Double Star of the Month in Corona Australis

• 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 2018 - Double Star of the Month

  On a straight line between alpha Oph and 93 Her, but two-thirds of the way towards 93, and therefore just in Hercules, is STT 338 (17 51 58.46 +15 19 34.9) a neat close pair which was discovered by Otto Struve at Pulkova.

  At the time of discovery the stars of magnitudes 7.2 and 7.4 were separated by only 0".6 in position angle (PA) 223 degrees. Since then orbital motion has taken them almost 60 degrees retrograde in angle and the separation has increased to 0".8.

  Sissy Haas, recalling T. W. Webb, describes them as them gold and green white, but in fact the latter term was abbreviated in Webb to mean Greenwich. A recent orbit by Dr. Jean-Louis Prieur and colleagues assigns a period of 1276 years.

  In a small rectangular area of about 20 x 9 degrees, just below the Teapot of Sagittarius, is the constellation of Corona Australis. It has a number of attractive double stars two of which HJ 5014 (August 2009) and gamma CrA (August 2010) have already been described in this column.

  Kappa CrA (18 33 23.13 -38 43 33.6) is a fine pair which was noted by James Dunlop and is number 222 in his catalogue. The stars are magnitudes 5.9 and 6.2 and the current PA and separation are 358 degrees and 21".5.

  Dunlop's 1826 separation of 30" must be an error, as the stars appear to have common proper motion and Gaia DR2 also indicates that they are both around 695 light-years away. Not connected however, are two fainter and more distant stars, a 13.1 magnitude at 202 degrees and 33", and an 11.6 magnitude at 247 degrees separated by 96".

  About 2 degrees following is lambda CrA (COO 227), a pair of stars of magnitudes 5.1 and 10.0 at PA 213 degrees and separated by 30". It is, nevertheless, a physical pair and DR2 gives distances of 205 and 200 light-years respectively, with similar proper motions. A third star (mag. 9.9) is at 51 degrees and 43".

  Bob Argyle - Double Star Section Director

• August 2014 - Double Star of the Month

  STF2375 (18 45 28.36 +05 30 00.4) is a pair of 6th magnitude stars found about 2.5 degrees north preceding the beautiful pair theta Serpentis. At discovery in 1825 F. G. W. Struve found 108° and 2".2. Orbital motion, for it appears to be a binary, has been rather slow. By 2010 the position angle had advanced to 120° and the separation to 2".6. Interest in the system was renewed in 1952 when Dr. William Finsen was observing the pair with his newly constructed eyepiece interferometer on the 26.5-inch refractor in Johannesburg. When an apparently single star is examined with the interferometer if it is in fact a close double there will be formed a set of fringes which disappear when the instrument is rotated so that the slits are parallel to the line joining the stars, in other words the position angle. Finsen was somewhat surprised when he found that there were fringes on both stars and they disappeared at exactly the same angle of the interferometer. It transpires that both stars were equally close pairs with identical position angles. It led Finsen to call them Tweedledum and Tweedledee. Since then orbital motion has destroyed the symmetry of the pairs and Aa-Ab is currently at 0".13, whilst Ba,Bb is now only separated by 0".08. Aa,Ab has a period of 27 years whilst that of Ba,Bb is 38 years.

  DUN 224 (18 54 01.4 -47 16 27.4) is a beautiful triple star in the north of Telescopium, about 2 degrees south of the border with Corona Australis. Dunlop found the wide pair in 1826. The stars are mags 7.1 and 7.3 and currently separated by 87" so its likely that they could be seen in binoculars of sufficient aperture. R. T. A. Innes found the primary to have a closer, unequal companion - mag 9.1 at 1".8. There has been little change in separation since then but the PA has moved on to 192°. The two bright stars are unassociated. Both have parallaxes determined from Hipparcos; star A is 225 light years distant whilst B is 626 light years away. Ross Gould notes colours of yellow and white.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - August 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.

  mu Cyg (21 44 08.57 +28 44 33.4) is a fine binary, currently well-seen in 10-cm aperture. It is number 72 in Christian Mayer's 1780 catalogue of double stars and since that time has been well followed by double star observers with the WDS listing more than 700 measurements. The apparent orbit is quite eccentric (0.66) and the companion spends about one-third of the orbital period of 789 years near periastron and the remaining two-thirds near apastron. A close approach occurred in 1936 when the distance dropped to about 0".5 and so the pair having reached an elongation will now start to close slowly, reaching around 1".2 in 40 years time and then widening to 7".2 in the 24th century.

  According to the WDS the stars are both F dwarfs with the companion somewhat earlier than the primary. This is an unequal pair with the stars being magnitudes 4.8 and 6.2. The primary is a double-line spectroscopic binary and both the distant companions are field stars.

  One of the finest sights in a small telescope in the southern hemisphere is gamma CrA (19 06 25.14 -37 03 48.5) whose binary nature was discovered by John Herschel from South Africa in 1834. With the separation ranging between 1".3 and 2".5 and a period of 122 years this is a good system to follow over a period of a few years. At the moment the companion is at 9 + and 1".35 with the position angle reducing at the rate of about 4 degrees per year. Like mu Cyg (above) it is an unequal pair of magnitudes 4.5 and 6.4. Hartung does not give any colours but Sissy Haas mentions that Gould finds pale yellow.

  gamma CrA is 56 light years distant according to Hipparcos and located near the border with Sagittarius. The bright globular NGC 6723 appears in the same wide-field finder view when acquiring the star.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - August 2009

  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.

  100 Her (18 07 49.6 +26 06 04) is a bright and wide pair of white stars in eastern Hercules, about 15 degrees south preceding Vega. It forms a fine sight for the small telescope. The original separation derived by Herschel in 1777 showed the stars to be 17".0 apart whilst a more recent measure by the author in 1995 indicates that the stars have closed to 14".2 with very little change in angle.

  These stars caused the Hipparcos satellite some difficulty as the errors in parallax, even in the revised version, are some ten times worse than might be expected but they still show that the parallaxes of the two stars are the same within the (large) errors and thus indicate that the two stars are probably physically related. In 1985 the CHARA team from Georgia State

  University discovered that component A was a close binary. Subsequently, the period turned out to be less than 16 years. Perhaps the same situation might apply to star B, as an explanation for the large error in parallax. Both stars are noted as white by several observers.

  HJ 5014 (18 06 49.9 -43 25 30) was another product of John Herschel's fecund search for new double stars at the Cape of Good Hope. Unfortunately there were few observations in the following 50 years when the pair moved through almost 180 degrees of position angle, widening considerably as it did so. Wierzbinski produced some orbital elements in 1958 with a period of 191.2 years. It was clear around the beginning of this century that the real period was much longer and Andreas Alzner produced an orbit in 2002 increasing the period to 450 years.

  This is a beautiful pair of white stars (both A5 dwarfs) each of visual magnitude 5.7 which can be well seen in 10-cm aperture. The position for 2010.0 is 2.4 degrees and 1".72, and it will continue to widen until 2170 or so.

  Bob Argyle - Double Star Section Director