December 2014 - Double Star of the Month

The double stars being featured this month require apertures around the 30-cm mark. Both are close binaries with periods in excess of 200 years and both are slowly widening, but at the time of writing each pair is at or below 0".5 separation.

52 Arietis (03 05 26.69 +25 15 18.7) is also known as STF 346. At discovery this was an 0".7 pair and motion during the remainder of the century was slow but by the early 1930's the pair was out of reach of all telescopes and circling around each other at the rate of 1 degree per week.

Since then the companion has been heading out towards the discovery position but even now it is still a difficult pair. The ephemeris for 2014.6 gives 258 degs and 0".49, and the stars are almost equally bright, both being around 6.2 visual. The writer was just able to measure it for the first time last autumn with a 20-cm Cooke refractor and will attempt to get some confirming measures later this year.

The two stars are accompanied by a 10.8 at 5" distance which is physically attached to the system. Another star of mag 13.2 at 105" appears to have been found by Smyth in 1835 and is mentioned in the Cycle of Celestial Objects. Smyth gave the magnitudes of C and D as 15 and 13, no doubt reflecting the difficulty of seeing C rather close to the bright binary components.

At the turn of the nineteenth century, Walter F. Gale was an active amateur astronomer living in Paddington, New South Wales and found a few double stars with an 8.5-inch reflecting telecope made by George With. He published a short list of discoveries in Astronomische Nachrichten in 1896, consisting of 5 double stars and a ring planetary nebula (IC 5148 - the Spare Tyre Nebula). Two of the pairs turned to be already known so the WDS now contains but three of his double stars.

The second object on the list was a close pair in Reticulum now called GLE 1 (04 16 20.92 -60 56 54.8). The stars, whose visual magnitudes are 6.8 and 7.5, passed through periastron in 2002 and are now slowly widening again although the current separation (0".35 at 218 degs) does require at least 30-cm and a good night. This system also contains the star TT Ret which is an alpha CVn variable with small amplitude and period of 2.8 days.

Bob Argyle - Double Star Section Director

November 2014 - Double Star of the Month

10 Arietis (=STF 208, 02 03 39.26 +25 56 07.6) is a marginally naked-eye star at the preceding end of a string of similar stars some 2.5 degrees north of alpha Arietis. Found by F G W Struve in 1821, when the separation was 2", the motion of the companion over the next 80 years appeared more or less linear but it became progressively more curved as the pair closed to 1". The companion then made a closest approach at 0".3 in 1920 or so and since then has widened and can be found at 347 degs, 1".49 in 2014.8. The currently accepted period is 325 years but this could well be an underestimate as nothing is known about the motion at apastron. The pair should be divided in 15-cm but a good night is needed because the magnitudes of the stars are 5.8 and 7.8. A 13.5 mag. can be glimpsed 95" distant in PA 150 degs.

kappa Tucanae (01 25 45.50 -68 52 34.5) is located about 5 degrees north following the Small Magellanic Cloud and Burnham's Handbook spends a lot of time on the latter but only notes in passing the former which is a splendid object for both binoculars and telescope. For the binocular user there is a mag 7.8 star which is 319" from the mag 5.00 primary. John Herschel found A to be a fine pair (HJ 3423, mags 5.0 and 7.7) - 'very beautiful' was his verdict. E J Hartung gives the colours as yellow and orange. Two measures at Feldhausen yielded separations of 2" and 2.5" and a mean position angle of 11.7 degrees. Recent measures in 2009 show that the separation has doubled to 4".8 and the position angle is now 319 degs. This is a binary system with a period of about 850 years.

In 1895 Robert Innes found that the distant 7.8 mag star was also double but much more equal in brightness (7.8 and 8.4) and also a binary of fairly short period. He was using a reflecting telescope owned by Mr. F. Dixon Edmonds, an early member of the BAA, and in presenting his list of 16 new pairs Innes said that the telescope had enabled the 'discovery of more new double stars than all the rest of the silver-on-glass reflectors ever made'. I 27 CD revolves every 85.2 years in a circular orbit only slightly tilted to the line of sight so the separation ranges between 0".9 an 1".1 and the angular motion is a fairly constant 4 degrees or so. Innes also noted that both AB and CD have similar proper motions so this is likely a quadruple system.

Bob Argyle - Double Star Section Director

October 2014 - Double Star of the Month

About 2 degrees north of iota Cephei can be found the pair STF2947 (22 49 00.68 +68 34 12.2). This neat 4".6 pair can be well seen in 15-cm and the magnitudes are 6.9 and 7.2. Sissy Haas notes that iota is golden colour and STF2947 is a pair of yellowish-peach stars. Hipparcos does not appear to have observed this pair but it does appear in the 1952 Yale Catalogue of Parallax where the distance is given as 120 light years but with an uncertainty of 20%. A third star of magnitude 12.5 can be found at 208 degrees and 121" but it does not share the space motion of the close pair. About two degrees south is STF2948 (7.3, 8.6, 4 degs, 2".6))

Theta Gruis (23 06 52.77 -43 31 17.2) is the brighter component of the very wide pair SHY 366. The nomenclature refers to Shaya and Olling who in 2010 made a study of wide pairs in the Hipparcos catalogue for which the proper motions were very similar. In the case of Theta Gruis they concluded that the likelihood that A and C (mags 4.5 and 7.8, 292 degs, 159") were physical was 100%. The distances to A and C are respectively 131.9 and 130.4 light years. Jacob then discovered that A itself was a close pair with star B of magnitude 6.6, being found at 114 degrees and 1".5 in 2009. William Stephen Jacob was an Army engineer with a deep interest in astronomy an during secondment on duty in India in the 1840s managed to make some observations of double stars. He used the 6.3-inch Lerebours refractor at Madras to make some micrometric measurements and also discovered a number of new pairs. The WDS contains 24 pairs bearing his discovery number which also includes the binary JC 8 and the Antares lookalike-pair 21 Sgr = JC 6 (see the column for Sep 2008).

Bob Argyle - Double Star Section Director

September 2014 - Double Star of the Month

H 1 48 (21 13 42.46 +64 24 15.1) is a very rare example of one of William Herschel's close discoveries which retains its original designation instead of being absorbed into the catalogue of F. G. W. Struve, as many of his pairs were. It is also remarkable as being a fairly short period system which has both high eccentricity and high inclination. It is characterized by periods of rapid angular motion at very small separations and then stretches of decades when it is visible to the medium aperture. The brightness of the stars (7.2 and 7.3) means that it is never an easy object in small telescopes but may well be visible in 15-cm after 2020 or so when the separation slowly increases to 0".9. The ephemeris for the 81.7 year orbit by Marco Scardia and colleagues gives a separation of 0".72 and PA 243° for 2015.0. The pair can be found 1 deg south preceding 6 Cephei which is, in turn, 3 degrees north of alpha Cephei.

BSO 15 (21 48 15.75 -47 18 13.0) is a naked eye star in Gruis about 4 degrees preceding and slightly south of alpha. It was found to be double by Thomas Brisbane in the early 1830s. The primary is a GO dwarf of V = 5.6. Hipparcos places this star at a distance of only 52 light years and as a consequence it has a fairly substantial proper motion of almost one-third of an arc second per year. The mag 8.8 companion is not connected and is being rapidly left behind by the proper motion of A. The pair was first accurately measured by John Herschel in 1836 when he found B at a separation of 30.3" in PA 14°. Last year the writer measured the pair again and found a distance of 78".9 in PA 350.2°, in good agreement with the prediction given in the USNO Linear Elements Catalogue. This pair has not been observed by either Hartung or Haas and the writer didn't note any significant colour in either component. Recent observations by the infra-red Herschel telescope show there is a large proto-planetary dust ring around this star stretching from about 100 to 180 AU.

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

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

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

May 2014 - Double Star of the Month

The northern target for this month's column is one of the most popular of all double stars - zeta UMa (13 23 55.42 +54 55 31.5). Known equally well as Mizar, this beautiful pair of early A stars has the longest history of all telescopic pairs. First seen by Castelli in 1617, the two stars, whose magnitudes are 2.2 and 3.4 and which are separated by 14".5 were also the first double star to be photographed by Bond in 1857, and Mizar A was the first star to be shown to be a spectroscopic binary in 1889. Mizar A was also high on the list of pairs observed by Michelson with the interferometer on the 100-inch telescope at Mount Wilson in the 1920s, and was duly resolved. Some 706" distant is Alcor (mag 4.0) which forms a naked-eye pair with the Mizar system. It seems likely that the three stars are moving through space together in common with other members of the UMa moving group. In addition to Mizar A, Mizar B is also a single-lined spectroscopic binary and Alcor was also suspected a number of times of being a close binary but no proof has yet been put forward. There is, however, a faint, low-mass star only 1".1 distant from Alcor, discovered in 2009 which does seem physically connected, making the whole group a sextuple system.

Another easy pair of B stars can be found in 3 Cen (13 51 49.58 -32 59 38.6). Discovered by William Herschel it was also picked up by James Dunlop in Australia and appears in his catalogue as Dun 148. With 32.5-cm Gould finds the colours are pale yellow and white. The stars have closed slightly since discovery and are now separated by 7".8 in 104° as measured by the author in 2013. The parallax of A, determined by Hipparcos, puts the star at a distance of 343 light years and the system is part of the Sco/Cen moving group, as is the neighbouring 4 Cen which is barely 1° north following. Another Herschel discovery, in 2013 these stars were 14".8 apart in PA 185°. Hartung notes the colours as pale yellow and ashy. Hartung notes both stars are spectroscopic binaries but the WDS only mentions the duplicity of the brighter component which has a period of 6.93 days.

Bob Argyle - Double Star Section Director

April 2014 - Double Star of the Month

STT 235 (11 32 20.76 +61 04 57.9) is in UMa close to the bowl of the Big Dipper and about 5 degrees slightly south preceding alpha UMa. The pair has a period of 72.7 years and is presently opening, reaching maximum separation of 1" in 2027. At the time of writing the stars are separated by 0".88 so this is a good opportunity to resolve this pair. The components have visual magnitudes of 5.7 and 7.6 so pick a night when the seeing is good and use at least 20-cm, although 15-cm, if the optics are particularly fine, would probably show the object as double. The star appears in the Hipparcos catalogue as HIC 56290 and it has an annual proper motion in declination of about 0".1 towards the south. The mag. 11.3 star some 195" away would seem to be travelling through space with a similar motion, and was noticed by Helmut Abt. STT 235 has a parallax of 35.73 mas putting it at a distance of 91 light years.

Far down in the southern sky, epsilon Cha (11 59 37.58 -78 13 18.5) is the brightest member at the centre of a small cluster of stars some 111 pc distant. Its nearby co-moving companion, HD 104237 (mag 6.6) is also called DX Cha and is the nearest Herbig Ae star. This is a stellar quintet with most of the companions being very young stars. In 1836, John Herschel divided eps Cha itself into two components 1".6 apart, and the pair is known as HJ 4486. The WDS gives magnitudes of 5.3 and 6.0 but orbital motion has taken the fainter star to within about 0".4 of A.

The author made a measure of this pair from Johannesburg in 2008 and obtained 210° and 0".37 very similar to the last measure in the WDS dated 1997. A substantial aperture will be required to see this pair and it would be interesting to have a confirmatory sighting.

Unlike many of the stars in the cluster and a wider association which are spectral class M, eps Cha is a late B star.

Bob Argyle - Double Star Section Director

March 2014 - Double Star of the Month

24 CBe (12 35 07.76 +18 22 37.4) sits at the north edge of the great Coma cluster of galaxies about 3 degrees following M88, but it is in a poor naked-eye star field so that locating it is not straightforward. One way would be to find beta CVn (V = 4.3) and move 23 degrees south. The effort is worth it - this is a very pretty, bright and easy pair for the small telescope. The contrasting colours have been noted by many observers. Webb thought the stars yellow and very blue, Smyth found orange and emerald and Sissy Haas made them citrus orange and fainter royal blue. The pair also serves another purpose - as a scale and orientation calibrator. At present the position angle is 270 degrees and the separation 20" (actually 270.3 and 20.14 for 2015.0). Many observing guides list the pair as optical but the evidence is not very persuasive. The Hipparcos satellite gives the parallaxes as 7.24 +/- 2.74 milliarcseconds (mas) for A and 19.29 +/- 14.58 mas for B. The proper motions are small but very similar. A is a K2 giant and B is a metallic-lined A9 dwarf which is also a spectroscopic binary.

x Vel = DUN 95 (10 39 18.39 -55 36 11.8) is in a rich area of the southern Milky Way just 5 degrees north of the Eta Carina Nebula, NGC 3372. This pair was found by John Dunlop at Paramatta in 1826 and is a glorious sight in small telescope. The stars (V = 4.38 and 6.06), according to Ross Gould using a 35-cm reflector, are yellow and deep-yellow and the low power field contains two small asterisms. Andrew James, on the other hand, is an experienced Australian observer with a very extensive website devoted to double stars and especially those of Dunlop. He reports that Russell in 1873 made the colours straw-yellow and greenish blue and around 1980 members of the AS of New South Wales reported orange and pale blue. Given the spectral type of B is B8V, the reported deep yellow is rather unexpected. There has been little motion between the two components over the last two centuries. In 2000 the position angle was 105° and the separation 51".7. The primary is an early G-type supergiant which is also a semi-regular pulsating star. Hipparcos puts the primary at a distance of 840 light years. In 1834 John Herschel found a faint companion to B, V = 11.9, some 15" away in position angle 178°. The distance has widened to 20" today.

Bob Argyle - Double Star Section Director

February 2014 - Double Star of the Month

STF 1126 (07 04 06.99 31 51.9) is easily to locate - it lies in the same low power field as Procyon, preceding the bright star. Since discovery by Herschel in 1781 the stars widened slightly but at the mid-point of the 19th century they slowly began to close. Motion is very slow and at present the companion can be found at 174° and 0".9. With magnitudes of 6.6 and 7.0 this is a relatively easy object for 20-cm although being fairly low in the sky it is not often seen to full advantage. Observations by the writer with the 20-cm at Cambridge were made in 1992, 2002 and 2012 and over that period the position angle increased by 10 degrees with no change in separation. Either A or B is a spectroscopic binary according to the WDS. A third star of magnitude 11.4 can be found at 251°, 43". The primary star is an AO giant, and the catalogue value for the parallax is given as 12.6 13.7 milliarcseconds.

STF1146 (07 47 56.71 -12 11 33.8) is also known as 5 Pup. It lies near the extreme northern border of Puppis about 3 degrees north following M46 and M47. During a winter evening of very good seeing in early January the writer made a rare foray around the stars of Puppis and saw this star well resolved with the 20-inch Thorrowgood OG. After discovery by F. G. W. Struve when separated by 3".3, the pair started to slowly close. In the 1960s Ernst Hartung noted that it was 'a fine object in a starry field and 75-mm shows it well. In recent years, however, it has been closing more quickly and now is separated by barely 1". This is a long period binary with a highly inclined orbit and the stars will reach a separation below 0".5 before widening again. Thomas Lewis gave the colours as yellowish and blue whilst Webb thought the fainter star to be ruddy in 1851. The primary star is a F5 dwarf, and this pair lies 93 light years away according to Hipparcos.

Bob Argyle - Double Star Section Director

January 2014 - Double Star of the Month

The two stars to be highlighted this month are both systems of higher multiplicity although the closest visual components are at the limit of amateur instrumentation.

2 Cam (04 39 58.03 +53 28 23.7) is a Struve pair (No 566) which has somehow eluded me for more than 40 years. I first saw it in 2012 with the Cambridge 8-inch Cooke and the companion was quite well seen and measurable. Since it was first measured in 1828 at 312° and 1".5, the two stars have slowly closed and moved in a retrograde manner and last year I obtained 169.7° and 0".89. A preliminary orbit gives a period of 425 years for this unequal (5.6 and 7.5) pair of stars. In 1901, using the Yerkes 40-inch OG, Burnham noted that the primary was a close pair in itself with the new component (V = 7.4) being found at 317° and 0".2. This turned out to be a rapid binary and the currently accepted period is 26.89 years, whilst the separation of the stars never exceeds 0".3. Dembowski adds another pair in the field (D 4) and it can be found about 4 minutes preceding - mags 9.0, 10.3 and separation 5".8. Whilst in this area look at 1 Cam = STF 550, a fine bright pair.

eta Orionis (05 24 28.62 -02 23 49.7) This fine, bright pair of white stars is a good test for the 20-cm aperture. It was first resolved, when separated by 0".9, by W. R. Dawes who noted 'This close and beautiful object was discovered by me on Jan 15, 1848, with an aperture of only 4.25-in which I happened to be using on my 6.33-in refractor'. It is one of 13 pairs in the WDS under his discovery code. The current separation is around 1".8 and the stars have been slowly separating since discovery. Notwithstanding the fact that they are similarly hot, young and massive stars, the WDS notes that A and B form an optical system; the more distant mag 9.4 star at 114" is also believed to be unconnected with the bright stars. DA 5 was not known to Smyth but Webb notes colours of white and purplish (this may be the observation of Dawes) whilst others see only two white components. The A star is actually a massive triple system. Many years ago it was found to be an eclipsing spectroscopic binary of 7.88 days period with both stars possessing about 12 solar masses. More recently, speckle interferometer observations by H. A. McAlister and colleagues found a third companion with a period of 9.9 years, whose mass is about 1.5 solar.

Bob Argyle - Double Star Section Director