Double Star of the Month Archive 2015

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.

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

November 2015 - Double Star of the Month

56 And (01 56 09.23 +37 15 06.5) is a very wide pair of stars which can be found 5 degrees south and a little preceding that spectacle of the autumn sky, gamma Andromedae.

It is notable for the colours of the stars involved.

Here we have a K0 giant (A) and an M0 giant (B) within 200" of each other but actually not associated at all. The M star is about 3 times further away than the other according to Hipparcos.

Although the WDS gives visual magnitudes of 5.7 and 6.0, historical reports sometimes put star B as the brighter of the two - it is likely that either or both stars are liable to some small variation in brightness of a size such as to blur the difference between the two altogether.

Burnham, who would have been interested in this system because of the large proper motion of A (0".2 per year), found a faint companion of mag 11.9 at 18" and 77 degrees from A.

Also nearby (two-thirds of a degree preceding and a little north, according to the discoverer William Herschel), is STF 179, containing stars of magnitudes 7.6 and 8.1 separated by 3".5, and the open cluster NGC 752 is less than a degree north following.

Bernhard Hildebrande Dawson was an Argentine astronomer who was born in Kansas City in 1891 and who worked initially at La Plata Observatory in Argentina. He was perhaps best known as the discoverer of Nova Puppis 1942.

His double star discoveries are denoted by a small greek delta and number 31 is perhaps his most interesting find - a visual pair with a period of 4.56 years.

He started using the 17-inch refractor at La Plata in 1912 to pursue a programme of re-measurement of the John Herschel pairs. The first star in his catalogue (DAW 1 at 02 27 57.34 -58 08 22.3) was found in 1916 and sits in a rather blank area of sky in Horologium about 13 degrees following the first magnitude Achernar. This is a triple system with the two widest components (17" apart) being found by Sir John Herschel in South Africa.

In 1916 Dawson divided A and found a separation of 0".8. The current value is 1".2 so assuming it is still widening this would explain why Herschel did not see it during his sweeps.

The WDS gives A and B as 8.0 and 8.5 whilst C is 9.6. The range of position angle of AB is close to 180 degrees, a sign that the magnitudes of the two stars must be close enough to cause uncertainty about the quadrant in which B sits.

Ross Gould from Australia using a 40-cm sees A as a bright yellow star whilst x140 will show the Dawson companion.

Bob Argyle - Double Star Section Director

October 2015 - Double Star of the Month

STF 2816 (21 38 57.62 +57 29 20.5) which sits within the cluster Trumpler 37 (and the HII region IC 1396) and is a multiple star with three stars visible to the small aperture.

AC are stars of magnitudes 5.7 and 7.5 separated by 12" in PA 120 and with little motion visible. The primary is a very hot star of spectral type O6f. Some 20" away in PA 338 is a further star (D) of magnitude 7.8. This in turn has a 13.2 mag comes at 351 degs and 55". In 1889, using the Lick 36-inch refractor, S. W. Burnham added a 13.3 star to A at a distance of 1".7. At the highest angular resolutions A is double again with a companion at a distance of 0".1 and it is also a double-lined spectroscopic binary with a period of 3.7 days so it seems likely that this is a very close triple.

For the binocular user, the stars that make up the wide pair of pi1 and pi2 Gruis make a splendid sight. They lie about halfway between alpha and beta Gruis and a degree north of that line.

The brighter of the two is pi1 Gru (22 22 44.2 -45 56 52.61) at V = 5.62. This is a F3 star which is a giant or sub-giant and which has a large annual proper motion. Its distance is correspondly small - 130 light years. Some 269" east and slightly north is pi2 at V = 6.55 but extremely red in colour. It is a member of the rare spectral class S and has a (B - V) index of 2.1. Looking at it on the POSS images it appears slightly brighter than pi1 but when the 2MASS survey image is examined, its overwhelmingly bright image practically obliterates that of its companion.

Both stars are double and both were discovered by Robert Innes. Pi1 is I 135 - the companion is a GO dwarf, magnitude 10.7, distant 2".5 in PA 200, although observations of the system with VLTI show a spiral-shaped arc of emission which may be due to orbital motion of this dwarf and indicates the possibility of a third star, much closer in with a period of less than 10 years. Pi2 is I 382 where the 11.4 mag companion has moved from a separation of 4".6 to 11" over the course of a century, due to the large motion of A.

Bob Argyle - Double Star Section Director

September 2015 - Double Star of the Month

Nestling in the region about 4 degrees north following the upper left-hand corner of the Square of Pegasus, AC 1 (HIP1669) (00 20 54.10 +32 58 40.9) seems to have been a little neglected by the double star community. It doesn't even have a note in the WDS catalogue but things may change now that Henry Zirm has published an orbit for it.

It was discovered by Alvan Clark using a 7.125-inch object glass of his own make in October 1856, and published in MNRAS a year later with additional notes by W. R. Dawes. At the time of discovery it was 0".4 apart and Dawes was of the opinion that it escaped the attention of F. G. W. Struve at Pulkovo because it was too close at that time.

This view has been borne out - the stars are now easily measurable with a 20-cm refractor. The magnitudes are 7.3 and 8.3. Hartung notes that the stars are 'deep-yellow' and also points out the presence of a orange-red star some 4'.5 SW (HR 59 - spectral type K5III). Zirm's orbit with a projected period of 525 years is clearly provisional as the stars are close to maximum separation and only 12 degrees has been described in the apparent orbit. The author found 289°s; and 1".9 in autumn 2014.

Psi 1, 2, and 3 Aquarii are three bright stars of visual magnitude 4.2, 4.4 and 5.0 respectively. They can be found about 3 degrees east of the centre of the line joining alpha Peg to Fomalhaut, in a region rather low down from the UK but filled with interesting visual doubles.

Some 4 degrees directly below psi 3 is 94 Aquarii (23 19 06.51 -13 27 30.4) a fine, wide pair which is worth seeking out. Its mag 5.3 and 7.0 stars appeared yellow and orange to Hartung who was able to observe them close to the zenith whilst the Reverend Webb noted a reddish glare in A whilst B appeared greenish.

The proper motion of the pair amounts to more than 0".3 per year so the fact that the change recorded in separation over 200 years amounts to only 2 arc seconds tells us that this is a physical pair, and it is located 69 light years away. In 1976 McAlister and colleagues discovered that B was a close pair (MCA 74) and it has subsequently turned out to be a binary of short period. It rotates every 6.3 years and the separation is never more than 0".2.

Bob Argyle - Double Star Section Director

August 2015 - Double Star of the Month

Lambda Cygni (20 47 24.563 +36 29 26.7) is easily found. Just move 3 degrees due north from epsilon Cygni, the left-hand star in the cross. One of Otto Struve's discoveries from Pulkova the seeing needs to be good to get a measure of the companion. The stars are magnitudes 4.7 and 6.3 but have always been separated by less than 1" although the angular distance between the stars is now roughly double that at discovery. Since 1990 the writer has observed the pair in 12 seasons and in that time the position angle has decreased by about 10 degrees. The current orbit has a period of 391 years but this is complicated by the fact that Hal McAlister and colleagues found the primary to be an interferometric binary with a period of 11 years and maximum separation of 0".05. There is some evidence that one of the three stars is also a single-lined spectroscopic binary. Lambda Cyg has a spectral type of B5Ve and is a rapid rotator surrounded by a circumstellar disk. Sir James South adds a faint companion, mag 9.7, at 106° and 83".

The sprawling constellation of Capricornus sits near the meridian on a northern summer night but locating stars in it apart from the third magnitude alpha and beta needs the help of a star atlas. However, starting with the brightest star of all, beta, by moving 3 degrees south and slightly east a trio of stars is encountered, all enclosed by a 1 degree field. Each of these is a visual double star and the subject of this column is omicron Cap = SHJ 324 (20 29 53.91 -18 34 59.4) the most southerly of the three. Smyth calls it omicron2 and notes that both components are to be found in Piazzi's catalogue. The WDS gives magnitudes of 5.9 and 6.7 and the separation is currently 21".9, down from 25" when found by William Herschel with a small decrease in the position angle, currently 238°. The Hipparcos catalogue gives the distance as 216 light years but with a formal error of 27% this is an indicator that there could possibly be another star nestling in the system.

Smyth calls both stars bluish, and whilst Sissy Haas regards them as almost equal, the report by Hartung notes that they are an 'unequal white pair'.

Bob Argyle - Double Star Section Director

July 2015 - Double Star of the Month

lambda Ophiuchi - STF2055 (16 30 54.84 +01 59 02.8) lies 20 degrees due south of the south-easternmost star in the Keystone of Hercules, beta Herculis. During the 19th century, measurements showed the two stars slowly separating and reaching a maximum distance of about 1".9 in the early 1880's. In 1906, Thomas Lewis, in his book on the Struve stars, considered that the period was about 130 years which as it turns out was a good attempt. The current period from observations of almost 2 revolutions gives a period of 129 years and a predicted position for mid-2015 of 42° and 1".43. The pair is not particularly easy to observe, partly because of its altitude in the sky - it is barely above the celestial equator - and partly because of the magnitude difference between the two components (4.2 and 5.2). Both stars are A0 dwarfs, according to the WDS, and lie at a distance of 173 light years. There are two much fainter stars at 120" and 308" which appear unconnected to the system.

DUN 219 (17 58 55.69 -36 51 30.2) lies not far from the spectacular open cluster Messier 7 and follows G Sco by about 2 degrees. It has not been included in either Hartung or the book by Sissy Haas but is nevertheless is a splendid sight. The stars are magnitudes 5.8 and 7.8 and the separation derived by Dunlop in 1836 of 47".1 has now increased to 53".5, (with the position angle changing from 265° to 252° over the same interval of time), making it a striking object object in small telescopes. The USNO include the pair in their linear elements catalogue, confirming that the two stars are entirely unrelated. A third star of mag 11.3 can be found at 40" from A. Just 16 seconds of time following and 8 arc minutes south is HJ 5000 AB, stars of magnitudes 7.1 and 8.9 separated by 7".3 but apparently in orbital motion. The writer measured this pair in 2010 and noted pale yellow and pale blue - pretty.

Bob Argyle - Double Star Section Director

June 2015 - Double Star of the Month

Two bright double stars, each with its own observational difficulties, feature this month.

In the constellation of Draco, eta Draconis (16 23 59.51 +61 30 50.7) is circumpolar for observers in the UK but as a classical refractor user I find it is easiest to observe when in the north-west in the early evening. The primary is a G8 giant (V = 2.8) which is 92 light years distant and as a relatively nearby object it has attracted the attention of the astronomers wishing to determine its diameter. The result is that it is a shade under 10 million miles in diameter or about 11 solar diameters. The mag. 8.2 companion was first noted by Otto Struve at Pulkova in 1843 and he found it at PA 150° and separation 4".4. Since then it has moved retrograde by 11 degrees and the separation has slightly increased. I measured the pair on 4 nights in 1994 but in recent years have not been able to see the companion. The primary is also recorded as being variable so here is a pair to keep an eye on.

epsilon Lupi (15 22 40.89 -44 41 22.5) first attracted the attention of James Dunlop from Paramatta as a fine unequal and wide pair (DUN 182 AC). The stars are magnitudes 3.6 and 9.1 and today are separated by more than 26", a considerable increase on Dunlop's figure of 19". When Ralph Copeland, who later became Astronomer Royal for Scotland, visited the Andes with a 6-inch refractor for site-testing purposes, he found the primary star was a close and unequal pair which has since turned out to be a binary of period 319 years. Star B, mag 5.1, ranges in distance from A between 1".2 (as in 1834) to 0".18 (in 2026); in 2015 the pair are separated by only 0".26 and needs a large aperture. More recently, in a survey of stars in the Sco-Cen-Lup-Cru association, Rizzuto and colleagues, using the Sydney University Stellar Interferometer (SUSI) found another star of magnitude 5.1 but closer in at a distance of 0".05. It is not clear whether this is the known spectroscopic binary component of A (which has a period of 4.55 days), but epsilon appears to be at least a massive triple system.

Bob Argyle - Double Star Section Director

May 2015 - Double Star of the Month

STF 1694 (12 49 13.80 +83 24 46.3) is one of the most northerly of the bright double stars and lies in a rather sparse area of the sky. It is worth making the effort to find it as the planetary nebula IC 3568 lies 1.5 degrees south-west. Both stars are in the Hipparcos Catalogue and though the parallaxes seem significantly different (8.14 mas for A and 5.57 mas for B), the large error on that of B means that they are the same within the quoted uncertainties. However, B is a spectroscopic binary and this may well account for the Hipparcos satellite's problems with defining its distance accurately. The primary star (V = 5.3) is an A1 giant whilst the companion (V = 5.7) consists of a pair of A0 and A2 dwarfs. The current position angle and separation is 324° and 20".9, little changed from the epoch of discovery. The proper motion of RA of both stars is significant and similar so it would appear that this is a common-proper-motion pair. In 1944 Wallenquist found a third star of magnitude 11.5 at 223° and 73" but both these values are decreasing quite quickly. Sissy Haas calls the primary 32 Cam and notes that both stars are lucid-white.

4 Centauri (H N 51) is located at 13 53 12.54 -31 55 39.4. Also known as h Cen, this is one of the pairs found by William Herschel in his last concerted campaign of double star observation and was observed on 1787, Mar 15. It is in an arc of faint naked-eye stars some 5 degrees north-west of theta Centauri and is a splendid sight in a small aperture. The stars, magnitudes 4.7 and 8.5, were measured by the writer in 2013 and were found to be at 185 degrees and 14".8. Hartung notes that the colours are pale yellow and ashy, and that both stars are spectroscopic binaries. The primary star is a subgiant of spectral class B4 which Hipparcos puts at a distance of 637 light years.

Bob Argyle - Double Star Section Director

April 2015 - Double Star of the Month

STF1555 (11 36 17.94 +27 46 52.7) is in Ursa Major in a fairly sparse part of the constellation down by the Bear's front foot. It is perhaps most eassily found by moving 5 degrees south-east from the bright binary xi UMa (STF1523). This was an easy pair for the small aperture at discovery at 1".4 but during the next one hundred years the two stars approached each other until minimum separation of about 0".1 was reached in the 1930s and since then they have been separating. The nature of this system is a little unclear but observations over the next decade or so will show whether it is an optical pair (as classified in the WDS, or a highly inclined binary system, as suggested by Docobo in 2007 when he derived an orbit of 916 years for it. The orbit predicts star B beginning to turn back towards A with the separation slowly decreasing again. In spring 2015 B can be found at 150°, 0".67. The stars are magnitudes 6.4 and 6.8, and a third component of magnitude 11.2 which is listed as HJ 503, can be seen at 158° 22".5, both values are slowly increasing.

RMK 14 (12 14 02.71 -45 43 26.1) can also be found by reference to a nearby bright binary star. It forms an isoceles triangle of side about 5 degrees with gamma and delta Centauri to the south. Unfortunately, as of early 2015, gamma is near closest separation and needs a large aperture to resolve but RMK 14 (D Cen) is a beautiful pair which is worth searching out. The primary is a K3 giant of visual magnitude 5.8 and is also known to be a spectroscopic binary. The companion can be found at 243° and 2".7 having closed up from 4" at discovery. This is a distant pair - Hipparcos lists the parallax as 5.71 mas which, as it happens, translates to 571 light years. The colours seem to be well determined. E. J. Hartung gives orange and white whilst more recently, Richard Jaworski finds yellowish-orange and white. Sissy Haas notes it as a showcase pair.

Bob Argyle - Double Star Section Director

March 2015 - Double Star of the Month

With Cancer now moving inexorably towards the western horizon, its stars are becoming more comfortably accessible to the classical refractor user and one of its finest doubles is 57 Cnc (08 54 14.70 +30 34 45.0).

According to Webb, it is a pair of crocus-yellow stars which are not quite equal in brightness (magnitudes 6.1 and 6.4). There has been little motion of any kind save a small increase in both position angle and separation with the result that the separation is now well over 1".

It is an ideal test for a 7.5-cm refractor. Webb was able to divide it at x144 on his 3.7-inch Tulley OG, and the writer found 1".55 with the Cambridge 8-inch in 2006.

The pair is almost certainly binary in nature as the proper motion of the system would have separated them a long time ago. A faint component can be found at 55" and PA 202° which appears fixed. The WDS gives its magnitude as 9.2.

57 Cnc is about 2.5 degrees north following the spectacular wide pair iota Cnc.

Gamma Sextantis, also called 8 Sextantis (09 53 30.47 -08 06 17.7) was discovered on 1852 Apr 7 by Alvan Clark with a 12-cm aperture. As it happens he observed the star at a suitable part of its orbit but this discovery is a good indication of the quality of Clark's telescopes.

He said, reporting his finds in Monthly Notices for 1857, Notwthstanding the moderate meridional altitude of of 8 Sextantis at Dorpat (about 24°), it may reasonably be doubted whether its duplicity would have been left to be discovered with a 4 3/4-inch object-glass, however perfect, if no change had occurred in its appearance since Struve's scrutiny of that part of the heavens.

The separation of this close pair is never wider than 0".6. It reached maximum separation in its 77.8 year cycle in 2002 and is now closing again. By Spring 2015 it will be at 44°, 0".54 and the low declination means that it will be a significant test for 20-cm and its more likely that a good, steady air will be essential to resolve it, especially as there is a considerable difference in brightness between the components - A is 5.4 and B is 6.4. Hipparcos places the system at 278 light years distance and the primary is an A1 dwarf.

To find gamma Sextantis, locate alpha Hya and move 5 degrees east.

Bob Argyle - Double Star Section Director

February 2015 - Double Star of the Month

In mid-February, Gemini passes the meridian at 10pm and offers an excellent opportunity to view some of the fine double and binary stars in this constellation. One of the less-well known systems is 20 Gem (06 32 18.52 +17 47 03.4) which is eaily found about 3 degrees north preceding the bright star gamma Gem. Wil Tirion's Cambridge Star Atlas shows another double star about a degree south of 20. This is STT143 (6.2, 10.4, 103°, 7".6 with a fainter third component at 345° and 47"). Also known as STF924, 20 Gem offers a beautiful pair of stars which Webb noted as topaz yellow and cerulean blue, but as these are the exact hues noted by Smyth in the Bedford catalogue it may be assumed that Webb was merely repeating the Bedford colours. More recently Sissy Haas finds both stars to be gloss-white.

There is little relative motion between the stars and the current situation is that the PA is 211° and the separation 20".2. Hipparcos found the determination of the parallax of star A difficult and finds a distance of 262 light years with an error of 30%. This is no doubt due to the motion induced by the duplicity of A confirmed by both occultation observations and also directly as a spectroscopic binary.

Looking about 1° slightly north preceding the V = 4.4 star q Puppis, one will alight on the wide pairs DUN 67 (08 13 58.31 -36 19 20.2) and DUN 68 about 1.5 arc mins south and west of it. DUN 67 consists of stars of magnitude 5.0 and 6.0 which are currently separated by 66 arc seconds and PA 174°. The separation is slowly decreasing. Hartung chooses not to include them is his book 'Astronomical Objects for Southern Telescopes' but Gould with 175-mm finds both stars to be pale yellow and notes the existence of several fainter pairs in the area. The components of DUN 68 are both mag 7.3 but form a wider pair than its neighbour. The PA is 25° and the separation is 125" and increasing.

The whole region is fine with the large scattered star cluster NGC 2546 about 1.5° further south adding further reason to take in the area with a pair of binoculars as well. Both pairs are at the same distance from us within the stated errors of the Hipparcos parallaxes.

Bob Argyle - Double Star Section Director

January 2015 - Double Star of the Month

STF 644 in Auriga (05 10 18.81 +37 18 06.7) is a pair noted for the contrasting colours of its components. The spectral types of the two stars are respectively B2II and K3 and when W. H. van den Bos observed the pair with the Lick observatory 12-inch refractor in 1962 he noted the colours were white and reddish. E. J. Hartung noticed this colour disparity too - he found orange amd white with 10.5-cm aperture adding if merely elongated the appearance of the ellipse is striking with one end orange and the other white. Sissy Haas on the other hand with 125-mm at x200 found both stars to be yellow-white.

This is a close pair which has remained virtually motionless since it was discovered by F. G .W. Struve. The magnitudes are 7.0 and 6.8 with the orange component being slightly brighter visually. G. van Biesbroeck added a distant mag 10.5, K3 dwarf at 192 degs, 72". To find STF 644 look about 1.5 degree south of mu Aurigae. It is the preceding of two mag 6 stars - the other being the unequal wide pair SEI 105 (6.5, 11, 27 degs, 35"), about 1 degree following.

HJ 3683 (04 40 17.72 -58 56 39.5). Very fine noted John Herschel from South Africa in 1836 when he first laid eyes on this pair of stars and gave them both magnitude 8. The WDS catalogue says they are 7.3 and 7.5 but the main interest lies in the nature of the apparent orbit of this 326 year binary. The high inclination means that motion is restricted to a narrow range of position angles and so mainly manifests itself in separation, whilst the very high eccentricity (0.95) means that the maximum separation is 4".3 whilst the minimum is only 0".03 which, given the Hipparcos parallax of 32.77 mas, indicates that the two stars are only 3.7 AU apart at periastron but 144 AU apart at apastron. When at their nearest separation the position angle changes by 1 degree per day, almost 5 times faster than that managed by the two components of gamma Virginis at periastron.

The pair is currently at 3".7 and so is an easy object for the small telescope. Gould with 175-mm calls both stars yellow and the pair can be found about three degrees south and slightly following alpha Doradus.

Bob Argyle - Double Star Section Director