Double Star of the Month in Aries

• December 2018 - Double Star of the Month

  Both pairs in this month's columns are long period binaries accompanied by distant and faint, but co-moving companions.

  STF 326 (02 55 39.06 +26 52 23.6) was unknown to me until recently. Observations of it on the web indicate that the stars are yellow-orange and reddish. It is 1.5 degrees ESE of 41 Arietis, itself a wide pair found by Herschel (3.6, 8.8, 237 degrees, 123"), but also a more complex system according to the Washington Double Star catalogue (WDS).

  Despite having moved just 5 degrees in position angle (PA) since 1831, STF 326 was allocated a hyperbolic orbit in the 1960s - suggesting that the stars make one close approach and then fly off into different directions in space. The existing astrometry hardly supports this theory but the stars have certainly closed since discovery and are now at 221 degrees and 5".5 with the K2 primary at magnitude 7.7 and the M0V secondary at V = 10. They are thus rather faint but the fine colours make this a system worth looking out for.

  There is a background star (C) at 171 degrees and 41" (magnitude 11.9), but the 13.9 magnitude comes at 266 degrees and 44" is LDS 883 D. It is at the same distance as the AB pair, and moving with the same substantial proper motion- 0".3 per year.

  Gaia DR2 puts them all at 73.5 light years.

  BU 1004 is in the constellation of Eridanus (04 02 03.44 -34 28 55.7) and located about 3.5 degrees west of 41 Eri. It was found by Burnham in 1881 and with magnitudes of 7.3 and 7.9 it must have been an easy object in the Mount Hamilton 12-inch.

  Since then the position angle has reduced by 100 degrees and the separation has changed from 1".7 to the current value of 1".2 at PA 50 degrees, making it a rather tricky object from the UK due to the very low altitude.

  J. Docobo finds an orbital period of 410 years and predicts that the stars are now close to minimum separation and may reach a maximum of 1".8 by around 2280.

  In the last century W. J. Luyten found a faint star, probably a white dwarf, moving through space with a similar proper motion to AB. LDS 3551 B is visual magnitude 18 and lies 64" distant in PA 313 degrees.

  DR2 pins all three stars down to 151 light years give or take 0.1 or 0.2 light years so this is a physical triple star.

  Bob Argyle - Double Star Section Director

• December 2016 - Double Star of the Month

  pi Arietis (02 49 17.55 +17 27 51.5), also known as STF 311, is a very unequal triple and as such probably needs 10-cm to see all three well. The stars are magnitudes 5.3, 8.0 and 10.7, and the separations of AB and AC are 3".2 and 24".1.

  AB first came to the attention of the elder Herschel in 178x and is listed as H I 64. Sissy Haas notes that this pair was not seen in 12.5-cm, but Admiral Smyth is enthusiastic about this superb trio. Pale yellow, flushed and dusky is his conclusion on the star hues.

  The proximity of pi Ari to the ecliptic has resulted in lunar occultations occurring and it was during one of these events that a close companion to A was found. A also appears to be an SB1 but it seems unlikely that this is the occultation pair so A would appear to be a group of 4 stars.

  Looking much further out David Arnold finds a mag 10.5 star at 220" whilst Tofol Tobal has imaged stars of mags 14 and 15, at respectively 14".2 and 17".5 from A.

  DUN 16 = f Eri (03 48 35.88 -37 37 12.6) is a member of the Tucana/Horologium + Columba moving group - a cadre of bright stars with similar space motions which are between 37 and 65 light years away from the Sun.

  The pair consists of a magnitude 4.7 B8 dwarf paired with a 5.3 magnitude A1 dwarf. The separation has increased from 7" in 1826 to 8".4 in 2009 whilst the position angle has changed from 202° to 216° in the same interval.

  The significant proper motion of star A would have moved it further away from B had B been a field star, rather than a binary companion, as seems to be the case.

  One star is possibly a beta Lyrae-type eclipsing binary. The revised Hipparcos parallax gives a distance of 50.8 parcsecs. Ernst Hartung says that This beautiful pale yellow pair dominates a field of scattered stars and is a fine sight with 7.5 cm. Sissy Haas calls it a Showcase pair.

  Bob Argyle - Double Star Section Director

• December 2015 - Double Star of the Month

  Epsilon Arietis = STF 333 (02 59 12.73 +21 20 25.6) can be found 17 degrees following beta Arietis. It is in a rather sparse area of sky and for those without setting circles this is probably the easiest way to find it.

  It is a splendid binary star with both components of mags 5.2 and 5.6 being white to most eyes although W. H. Smyth fancied that he saw a pale yellow hue to the primary star and the secondary was 'whitish'. Smyth also noted that the Reverend Dawes first saw this pair at his observatory at Bedford.

  Struve noted is as being amongst the closest of his discoveries (0".5) and subsequent observations showed it widening with a small increase in position angle. Over the last century there has been little motion of note except that the companion now seems to have reached greatest elongation and is slowly heading back towards the primary star.

  In Webb Society Double Star Circular No 17 (2009) Ian Coster produced an orbit for STF 333 with a period of 313 years and three years ago Francisco Rica published one with a period of over 1200 years, so the future motion is almost entirely indeterminate. The pair will certainly remain an excellent test for 10-cm aperture for a few years yet, and the current separation is 1".34. A faint field star of mag 12.7 is 146" distant.

  John Herschel swept up the coarse triple HJ 3644 (04 21 31.29 -25 43 42.4) in 1836. It is located in an empty region in Eridanus and can be found by firstly locating the bright pair nu3 and nu4 Eri and moving about 9 degrees due north.

  Herschel noted the stars had magnitudes 6, 8 and 14 and only estimated angles and separations. Modern catalogues give the brighnesses as 6.2, 8.2(C) and 13.0(D) and the distances between AC and AD are now 41" and 44". Burnham found the A star to be a close double in 1879 when using his 6-inch refractor on Mount Hamilton in California, fortuitously as it turns out because the pair was then at its widest separation of 0".65.

  For a few years after discovery motion appeared rather slow but accelerated considerably in the second decade of the last century and the pair closed up to 0".2 by 1920. Modern computations give orbital period as 81 years and at the present time the stars are slowly widening. The position for 2016.0 is 222°, 0".40 so at least 25-cm is necessary to see the pair divided. The WDS lists the magnitudes of AB as 6.6 and 7.3.

  Bob Argyle - Double Star Section Director

• 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

• Double Star of the Month - November 2013

  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.

  Three degrees north of alpha Ari is a line of four stars of marginal naked-eye visibility. The most westerly of these stars is 10 Arietis, a close visual binary. The third star is 14 Ari - a coarse triple. 10 Ari (02 03 39.26 +35 56 07.6) was found by Wilhelm Struve at Dorpat and during the remainder of the nineteenth century the star closed slowly, leading Burnham to comment (in 1906) 'Probably orbital motion, although the measures are well represented on the hypothesis of rectilinear motion'. From a distance of 1".98 in 1833, the companion passed by the primary around 1920 at a distance of about 0".3 and has been widening since. The current catalogue period is 325 years and gives a position of for 2014.0 of 346° and 1".48. It is a nice pair in the Cambridge 8-inch but needs a night of good seeing to see the companion of magnitude 7.9 close against the mag 5.8 primary.

  omega Fornacis (02 33 50.71 -28 13 56.4) was swept up by John Herschel at Feldhausen in 1834 and is catalogued as HJ 3506. He noted that it was a 'very fine star but ill-defined'. The stars are magnitudes 5.0 and 7.5 and there has been little motion since discovery, the latest position in the WDS gives 10".8 and 245°. Sissy Haas calls this a 'showcase pair' and gives colours of goldish-white and smoke-grey whilst Magda Streicher with a larger aperture notes yellow-white and light grey-blue. Hartung does not pronounce on colours but notes a similar pair about 2' west. It seems likely that the two stars are physically connected. Hipparcos gives a distance of 484 light years and the primary is a subgiant of spectral type B9.5.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - November 2006

  In this new 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.

  Well placed in mid-November in the north is the constellation of Aries, its three brightest stars nestling midway betweeen the Square of Pegasus and the Pleiades.

  The brightest is Alpha Ari, V=2.0 also known as Hamal. It is a K2 giant at a distance of 20.2 parcsecs.

  Beta Ari (Sheratan, V=2.65) is a close pair - it was first found as a spectroscopic binary over a century ago by Vogel at Potsdam. More recently, it has been resolved by long base-line interferometers. The apparent orbit is very eccentric and the separation varies between 4 and 60 milli-arcseconds.

  The primary is an A5 dwarf and the difference in magnitude is 2.6. The system is 18.9 parsecs distant.

  But for the small telescope user, the best double star in Aries is gamma, first found by Robert Hooke in 1664 whilst following a comet. Also known as Mesarthim the proper motion of star A is about 0".1 per year but as the relative positions have changed by less than 2 arc seconds over the past 170 years, it can be assumed that this is a long period binary. Both appear to be pure white and can be separated in a 2-inch telescope or possibly in stabilized binoculars. The separation has slowly diminished since the first measures (8".6 in 1830) and is now 7".6 with the position angle almost exactly 0 degrees.

  Fornax occupies the area of sky from about 2 to 4 hours RA and between declinations -23 and -39 approximately. It is more notable for extragalactic objects - hosting as it does the Fornax cluster of galaxies, the Fornax dwarf gakaxy and NGC 1365, a spectacular barred spiral galaxy. The brightest star alpha For, a yellow F6 subgiant at 03 12.1 -28 59 (2000) is of considerable interest to the double star observer. Found by John Herschel in 1835 (HJ 3555) at PA 310 and separation 5".30, the pair continued to close and was single to Robert Innes at Johannesburg using a 9-inch refractor in the first years of the last century. It was not until 1925 that the companion appeared on the other side of the primary at a distance of 0".8 when van den Bos found it to be an easy object, but the following year, as it widened, it seemed to have faded by as much as 1 magnitude. Hipparcos notes a range of 0.04 magnitude in the visual brightness of the system, which, if it occurs only in star B implies that B varies about 0.13 mag. This is small compared to observed variations which perhaps support a period of variation of decades. The Hipparcos mission lasted only 3 years. The apparent orbit is a very narrow elongated ellipse and the companion is currently now near maximum separation which will occur in 2042 so should be easily visible in a 20-cm telescope.

  The period is about 290 years and the pair is 14 parsecs distant. Burnham identifies the star as 12 Eridani in his General Catalogue. The colours have been noted as yellow and greenish.

  Bob Argyle - Double Star Section Director