Double Star of the Month in Serpens

• 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

• August 2019 - Double Star of the Month

  16 Cyg (19 41 49.1 +50 31 32) is a beautiful, wide double star easily found by heading due north from delta Cygni, itself a bright although unequal and much closer pair (see the column for Aug 2011).

  16 Cyg is a much measured pair and the Washington Double Star (WDS) observations catalogue has almost 600 entries.

  It is clear from the astrometry of both stars given by the Gaia DR2 mission that they at the same distance from Earth (A is 68.8 light years away whilst B is 69.2). It seems certain that they form a binary system of long period where here long is taken to mean anywhere from 18 centuries to 485 centuries. This is the range of possible solutions from three different research groups.

  In 1998 a star of visual magnitude 13, thought to be a M dwarf was found just 3 arc seconds from A and it is clear that it shares the large space motion of the bright stars and is therefore physically associated with A. In 1996 a planet associated with A was independently found at Lick and McDonald observatories which has a period of 2.19 years.

  16 Cygni is a binocular pair but best seen in telescopic apertures. The components are spectral types G1 and G3, so slightly larger and more luminous than the Sun, and they shine with magnitudes 6.0 and 6.2. Smyth calls them pale fawn colour, whilst Webb just notes that they are yellow.

  AC11 (18 24 57.2 -01 34 46) is in Serpens, about 2 degrees north-west of the 3.3 mag eta Serpentis. It was found by Alvan Clark on 30th July 1854, and reported by W. R. Dawes to the Monthly Notices of the RAS. "A very difficult object", he reported, "though decidedly elongated with a 7.5-inch aperture".

  The stars are of magnitudes 6.7 and 7.2 and at present this 248 year binary is just closing from maximum separation. The orbital position in late 2019 is 354 degrees and 0".8 and it is well seen in the Cambridge 20-cm refractor. It remains above 0".6 for another 20 years or so and then dives down to about 0".03 towards the end of this century.

  Continue another 2 degrees NW to find 59 Ser, a pretty pair separated by just under 4".

  Bob Argyle - Double Star Section Director

• June 2017 - Double Star of the Month

  beta Serpentis (15 46 11.21 +15 25 28.9) is a magnitude 3.7 star of spectral type A2IV some 155 light years away.

  About 30" west of it is a magnitude 10 companion which is unrelated. This pairing was seen by William Herschel in 1781. The companion should be visible in 7.5-cm on a fair night as it is not too close to the glare of the bright primary.

  About 25 arcminutes further west is an 8th magnitude star which is moving through space with the same motion as beta and it is almost certainly physically associated. This star, in turn, has a companion and the system is known as ROE 75.

  E. D. Roe was an American astronomer who had access to the 12 and 40-inch refractors of Yerkes Observatory for double star measurement, but in this case he discovered number 75 on 1911 May 20 with his own 6.5-inch Clark refractor.The secondary is magnitude 10.7 at only 6" distance so a larger aperture may be needed to see this unequal pairing.

  Ara lies below Scorpius in the southern sky and the north-east of the constellation is rich in open clusters.

  NGC 6193 is part of the Ara OB1 association and is thought to be 4200 light years away. Buried in the cluster is the bright and wide pair DUN 206 (16 41 20.42 -48 45 46.7).

  This is an easy pair for the small aperture with the white components of magnitudes 5.7 and 6.7 being 10" apart.

  The brighter component is itself quadruple. Owners of 10-cm or more may be able to see the mag 8.4 star at 1".7 from A, but A itself is a massive triple consisting of three O stars, two of which form a close SB of 2,67 days period and they are in turn circled by a third star every 8.1 years. The total mass of these three stars is thought to be about 140 solar.

  The whole region is spectacular for the small telescope and there are fainter and more distant stars to be seen with small apertures.

  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 - June 2011

  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.

  Beta Serpentis (15 46 11.21 +15 25 18.9) lies about 11 degrees south of the crown of Corona Borealis. The primary is a mag 3.7 star of spectral type A2IV and many observers find it to be yellowish. The faint (mag 10.0) companion was found by the elder Herschel on 1781 Aug 13, and he did not allocate it a colour, noting it only as `extremely faint'. Today 15-cm aperture will show it clearly enough - a slighter harder test is the 10.7 mag star some 200" away in PA 212°. A very recent study has been using Hipparcos data to physically link apparently unrelated pairs of stars and in Beta's case the study found that there is a very high probability that the faint wide pair ROE 75 (mags 8.2, 10.7, PA 327°, separation 6".2) which can be found some 20 arc mins south preceding beta Ser is physically connected to Beta by dint of having a very similar parallax and proper motion.

  The small constellation of Ara is seen against the Milky Way near the tail of the Scorpion, and Hartung lists numerous objects of interest within its borders. The coarse cluster NGC 6193 is involved with the emission nebula NGC 6188 and in addition the brightest star MLO 8 (16 41 20.42 -48 45 46.7) is a multiple system of some interest. In 1878 observers at Melbourne divided A (mag 5.1) and found a mag 8.4 companion at 2".1 distance. The distance between the two has been closing since and the last recorded measure in 1938 placed the companion 1".6 from the primary, although Hartung reports seeing the star clearly resolved with 10.5-cm in 1963. It is not clear if B is a background star or an orbital companion - the next positive observation will help to decide this question. For the small telescope user there is star C (mag 6.8) some 10" away and medium apertures should easily find two fainter (mags 10.5 and 11.4) and more distant comites found by Herschel and another of mag 12.5 discovered by Innes.

  Bob Argyle - Double Star Section Director