Double Star of the Month in Ophiuchus

• July 2022 - Double Star of the Month

  Delta Serpentis (15 34 48.14 +10 32 20.0) is a bright, easy, and attractive pair in small apertures. It lies in a rather undistinguished part of the sky below Corona Borealis and a quick way to find it is to locate alpha CrB and then move 16 degrees south in declination.

  

  Since it was found by Herschel in 1782, when he noted that it was a fine double star with colours of white and greyish, it had been slowly widening until about 2020 since when it has begun to close.

  Looking at the apparent orbit, there is no question that this is a binary system and yet Gaia EDR3 puts the stars at 302±14 and 172±1 light-years respectively for the primary and secondary components. Neither of these stars appears in the Hipparcos catalogue and the large error for the primary star suggests that it may be a system of higher multiplicity. The plot of the apparent orbit in the Washington Double Star (WDS) Sixth Orbit Catalogue shows rather large errors on the earlier observations of this pair which are difficult to reconcile with the ease of measurement that such a system should present, again possibly evidence for an oscillation in the movement of one of the stars.

  I have made eight sets of mean measures since 1993 and the separation has ranged between 3.94 to 4.43 arc-seconds. The orbit gives 171 degrees 4".0 for mid-2022.

  H.F. Donner was a student at the University of Michigan when he was sent out to South Africa to assist Robert Rossiter with the survey for southern double stars at the Lamont-Hussey Observatory in Bloemfontein using a 27-inch refractor. He stayed for 6 years and accumulated a total of 1030 new pairs, many of which are faint and close. Xi Ophiuchi (17 21 00.37 -21 06 46.5) is number 832 in his catalogue of discoveries.

  

  The primary magnitude is V=4.4 and companion V=8.9. This is a difficult pair for the small aperture and probably needs at least 15-cm to see distinctly. The current separation is only 1".7 with the corresponding position angle 20 degrees. A orbit was calculated with a period of 421.5 years which is clearly rather premature since the motion since discovery amounts to only 40 degrees.

  There is a very faint star at 261 degrees, 11" which is K=13.7 and was discovered during an imaging survey for exoplanets with the 200-inch reflector, but it remains unconfirmed and there is no subsequent astrometry.

  Bob Argyle - Double Star Section Director

• July 2021 - Double Star of the Month

  On summer evenings the beautiful binary star 70 Ophuichi (see this column for July 2008) is well placed for observation. It is part of a triangle of naked-eye stars which also include 68 Oph (= H VI 2), a wide and unequal Herschel pair) and 67 Oph (= BU 1125), a very unequal and close Burnham pair which is now closing. Draw a line from 70 through 68 and extend it by the same distance and you will alight on STF 2244 (17 57 04.32 +00 04 00.0), also a close binary, but more tractable than BU 1125 for the observer with moderate aperture.

  

  The period is 475 years and in mid-2021 the separation is 0".69. The stars are magnitudes 6.6 and 6.9 so 20-cm should be sufficient to see them separated. By 2027 they will start to slowly close again so this is a good opportunity for observation. Note that if you are using the second edition of the Cambridge Double Star Atlas, the star appears as STF 2224. Gaia EDR3 offers no up-to-date information on parallax, but Hipparcos found a distance of 386 light-years but with an uncertainty of 33 light-years.

  The components of kappa CrA (alias DUN 222 - 18 33 23.13 -38 43 33.6), on the other hand, probably do form a physical system. The stars, of magnitudes 5.6 and 6.2, are currently 20" apart and lie almost in a north-south direction. When James Dunlop discovered this pair they were closer to 30".

  

  The proper motions from Gaia EDR3 show that the stars are moving in similar directions through space, the radial velocities are very close, and the parallax measurements from the satellite tell us that both stars are about 717 light-years away. Both components are white, being late B in spectral type. The system lies close to the 'Teapot' of Sagittarius and can be found 5 degrees south and slightly east of epsilon Sgr (V = 1.9).

  Incidentally, epsilon Sgr has a companion of magnitude 14.3 - discovered by T. J. J. See in 1896 which has been measured only four times since 1896. The current distance is 39".

  Bob Argyle - Double Star Section Director

• July 2020 - Double Star of the Month

  

  Mira was not the first Long Period Variable to be shown to be a visual binary. T. H. E. C. Espin found X Oph (18 38 21.13 +08 50 02.6) in 1886 and in 1900 William Hussey, observing at Lick Observatory, noted that the star was a close pair and catalogued it as HU 198. On several nights with the 36-inch refractor he saw two equally bright stars at a separation of 0".2. Later spectroscopic observation showed that the star spectrum was composite with a K giant combined with an M6 giant. George van Biesbroeck, when observing X Oph visually noted its very deep orange colour.

  Since then the stars have separated somewhat and in 2018 the companion was found to be at 126 degrees and 0".5. This fact, and also that X Oph has only a 3.3 magnitude amplitude, and the companion is significantly brighter than Mira B makes this star easier to resolve than Mira, although at least 30-cm would be needed, preferably.

  An ephemeris for X Oph gives a maximum brightness on Feb 5, 2020 and Jan 8, 2021 so during July 2020 the variable should be near minimum and therefore helps visual resolution. It is also 10 degrees higher in the sky than Mira. X Oph can be found 4 degrees due north of the open cluster IC 4756 or 18 degrees due west of Altair. According to David Boyd of the BAA Variable Star Section, the primary ranges from V = 6.5 to 9.8 whilst the B star is magnitude 9.0. Gaia DR2 gives a parallax for the variable of 4.66 ± 0.30 mas or 700 ± 45 light-years but there is no entry for the companion.

  

  SEE 316 (17 00 26.96 -48 38 52.2) is in northern Ara and lies 2.5 degrees due east of DUN 211 which was described in May's column.

  A discovery of T. J. J. See, this fine pair can be well resolved in 15-cm. The stars are magnitudes 6.3 and 7.7 according to the Washington Double Star Catalog (WDS) and when the writer last measured them in 2016 the position angle and separation were 173 degrees and 1 arc-second. This represents a small increase in angle and a doubling of separation since the first observation in 1897.

  Hipparcos gives a distance of about 350 light-years but with a significant error. Surprisingly, the stars do not appear in the Gaia DR2 catalogue even though significantly closer pairs are included. E. J. Hartung noted that the field is rich in faint stars.

  Bob Argyle - Double Star Section Director

• July 2019 - Double Star of the Month

  STF 2398 (18 42 46.69 +59 37 49.4) is a pair of M dwarfs whose large proper motion has been known for well over a century. It was inevitable that the stars would be shown to be close to the Sun, and such indeed has proved the case.

  Gaia DR2 puts the primary star at 11.487 light-years whilst B comes out at 11.490 light-years, with formal errors of 0.002 and 0.004 light-years respectively. The large proper motion of 2.3 arc-seconds per annum means that two fainter and unassociated field stars (C = mag. 12.2 at 158 degs, 215", 2000) and D (13.5 at 110 degs, 100", 2008) are being rapidly left behind.

  The current orbital period is 408 years and in mid-2020 the companion can be found at 182 degrees, 10".9. This pair needs 20-cm to see well, but the writer has yet to measure it with the Cambridge 8-inch as the red field illumination tends to swamp the stars; 30-cm might show the colours of orange-red. it can be found 1 degree west and slightly north of omicron Dra, itself a colourful pair worth seeking out (4.8, 8.2, 317 degs, 38") which Burnham calls orange and blue and Haas finds yellow peach and clear grey.

  Just 3 degrees north of Antares and a little proceeding is the naked-eye star rho Ophiuchi (16 25 35.03 -23 26 47.0). This beautiful pair, whose components are magnitudes 5.1 and 5.7, is currently 3".0 apart in PA 334 degrees and has been slowly closing since discovery by Herschel with the position angle decreasing by 30 degrees over the same interval.

  Whether it is binary is not yet fully established, as the measures of distance by Gaia as given in the DR2 catalogue show that the parallaxes just overlap within the quoted errors but the proper motion of B is significantly larger than that of A.

  The stars are 467 light-years away putting it about 40 light-years beyond the rho Ophiuchi dark cloud which lies 1 degree to the south.

  Rho has a number of faint companions - C is 7.3 at 0 degrees, 149" and D is 6.8 at 252 degrees, 156"; both distances are slowly reducing. S. W. Burnham divided D into stars of 6.8 and 8.4 which are currently 0".28 apart in a binary orbit which takes 675 years.

  Bob Argyle - Double Star Section Director

• July 2015 - Double Star of the Month

  lambda Ophiuchi - STF2055 (16 30 54.84 +01 59 02.8) lies 20 degrees due south of the south-easternmost star in the Keystone of Hercules, beta Herculis. During the 19th century, measurements showed the two stars slowly separating and reaching a maximum distance of about 1".9 in the early 1880's. In 1906, Thomas Lewis, in his book on the Struve stars, considered that the period was about 130 years which as it turns out was a good attempt. The current period from observations of almost 2 revolutions gives a period of 129 years and a predicted position for mid-2015 of 42° and 1".43. The pair is not particularly easy to observe, partly because of its altitude in the sky - it is barely above the celestial equator - and partly because of the magnitude difference between the two components (4.2 and 5.2). Both stars are A0 dwarfs, according to the WDS, and lie at a distance of 173 light years. There are two much fainter stars at 120" and 308" which appear unconnected to the system.

  DUN 219 (17 58 55.69 -36 51 30.2) lies not far from the spectacular open cluster Messier 7 and follows G Sco by about 2 degrees. It has not been included in either Hartung or the book by Sissy Haas but is nevertheless is a splendid sight. The stars are magnitudes 5.8 and 7.8 and the separation derived by Dunlop in 1836 of 47".1 has now increased to 53".5, (with the position angle changing from 265° to 252° over the same interval of time), making it a striking object object in small telescopes. The USNO include the pair in their linear elements catalogue, confirming that the two stars are entirely unrelated. A third star of mag 11.3 can be found at 40" from A. Just 16 seconds of time following and 8 arc minutes south is HJ 5000 AB, stars of magnitudes 7.1 and 8.9 separated by 7".3 but apparently in orbital motion. The writer measured this pair in 2010 and noted pale yellow and pale blue - pretty.

  Bob Argyle - Double Star Section Director

• July 2014 - Double Star of the Month

  zeta Her (16 41 17.46 +31 36 07.0) is part of the Keystone of Hercules, the south preceding component of the four stars in the pattern. Only 35 light years from us, it has long been known to be a close and difficult pair, but at the time of writing it is opening up and will soon be as easy to resolve as it gets. The ephemeris for the 34.5 year period orbit shows that in summer 2014 the stars are at 140° and 1".20. The difficulty comes with the large difference in magnitude - in the visual the components are magnitudes 2.95 and 5.40. The writer has followed this pair since 1990 and has been able to measure it each year since apart from 2001 - 2004 inclusive when it was too difficult for the 20-cm Cooke refractor at Cambridge. Over many years there have been suggestions of a sub-period due to one of the stars being a close, unresolved pair, and in 1983 a third component was detected in the infra-red but since then no confirming observations have been made and at present it is assumed that zeta is a simple binary star. The primary star is of spectral type early G and sometimes appears orange to observers with the companion appearing green by contrast.

  eta Oph (17 10 22.66 -15 43 30.5) was one of S W Burnham's later discoveries and is also known as BU 1118. This bright, twin pair of white early A stars of visual magnitudes 3.05 and 3.27 was separated by 0".4 at discovery in 1889. Like zeta Her above this pair is now close to its maximum separation and is actually starting to close. In summer 2014 it will be found at 232° and 0".57. This needs 30-cm on a night of very good seeing as it is low from the UK. The orbital motion accelerates rapidly as periastron approaches in 2024 at which time the stars are 0".006 apart and moving at 15 degrees per DAY. The period of this highly inclined and very eccentric (e = 0.95) orbit is 88 years. There are two faint comites of magnitude 11.2 and 12.4 both about 100" distant. Eta (combined magnitude 2.4) can be found about 15 degrees north following Antares.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - July 2012

  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.

  With the summer constellations now becoming dominant, the two pairs selected this month are to be found in the constellations of Hercules and Ophiuchus.

  STF2194 (17 41 05.50 +24 30 43.2) sits in the preceding edge of a glorious 1 degree field which includes the bright stars 83 and 84 Herculis. The pretty pair itself is an optical system and since 1783 it has widened gradually from 14".3 to 16".3. The main stars are magnitudes 6.5 and 9.3 and the current position angle is near 7°. Webb noted them as orange and blue in 1850, using his 3.7-inch refractor whilst Franks early in the last century found them to be yellow and lilac. The primary star is a K0 giant which is also a spectroscopic binary. About 169" away in PA 163° is a third star, nominally the same brightness in the visual as B.

  STF2173 (17 30 23.78 -01 03 46.2) is a visual binary with a period of 46.4 years. Its highly inclined apparent orbit means there are two opportunities for small apertures to see the two stars before orbital motion takes them beyond the range of most amateur telescopes. The writer measured this pair in 1992 and 1997 with the Cambridge 8-inch Cooke telescope when the separation was respectively 1".1 and 0".7. The two stars are now separating reaching a separation of 0".8 in the south-eastern quadrant in 2012, they then close rapidly and open out again, reaching 1".1 in the north-west in 2037. Both stars are yellow and close to V = 6.0 so this pair forms a beautiful test object at present for 15-cm aperture. Whilst in this area, check out another target some 4 degrees slightly preceding this pair - 41 Oph, a close unequal binary which is one of R. G. Aitken's last discoveries. It needs at least 15-cm on a good night.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - July 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.

  At this time of year the constellations of Hercules and Ophiuchus straddle the northern hemisphere meridian in the evening. Ophiuchus contains one of the finest binaries in the sky in 70 Oph (18 05 27.21 +02 30 08.8) - a pair with a period of 88 years and a long history of observation. Its proximity to the Sun means that the apparent orbit is large and the two components can be seen in small telescopes at any point in the orbital cycle. The last periastron occurred in 1991 and the telescopic distance between the stars has more than

  tripled since then. In mid-2008 the apprent position of B is 132.9 and 5".50. The writer has been following this pair every year since 1990 since when the position angle has decreased from 217 to 134 degrees. Early attempts to compute the orbit led to suggestions that there was a third body in the system. Later and more accurate measurements, along with substantial radial velocity investigations, have not shown any evidence for this idea.

  Herschel found the pair in 1779 and called it H II 4. It is number 2272 in Struve's Dorpat catalogue. Hipparcos gives the distance as 5.1 pc and the magnitudes are 4.22 and 6.17. The colours are particularly splendid, the spectral types being K0V and K4V.

  Triangulum Australe crosses the southern meridian about an hour after alpha Centauri on July nights. The brightest stars are magnitudes 1.9, 2.8 and 2.9. About 3 degrees north of beta is Dunlop 194 (15 54 52.64 -60 44 37.1), two stars of magnitude 6.4 and 10.0 separated by about 44 arc seconds and having changed little since John Herschel's measures from Feldhausen. R. P. Sellors using an 11-inch refractor at Sydney found that A was a closer double, the 8.1 mag companion being located about 0.6 arc seconds E of the primary. In this system too there has been little change since discovery. A is a luminous B star which Hipparcos places 500 parsecs away with an uncertainty of perhaps half this distance.

  Bob Argyle - Double Star Section Director