Double Star of the Month Archive 2017

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.

October 2017 - Double Star of the Month

William Sadler Franks was an astronomer who assisted Isaac Roberts at his observatory in Crowborough from 1892 to 1906 and then went on the act as astronomer for Mr. F. J. Hanbury in East Grinstead from 1908.

One of his interests was in the micrometrical measurement of wide double stars and he undertook to re-observe all the pairs in the Dorpat and Pulkova appendix catalogues which are usually marked by the characters OΣΣ, Σ I and Σ II.

The results, which were achieved with a Cooke telescope a little over 6-inches in diameter and a filar micrometer, appear in eight lists published in Monthly Notices of the RAS between 1914 and 1920.

During the course of this work he swept up a number of new, wide systems. The WDS catalogue contains the pairs FRK 1 to FRK 13 but in fact there are only 10 pairs in this list now left in the catalogue.

FRK 11 (22 30 06.50 +49 21 23.1) lies in Lacerta just over 1 degree south and slightly preceding α Lac. The primary is V = 6.7 and it is a K2 giant star. The companion, which is optical but also a K giant, is currently 67 arc-seconds distance in PA 91 degrees and is considerably fainter - magnitude 10.7. The interest attached to this pair is that the brighter star is an RS CVn binary with a period of 17.76 days and an amplitude of 0.12 in V.

In the south-eastern corner of Grus are a number of double stars discovered by James Dunlop at Parramatta.

Three degrees east of ε Gruis is DUN 246, a fine pair of yellow stars of magnitudes 6.3 and 7.1 currently 9 arc-seconds apart in PA 254 degrees.

Move a further two degrees east and you will alight on DUN 248 (23 20 50.13 -50 18 23.0). The stars in this case are magnitude 6.2 and 6.6 and measured by me at 211.4 degrees and 16".90 in late 2013, but in 1947, Richard Rossiter discovered that the A star was a close pair.

At that time the separation was 0".5 but in the 70 years since the separation has widened and the stars are now 1".3 apart. The companion is magnitude 8.9 so it will need probably 25-cm to see this close pair.

Bob Argyle - Double Star Section Director

September 2017 - Double Star of the Month

Four degrees south-west of ζ Cep and also 4 degrees SSW of the Garnet star μ Cephei is the fine pair STF 2840 (21 52 01.02 +55 47 48.3). First noted by William Herschel in 1782 this wide and easy pair of white stars can be seen in small telescopes.

The stars, of V magnitudes 5.6 and 6.4, are currently 18 arcseconds apart in position angle 192°, and they have closed up up by just 3 arcseconds since being measured by Struve at Dorpat in 1830.

The stars have been observed by the Gaia satellite which gives the distances for A and B as 5.18 mas and 6.06 mas but the error bars (± 0.57 mas) just overlap which does not rule out the possibility that they are physically connected, although the declination proper motions of the two stars are significantly different.

There is a mag 13 star 55 arcseconds away in position angle 346°.

29 Aqr (22 02 26.49 -16 57 57.8) is 4 degrees east and 1 degree south of Deneb Algedi (δ Cap). A discovery of Sir James South it boasts an interesting colour combination.

The bright components both of magnitude 7.2 have a rare coupling of spectral types, A2V and KOIII. The Dutch astronomer Willem van den Bos noted that they were orange and white in 1963 and Ernst Hartung, also observing from the southern hemisphere makes them white and yellowish.

If binary, then this is a long period system, the most recent relative position in the WDS from 2011 is 247° and 3".8, having closed up from 4".5 at the time of South. I measured the pair last year and got a distance of 4".4 over 3 nights but with a large scatter.

The star is very low from the UK so the magnitude 13 star at 291° and 143 arcseconds may well be a challenge too far.

Bob Argyle - Double Star Section Director

August 2017 - Double Star of the Month

Located about 1.5 degrees south-west of Albireo in the Cambridge Double Star Atlas (2nd edition) is a single point marked magnitude 7 to 7.5 according to the scale. This is the binary star STF 2525 (19 26 33.71 +27 19 21.9) which consists of stars of mag 8.2 and 8.4.

Found by Struve at Dorpat at a separation of 1".3, over the next 60 years it gradually closed up until it was unresolved between the late 1880's and about 1895 when it began to widen again. It was therefore regarded as an optical pair undergoing a close approach until Thomas Lewis (1906) clarified the situation.

In the previous decade the stars had made a very close approach and the similarity in magnitude disguised the fact that the companion was heading out back to where it came from.

The stars were measured in 2016 with the Cambridge 20-cm refractor (291 degrees, 2".38) and are now visible in 15-cm. The uncertain period is 883 years and the maximum separation of 3".3 will be reached around 2320.

Pi Capricorni (20 27 19.20 -18 12 42.1) is the easterly of three faint naked-eye stars which form a triangle some 4 degrees south and somewhat east of beta Capricorni (V = 3.1). The other two stars in the triangle, omicron Capricorni and rho Capricorni, have already been dealt with in this column in August 2015 and August 2016 respectively.

Pi Cap. is one of S. W. Burnham's earliest discoveries using the 6-inch Clark, but it transpires that it was first seen by O. M. Mitchel in 1846 and not published until 5 years after Burnham noted it in print.

From northern latitudes this is not a particularly easy pair. The stars are of magnitude 5.1 and 8.5 and the current position is 3".3 in PA 160 degrees; there has been very little motion since 1846. Hartung notes that the companion is white and the primary is a B8 star.

The discovery of variable radial velocity in the A star later led to the discovery of a close companion which appears to have a period of the order of 40 years or so.

For a real test, try the 14.1 mag star found by Burnham with the Washington 26-inch refractor. It is being left behind by AB and is currently 38" away in PA 40 degs.

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

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

May 2017 - Double Star of the Month

24 CBe (12 35 07.76 +18 22 37.4) sits just on the northern edge of the Coma Cluster of galaxies and about 10 degrees to the south of the coarse open cluster of stars in Coma.

It is a fine pair for the small aperture and well worth looking for. The different spectral types of K2III and A9Vm promise a substantial colour contrast. Smyth notes A, 5.5 orange colour, B 7 - emerald tint,- the colours very brilliant. Hartung, from Australia finds deep yellow and white, whereas Sissy Haas records citrus orange and fainter royal blue.

The WDS gives the visual mags as 5.1 and 6.3. Hipparcos gives similar proper motions but differing parallaxes - 7.24 mas for A and 19.29 for B but the latter comes with an error of +/-14.58 mas possibly due to the fact that this star is known to be a spectroscopic binary (SB). The relative positions are very useful for calibration purposes. At present the PA and separation are close to 270 degs and 20".

Starting at beta Crucis and moving eastwards by about 3 degrees will bring you to a pair of barely naked-eye stars separated by about a degree in right ascension.

The first of these is the fine pair R 213 (6.6, 7.0, 22 degs, 0".7) whilst the second is the multiple star I 424 (13 12 17.63 -59 55 13.9). To the user with 15-cm or more, here is an unequal pair of stars of mags 4.6 and 8.4, separated by 2". This has opened slightly and moved 20 degrees in increasing PA since being found by R.T.A. Innes.

Two fainter and wider companions are also catalogued. C is mag 12 at 258 degs, 46" whilst D, a much greater test, is mag. 14.9 at 24 degs and 53".

Whilst in the neighbourhood, about 7' north-east is the wide pair COO 152 (mags 6.2, 9.4, 146 degs, 25").

If you are able to bring 50-cm aperture to bear then there are two further tests. I 424 A is the close binary SEE 170 - currently at 0".17 and with a period of 27.4 years; the mags are 5.3 and 6.2. One of these stars is also a SB - actually an eclipsing beta Lyrae-type system.

The A star of COO 152 was resolved by Hipparcos. The components of this system are 6.4, 9.7 and the separation is 0".4. One of these stars is a SB of 4.23 day period and the 25" component is also physical meaning that COO 152 is a quadruple system.

Bob Argyle - Double Star Section Director

April 2017 - Double Star of the Month

Iota Leo (11 23 55.37 +10 31 46.9) is a naked-eye star which sits below the hind legs of Leo. It is a relatively easy double star to resolve given 15-cm of aperture and it seems to have been missed by William Herschel during his double star surveys around 1780.

The current orbit, which has a period of 186 years predicts a separation of 1".05 for 1780 but the pair is a very unequal one: the visual magnitudes are 4.06 and 6.71 making it somewhat easier than zeta Her which Herschel did discover. At the present time, the stars are separated by 2".1 and they are now almost back in the configuration in which they would have appeared to F. G. W. Struve in 1827.

From the UK iota Leo is relatively low and I have never found them particularly easy to measure. The stars will continue to widen until 2060 when they are 2".7 apart and then closing to 0".63 in 2128.

Admiral Smyth found the colours to be pale yellow and light blue. T. W. Webb noted white and tawny in 1870, whilst Hartung found yellow and whitish.

Pz 3 Velorum (10 31 57.33 -45 04 00.1) consists of two luminous B-type giants which lie over 600 light years away. Pz 3 lies at the end of a 5 degree arc of third, fourth and fifth magnitude stars starting with bright binary star mu Vel.

The arc also contains t Vel (HJ 4330 5.2, 8.6, 163º, 40") the primary of which is a recently discovered close pair, separation 0".4) whilst about 6 arcminutes east is HJ 4332 (mags 7.1, 9.8, 162º, 28").

Both components of Pz 3 were observed by Hipparcos and the resulting parallaxes show agreement although the scatter in each case is large.

With such a distant system, relative motion, if any, is very small and the current position angle and separation are 219º and 13".7 are little different from the first measures in 1826.

Gould notes that the components of the double star itself are white.

Bob Argyle - Double Star Section Director

March 2017 - Double Star of the Month

STF 1282 in Lynx (08 50 44.28 +35 04 15.4) is a neat pair which can be found 7 degrees north of iota Cnc and a little east. The components are magnitude 7.6 and 7.8 and in 2015 I found the relative position to be 282 degrees and 3".41.

There has been little motion between the two stars since the pair was found by F. G. W. Struve. Also called S 582. Burnham records yellowish white and very white.

A faint and distant star, mag 12.4 at 49", has a common proper motion with AB and is therefore likely to be a physical member of the system. It was found by the French observer G. Soulie. It, in turn, has a companion of magnitude 14.6 some 19" away which was found by Robo-AO, the first robotic adaptive optics system which is mounted a 60-inch telescope on Palomar Mountain.

upsilon Carinae (09 47 06.12 -65 04 19.2) is a 3rd magnitude (actually V = 2.97) star which is brilliant white and is accompanied by a mag 6.0 companion which is also white.

The separation between the two has barely changed since the early 19th century when the pairing was first noted by Rumker in Australia. It is almost certain that this is a binary system and both stars are massive, luminous and hot.

The primary is an A8Ib supergiant with an absolute magnitude of -5 giving it a luminosity of about 9000 suns whilst the companion is somewhat earlier in spectral type (B3 or B4) and is about 600 times brighter than the Sun.

Not surprisingly Sissy Haas includes it as a showcase pair and small telescopes should suffice in dividing the stars although the significant brightness difference might militate against using too small an aperture - the separation is currently 5".

A paper written in 1986 speculates that the period of the pair might be close to 19,500 years, and gives the distance as 400 pc. In 2007 the Hipparcos satellite found 440 pc but with a formal error of 54 pc. E. J. Hartung notes the pair HJ 4252 (9.3, 9.5, 303 degrees, 12") about 5 arc mins south following.

Bob Argyle - Double Star Section Director

February 2017 - Double Star of the Month

STF 958 Lyncis (06 48 12.23 +55 42 16.0) is a neat, bright pair in SW Lynx near the border with Auriga.

Start by locating the bright trio of 12, 15 and 14 Lyn, all visual binaries of increasing difficulty, and move 2 degrees south of the beautiful triple 12. This brings you to 13 Lyn, an orange giant of magnitude 5.3. Move a further 1.5 degrees south and you will alight on STF 958.

One of William Herschel's finds, it is doubtless a binary, and even although the movement since 1782 amounts a slight decrease only in the apparent separation, the WDS 6th Orbit catalogue contains two orbits computed for it. With magnitudes of 6.3 and 6.3 it is not clear which star is the brightest and even SIMBAD appears uncertain. The SB9 catalogue assigns spectroscopic duplicity to the A star and the period is given as 4.26 days. The WDS notes that the other component has variable proper motion, thus indicating that it is a physical quadruple system.

Herschel called STF 958 A pretty double star and assigned colours of pale rose to both components. Struve, on the other hand, found both stars yellow, as did Webb in the 1850s, whilst Sissy Haas notes that both stars are khaki-white. A 11.2 magnitude star can be found 176" away in position angle 268 degrees.

19 Pup (08 11 16.32 -12 55 37.3) is in a fairly sparse area of sky but it forms an approximate isoceles triangle with Sirius and Procyon.

It is the bright star on the south edge of the galactic cluster NGC 2539 and not surprisingly it comes with a number of faint distant companions in the WDS catalogue.

The primary is a G8 giant of magnitude 4.8 and small telescopes will easily show two distant companions, a magnitude 8.9 at 58" and a 9.3 a further 12" out from the primary.

The WDS lists two more which are fainter but Burnham in 1899 found a very faint star at a distance of about 2" which became BU 1064 AB. It was also measured by Aitken but no further sightings seem to have been made since then. Van den Bos looked twice, in 1936 and 1939 without success. Steve Coe noted 19 Pup and recorded that it was a triple star with the primary yellow and the two brightest comites being white.

Bob Argyle - Double Star Section Director

January 2017 - Double Star of the Month

The two pairs selected for this month are ostensibly relatively, bright and wide pairs; easy for the small aperture, or even binoculars. In fact, both are more complex.

The northern object is S 503 in Orion (05 56 03.43 +13 55 29.7). The brightest stars are mags 6.7 and 8.4 and they are currently 76" apart in position angle 322 degrees. The WDS however lists 6 companions, most are field stars but at least one is travelling through space with S 503.

Component C (mag 11.6 at 310° and 55".3) and D (mag 8.2 at 334° and 299") both also found by South are optical and should be visible in 10-cm aperture. Willem Luyten found a 13 magnitude star at 205° and 47" (E) which has the same space motion as A and which the WDS notes is a white dwarf.

In late 2015, Marcel Fay, who has been publishing his CCD astrometry with a 283mm reflector in El Observador de Estrellas Dobles found two further stars of around magnitude 13 (F and G) at distances of 32" and 9" from A. The status of these is as yet unclear but further measures will show whether they are co-moving with A.

In 2011 Shaya and Olling published a list of widely separated pairs of stars with common proper motions. This list includes pairs such as Capella and 50 Per, and gamma and tau Persei which have almost 100% probability of being physically related.

Number 185 on this list is a wide pair of apparently unrelated stars in Columba: SHY 185 (06 36 54.07 -36 05 18.4). Star A is a G1 dwarf of visual mag 6.4. Some 288" away in PA 129 is a mag 7.3 star (B) of spectral type G0V. Both stars are close binaries of short period.

A is RST 4816 which has a period of 14 years and is currently at 99° and 0".1 according to the orbit, whilst B was found by W. S. Finsen (FIN 19) and is predicted to be 339° and 0".3 at present. The Hipparcos parallaxes for A and B agree well within the quoted errors and the proper motions are similar.

There are two other double stars to be found close by. One is UC 1454 (7.2 and 12, 181°, 33"), on the southern edge of SHY 185 and about 40 arc minutes south of the group is the fine pair BU 755 whose stars of mags 5.9 and 6.9 are now 1".5 apart and a faint third component which is mag 11.5 can be see 21" away (HJ 3875).

Bob Argyle - Double Star Section Director