24 light years. To find it, draw a line between beta and nu Draconis in the head of the Dragon and extend the line twice as far again.

  See 342 (17 53 23.47 -34 53 42.5) is also a close pair requiring a reasonable aperture to resolve, but it does have the additional attraction of being embedded in the open galactic cluster M7, close to the tail of Scorpius. Since discovery in 1897, the pair has moved in retrograde fashion by 80 degrees whilst the separation has remained close to 0".4. A provisional orbit by Andreas Alzner puts the period at 700 years and its parallax places it at the distance of M7. A good chart for identifying See 342 is given in Burnham's Celestial Objects Volume 3, page 1712. It is #1 and is located at the SW corner of the cluster. Robert Burnham quotes magnitudes of 6.5 for each star whilst the WDS lists 5.85 and 7.89 which also seems too unequal. Recent measures by Tokovinin give a delta m of 0.3 to 0.4. The primary star is a K giant and a spectroscopic binary.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - July 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.

  Located close to the head of the Dragon, mu Dra (17 05 20.12 +54 28 12) is a long period binary system, first found by William Herschel in 1781. With a period of 672 years, the apparent separation of the two stars ranges from 2".0, which last occurred about 40 years ago, and 5".7. At the present time it is almost 2".4 apart at position angle 5°. With components of magnitudes 5.66 and 5.69 the star is easily visible to the naked-eye and is an excellent target for small telescopes so it is included in James Mullaney's One Hundred Showpiece Double and Multiple Star list, where he gives the colours as yellowish-white. Sissy Haas also notes the stars as goldish-white, but Smyth sees them only as white. There is evidence for a spectroscopic companion to B which may be bright and wide enough to be resolvable in the optical, and there is a mag 13.7 star at 12" may also be part of the group. The system lies at a distance of 90 light years.

  Nu Scorpii (16 11 59.27 -19 06 53) is the southern equivalent of the Double-Double in Lyra, although the brighter pair is more difficult to divide than its northern equivalent, and as a consequence at least 150-mm is needed to see the four components clearly. The wide pair catalogued by Herschel as H V 6 consists of white stars of magnitude 4.2 and 6.0 separated by 41". In 1846 Mitchel, using an 11-inch refractor in Cincinnati, resolved the companion into two stars about 1".3 apart. Burnham then discovered that the primary was also double with his 6-inch Clark in 1873 when the separation was around 0".6. Since that time both pairs have slowly widened and the current values are 1°, 1".3 for A and 55°, 2".3 for B. This is a physical system of high multiplicity since A is double again at the sub-0".1 level and there is also a spectroscopic component of 5.5 day period. The writer has measured both bright pairs from the UK with 20-cm but it needs a night with very steady air to do this.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - June 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 two systems this month are both bright and relatively easy objects to see in small telescopes now but both have highly eccentric orbits resulting in large ranges of apparent separation during the orbital cycle.

  The apparent orbit of 44 Boötis (STF 1909 - 15 03 47.68 +47 39 14.5) is a very elongated ellipse with the primary star somewhat nearer the eastern end than the western end. At

  present the companion is perched at the eastern end of the ellipse having been virtually stationary for the last few years. It will soon be closing noticeably and accelerating to pass by the primary at a distance of 0.20 arc seconds in 2019. At this point the angular motion will be very pronounced - more than 1 degree per week. The pair was discovered by Herschel in 1781 at PA 60.1 degrees but he made no note of the separation. A measure made a few weeks ago (i.e. early July 2008) showed the companion to be at precisely 60.0 degrees.

  The current orbit by Soderhjelm gives a period of 206 years. The stars are F7 and K4 dwarfs and the visual magnitudes 5.20 and 6.10. The secondary is one of the brightest W UMa eclipsing systems known with a period of 6 hours.

  xi Sco (16 04 21.63 -11 22 24.8) was found to be triple by Herschel the year after he found 44 Boo. The wide pair was separated by about 6.7 arc seconds and since then the angle has decreased 50 degrees and the separation has increased somewhat. Herschel gave no separation for AB - although his measure for PA put the pair at 188 degrees. In Lewis' book (1906) several orbits were listed, all of them with a period near 100 years and very low eccentricity. However, the equality in brightness of the two stars led to differences of 180 degrees in some position angles and it was left to Aitken to show that the period was nearer 45 years and was highly eccentric. The currently accepted period is 45.9 years and the is pair is now approaching maximum distance (1.13 arc seconds in 2021) so this is a good chance to see it with 15-cm aperture. xi Sco is missing from the Hipparcos catalogue but it is still possible to estimate its distance because it shares a common proper motion and radial velocity with the 12 arc second pair STF1999 some 5 arc minutes SE so that the whole system is quintuple. STF1999 has a parallax of 33 mas but with a significant uncertainty. The WDS gives magnitudes of 5.16, 4.87, and 7.3 for the three stars of xi Sco.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - July 2007

  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 double stars selected for this month have a number of common features. Both are relatively close-by, both have short periods, and both are visible in small to medium-sized telescopes at the present time.

  zeta Herculis (16 41 17.48 +31 36 06.8) is a famous binary found by the elder Herschel on 1782 Jul 18. Although the separations at the closest and widest points in the apparent orbit are respectively 0".6 and 1".6, the large difference in magnitude, and the glare of the 3rd magnitude primary makes the system a difficult one to measure accurately.

  In his 1906 volume, Thomas Lewis devotes five pages to the orbit of this pair which he found could only be explained if the orbital period was changing with time. This seemed to imply the presence of a third component but no such star has been found and there is no evidence from the extensive radial velocity history of this star. Recent direct measurements of the diameter of star A, a G7 giant, show that it is some 2.5 times the diameter of the Sun. The distance from Hipparcos measurements is 35.2 light years and the V magnitudes are 2.9 and 5.6. The pair, whose orbit has a period of 34.45 years, is now widening and in 2007.5 the position will be 200°.2, 1".08.

  MLO 4 = BU 416 = R 298 (17 18 56.36 -34 59 22.5) is just visible to the naked-eye on a clear night and can be found sitting some 3 degrees north-west of lambda Sco in the tail of the Scorpion. It was found to be double by Burnham with the 6-inch Clark in 1876 and it appears as BU 416 in his 1906 General Catalogue. His estimates of the magnitudes were 6.0 and 8.5. The modern measures of the magnitude difference by Hipparcos are nearer 0.9. It was also found independently by Russell using the 11-inch refractor at Sydney. According to R. T. A. Innes however, the pair was `first noted at the Melbourne Observatory in 1867', hence its catalogue name.

  The distance to MLO 4 is 22.7 light-years, although, strangely, the quoted Hipparcos parallax error is almost 12%, with the annual proper motion exceeding 1 arc second. The period of the binary is 42.15 years, according to Soderhjelm in 1999 and for 2007.5 the position is 206°.0, 1".47.

  The orange hues of each component betray the late spectral types; they are dwarf stars of class K3 and K5. There is some evidence that a more distant member of the system (31") is a M-type star. It has similar proper motion to AB so is physically connected.

  Bob Argyle - Double Star Section Director