Double Star of the Month in Virgo

• March 2025 - Double Star of the Month

  The spectral type of the primary of STF 1689 (12 55 30.32 +11 29 46.4) is given as M4III in the Washington Double Star catalog (WDS). One might expect to see a deep-orange, or red star but observations of the pair in the literature seem to indicate that the primary colour reflects an earlier spectral type.

  T. W. Webb notes yellowish, whilst Admiral Smyth recorded pale white. All three observers noted that the companion was blue or bluish. More recently, I recorded orange and blue using a 10-inch reflector at x96 in 1970. Sissy Haas gives peach-white, and a note on TheSkySearchers website records colours of orange and bluish.

  

  Gaia has observed both components and the stars are unrelated. The primary is 960 light-years away whilst B is 474 light-years distant. The components are slowly separating due to the different proper motions and in 2019 the relative position was 222 degrees and 30".3. The pair can be found 2 degrees NNW of epsilon Virginis.

  R 213 (13 07 24.30 -59 51 37.8) was discovered in 1874 by Henry Chamberlain Russell. Russell was the Government Astronomer at Sydney Observatory from 1870 until 1905. He was using a 11.5-inch refractor made by Schröder to carry out a survey of some of the John Herschel double stars which the latter discovered at the Cape. It was one of the most difficult of his discoveries being found at 207 degrees, 0."3. His description of the stars was a very beautiful double star - both alike and orange coloured.

  

  When I observed the pair in 2016 using the 67-cm refractor at Johannesburg, the PA was very similar but the distance has increased to almost 0".8. With WDS magnitudes of 6.6 and 7.0, the pair should be divided in 15-cm. R 213 lies in Centaurus close tothe border with Crux and can be swept up by moving 2.5 degrees due east from beta Crucis (Mimosa), which is DUN 125 - magnitudes 1.3, 7.2, 23 degrees, 373" - a fine binocular double.

  Bob Argyle - Double Star Section Director

• April 2022 - Double Star of the Month

  The two pairs in this month's column have a common feature. Each is accompanied at a distance by a ninth magnitude companion.

  STF 1781 (13 46 06.75 +05 05 56.1) is a visual binary with a period of 179 years which is 206 light-years away, according to Gaia EDR3. It can be found in Virgo, near the border with Boötes, about 2 degrees NE of 84 Vir, itself a double star (5.6, 8.3, 227 degrees, 2".7).

  

  Although the components of STF1781 are rather faint (V magnitudes are 7.9 and 8.1) the current position, 200 degrees and 1".1, indicates that the stars should be resolvable in 15-cm aperture. They are currently closing and by 2050 they will be separated by 1".2. I have not observed this system - in the late 1970s the stars were less than 0".3 apart and have been widening ever since. More recently Dr. Andrei Tokovinin found a common proper motion companion of magnitude 9.8 at PA 128 degrees and distance 1594".

  The primary of JC 16 (11 29 38.35 -24 27 50.6) is just above naked-eye visibility at V = 5.8. Eight arc-seconds away in PA 82 degrees is the B component of V = 8.6. The catalogue number tells us it was discovered by Captain Jacob from India in 1847. He also added a third star of V = 8.9 at 120".

  

  The Washington Double Star Catalog (WDS) notes that of 2018 this had increased to 166", but it is difficult to reconcile this large change with the very similar proper motions that all three stars have. In fact Gaia EDR3 gives a significantly different parallax for B than it does for A and C.

  The coarse triple is a fine sight in small apertures.

  Bob Argyle - Double Star Section Director

• June 2021 - Double Star of the Month

  About three degrees north of zeta UMi (magnitude 4.3) is a group of naked eye stars, the most southerly of which is the binary pi¹ UMi = STF 1989.

  

  The brightest star in the group is pi² (magnitudes 6.6, 7.3) which is also a pretty, wide pair STF 1972 (15 29 11.19 +80 26 55.0). (This pair is listed in the notes of the second edition of the Cambridge Double Star Atlas (CDSA2) but not identified with its Struve number on page 1 of the Atlas). CDSA2 notes doubles 6' south-following and 10' north. William Herschel found the pair in 1782 when the separation was 26", whilst a recent measure in 2018 gives 32".

  This is actually a physical quadruple system as both bright stars are 71.1 light-years away and each is also a spectroscopic binary. A recent deep imaging investigation, looking for exoplanets has also noted two twentieth magnitude objects within a few arc-seconds of B but the single epoch of observations does not indicate whether they are co-moving or connected to B. The relatively large proper motions of the bright stars has increased the distance to a 11.4 magnitude field star by 55" since 1911.

  The southerly objects chose for this month's column are just south of the celestial equator, in the constellation Virgo. In 1781 William Herschel noted a wide unequal pair which appears as H 6 51 (14 57 29.32 -00 11 05.74) in his second list of double stars, published in 1784. He records the position as in Monte Maeneli Heveliana. In the Hevelius star atlas, Boötes appears to be standing on ground called Mons Maeneli or Mount Maenelus which was a constellation created by Hevelius in 1687.

  

  The Washington Double Star Catalog (WDS) catalogue gives V magnitudes of 5.6 and 10.4 with position angle 224 degrees and separation 86". The primary has a strong orange hue, a consequence of its spectral type of K1III. The SIMBAD catalogue calls the bright star 1 Ser and rather surprisingly Gaia EDR3 shows that both stars have the same parallax within the quoted errors, showing that they are 322 light-years away.

  Nearby (just over one degree due east) is the much more difficult BU 348 (2 Ser - 15 01 48.92 -00 08 24.9) which requires 30-cm to resolve clearly, although S. W. Burnham discovered it with his 6-inch Clark refractor. Here the stars are magnitudes 6.1 and 7.5 and the current separation is only 0".5. If this is not challenge enough, try and see the 14.5 magnitude star C, 37" distant, which was found by Burnham in 1899. The close pair has a parallax of 3.252 mas corresponding to 1002 light-years.

  Bob Argyle - Double Star Section Director

• April 2019 - Double Star of the Month

  This month's pairs consist of a very easy pair in the southern hemisphere, and a much more difficult one in the north.

  When I used the 28-inch refractor at Herstmonceux in 1970, I observed STF 1606, an orbital pair in Canes Venatici (12 10 47.34 +39 53 29.5), which was found at a separation of 0".41. The pair then closed to 0".29 in 1990 and is now opening.

  For 2019.0 the position angle and separation will be 141 degrees and 0".6, so it should be just resolvable in 20-cm and I will look forward to seeing this pair as double for the first time in 49 years.

  It sits in a little group of three Struve pairs which also includes STF 1622 (see the column for April 2012) and STF 1624. The group is four degrees preceding and slightly south of beta CVn. STF 1606 is also practically coincident with NGC 4145, abarred spiral galaxy of V=11.3.

  In 2011, Shaya and Olling published a paper in Astronomical Journal in which they identified over 800 very wide pairs which they concluded were physically connected. From that list, number 588 (SHY 588) (12 02 39.44 -10 42 48.9) is a pair of stars with V magnitudes of 7.5 and 8.6. The current separation is 331" at PA 115 degrees.

  Gaia DR2 indicates that the brighter star has a distance of 177.6 light-years whilst the fainter is 186.8 light-years distant. The proper motions are similar but the difference in distance is supiciously large for them to constitute a binary, although Shaya and Olling used the Hipparcos data which actually suggests that the two stars are further apart in distance than does DR2. The telling factor may be the quoted error on the parallax of A which is seven times that on the parallax of B and suggests higher multiplicity.

  For the binocular user this is an easy pair and the field is enlivened by a V=8.5 star some 19 degrees and 262" distant from A. DR2 indicates that this is more than three times more distant than the SHY pair and therefore unrelated.

  Bob Argyle - Double Star Section Director

• July 2017 - Double Star of the Month

  The northern star (STF 1575, 11 51 57.57 +08 49 48.0) is one of the few doubles to be found in Virgo north of the equator. It is a William Herschel discovery (H 4 49) and since 1782 there has been little change in separation which is currently 30".4.

  Hipparcos demonstrates that both stars have similar parallaxes and proper motions and Shaya and Olling in their 2011 paper have indicated that there is a near 100% chance of the stars being physical.

  The magnitudes are 7.4 and 7.9 and the pair can be found a little more than 5 degrees due south of Denebola (beta Leo) and it forms the vertex of an isoceles triangle of stars which include 4 and 6 Virginis.

  Thomas Lewis (1906) incorrectly calls this pair H III 51 and notes both components are white, which are the colours derived by F. G. W. Struve. In fact both stars have spectral type K0. Richard Harshaw using 20-cm finds orange and white.

  About 1.3 degrees below the bright star Kaus Australis (lambda Sgr) there is a triangle of 5th and 6th magnitude stars. Each of these is a visual pair which can be seen in 15-cm. At the peak of the triangle is WNO 6 (18 28 57.76 -26 34 55.0) with magnitudes of 6.7 and 8.0 and which appears to be unchanged in over 100 years. The writer found 182.0 degrees and 41".83 in September 2016.

  About 15 arcminutes preceding are the other two members of this triangle.

  The northernmost star is BU 133 found by the great Americam observer on 1873, July 6 using his famous 6-inch Clark refractor. He estimated that the stars were both 7.5 magnitude and indeed all the early estimates show no difference in brightness between the stars and yet the WDS gives 6.6 and 8.5. The writer recently measured it (September 2016) and found 233.2 degrees and 0".77 which would make the pair a stiff test in 15-cm.

  The remaining star is magnitude 6.7 and has a magnitude 8.7 companion at 54" and PA 135 degrees. This component was mentioned by Burnham and subsequently ignored.

  Bob Argyle - Double Star Section Director

• May 2016 - Double Star of the Month

  Virgo straddles the celestial equator and this month two pairs are included from this constellation - one north and the other south of the zero line of declination.

  The primary of STF 1764 (13 37 44.01 +02 22 56.5) is a K2 giant and the note in Sissy Haas' book says that the stars are yellow and blue and that the colours are vivid. This system has been left out of Hartung and the Rev. Webb dismisses it with the comment yellow and ash.

  The pair can be found about 3 degrees north and slightly following zeta Virginis. It forms an equilateral triangle with 84 and 78 Vir. There has been very little change since discovery. The stars are mags 6.8 and 8.6 and last year the writer measured the pair and found PA 32° and separation 16".2. The distance of A has been measured by Hipparcos but the resulting large value (2038 light years) is very uncertain. There are two further and fainter companions 10.4 at 139° and 172" (C) and D 10.7 at 143° and 207" (D), which together form the pair STF 1765.

  SHJ 162 (13 14 55.85 -11 22 07.3) misses the cut in both Webb and Hartung but Haas was obviously impressed by the colours of the two wide stars A and B - white and pale red. John Nanson, however, using a 6-inch f/10 lens and x84 in 2011, noted that they were yellow and white.

  It was measured by James South on 1823 May 7 using his 5-foot equatorial when the distance of AB was 45". The writer has not observed this pair but apart from AB there is another star in the system, close to A which was discovered by Richard Rossiter from Bloemfontein in 1937 using the 27-inch refractor. This has turned out to be a binary of period 122 years and in mid-2016 the companion (a) is at 161° and 0".55. It is about 1.5 magnitudes fainter than A but might be seen in 30-cm on a good night. The rapid proper motion of Aa (0".37 per year - distance 128 light years) is leaving behind star B which is now 112" away. There is a star of mag 13.3 at 67". The system is 3 degrees directly preceding Spica.

  Bob Argyle - Double Star Section Director

• June 2014 - Double Star of the Month

  STF1883 (14 48 53.22 +05 57 15.9) is in Virgo near the northern border with Boötes. In `Celestial Objects' however it appears in Boötes. In Thomas Lewis' work on the Struve stars he gives the magnitudes of both stars as 7.0. The WDS however lists 7.02 and 8.95. The current value for B must be somewhere between these limits as the writer has resolved this pair with the 8-inch Cooke at Cambridge which would have been much more difficult if B was near magnitude 9. The WDS notes than one star is variable - this is presumably B and the amplitude must be considerable if the stars were deemed to be equally bright a century ago. The star has only the designation SV ZI1089 and does not appear to be a fully-fledged IBVS variable star. STF1883 is a binary star of period 216 years which has a highly inclined and eccentric orbit. Near 1".2 at discovery in 1830, it closed to around 0".25 in the early 1930's before widening to 1" where it is today. This value will not increase very much before the stars begin to close again in about 30 years time.

  HJ 4788 is d Lupi (15 35 53.25 -44 57 30.0) which can be found as one of triangle of naked-eye stars some three degrees following epsilon Lupi. This pretty pair consists of pale and deep yellow components in a well-occupied field (according to E. J. Hartung). The stars are mags 4.60 and 6.51 and the separation has slowly decreased from 3".1 in 1836 to 1".9 in 2013 whilst the position angle has increased from 349° to 13° over the same time interval. The primary is an early B-type dwarf which is 428 light years away and the WDS notes it is a spectroscopic binary.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - April 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.

  Although not strictly a northern pair, gamma Virginis (12 41 40.0 -01 26 58) is one of the most spectacular pairs in the sky and the recent close approach, the first for almost 170 years, excited some interest in double star aficionados. The pair was certainly noted by Bradley in 1718 when the separation was about 6 arc seconds, the stars closed slowly until the 1830s when the motion accelerated considerably. It attracted the attention of Sir John Herschel who applied the new science of orbital analysis to the pair but his first attempt did not represent the observed motion. By 1835 the separation was down to 1 arc second, and in the UK Dawes and Smyth made measurements of the pair, whilst F.G.W.Struve, using the 9.6-inch Fraunhofer refractor at Dorpat also followed events. In early 1836, Herschel found the star single with the 20-foot reflector at the Cape and in the spring Struve found an elongation, giving a separation of 0.25 arc seconds. The pair rapidly widened from then on reaching 2 arc seconds by 1843. Since the recent closest approach of 0.37 arc second in mid-2005 the separation has now increased to 0.74 arc second at PA 53 degrees (2007.3) with the angular motion about 2 degrees per month at present. Both stars are F0 dwarfs and appear yellow to the telescope user. A 6-inch telescope should almost resolve them, while an 8-inch will definitely do so.

  The small and bright constellation of Crux bestrides the meridian in southern latitudes during the late evening in April. The brightest star, alpha or Acrux, (12 36 35.9 -63 05 57) is one of the most spectacular pairs in the sky and is accessible to small telescopes. Discovered by Jesuit priest-astronomers in Siam in 1685, it was measured by Dunlop in 1826 who found a separation of 5.4 arc seconds and a PA of 86 degrees. There has been a small decrease in distance to 4.0 arc seconds and an increase in PA to 114 degrees at the time of writing. The two stars form a binary pair and the bright 5th magnitude star HR 4729 some 90 arc seconds away also shares in the proper motion. According to Andrei Tokovinin, all 3 stars are spectroscopic binaries and coronagraphic imaging of the distant companion shows three very faint stars close to C of which one, some 2 arc seconds distant may be physical. If so, then Acrux is a septuple system. The three visual components of Acrux are luminous B stars and therefore appear blue-white in the telescope. The Hipparcos satellite determined a parallax for alpha¹ equivalent to a distance of 321 light years with an uncertainty of about 21 light years.

  Bob Argyle - Double Star Section Director