Double Star of the Month Archive 2025
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 2025 - Double Star of the Month
34 Piscium (00 10 02.18 +11 08 44.9) lies near the border between Pisces and Pegasus. It is 4.5 degrees south of, and slightly preceding gamma Pegasi (V = 2.8). It was found by F. G. W. Struve and included as number 5 in his Dorpat Catalogue. This is a fine, but very unequal pair, and 15-cm should give a good view of the stars.
The Washington Double Star catalog gives magnitudes of 5.5 and 9.4 making them a rather tricky prospect for a small aperture and classifies the primary as B9V. The stars have remained fixed in separation since discovery and are currently 7".5 apart in position angle 159 degrees.
Gaia DR3 gives parallaxes for the stars but the error on the primary star is almost five times that of the secondary and even allowing for the combined errors the parallaxes are different, although the proper motions are similar. Hopefully DR4 which is due out at the end of 2026 will throw more light on the situation.
HJ 3375 (00 44 43.80 -35 00 07.3) is 2.5 degrees SSE of eta Sculptoris and about 5.5 degrees west and slightly south of the Sculptor Dwarf Galaxy. It was found by John Herschel at the Cape. Over the past 190 years there has been little angular motion and in 2015 the companion was found in position angle 171 degrees at a separation of 4".4.
This is a relatively nearby binary system - the distance is 110.7 light-years and both components are moving across the sky at more than 0.5 arc-seconds per year.
Four degrees SE of HJ 3375 is the wide pair lambda 1,2 Scl. The easterly component (lam 1) is a K giant (V = 7.1) whilst 20 arc-minutes to the west is lam 2 (B9.5V). This is a visual binary which is now slowly separating and is currently near 0".8. The components have magnitudes 6.6 and 7.0.
Bob Argyle - Double Star Section Director
If you'd like to try out the Clear Skies Observing Guides (CSOG), you can download observing guide for the current Double Stars of the Month without the need to register. CSOG are not associated with the Webb Deep-Sky Society but the work of Victor van Wulfen.
September 2025 - Double Star of the Month
About 1.5 degrees NW of gamma Sagittae, the bright star at the tip of the arrow of Sagitta, is STF I 48, or more recently STFA 48 (19 53 22.65 +20 20 14.1). This is a wide and bright double star which is easily seen in small apertures.
It forms part of the additional list of wider pairs compiled by Wilhelm Struve during his Dorpat survey. It was, however, noted by William Herschel forty years earlier who dubbed it H V 106 (now H 5 106). I measured it in 2003 when the relative position was 147 degrees and 42".
There is a 10.5 magnitude star some 222" distant and more recently the Hipparcos satellite resolved the B star into a pair of stars with magnitudes 7.6 and 9.4 but with a current separation of 0".2 this probably requires 40-cm and a night of good seeing. It is a binary with a period of 85 years.
The double stars found by James Dunlop are bright, wide and sometimes colourful. DUN 236 (21 02 12,73 -43 00 07.7) lies in the constellation of Microscopium about 1.5 degrees SSW of the magnitude 5.5 star eta Microscopii which is an orange giant.
The stars (magnitudes 6.7 and 7.0) were 67" apart when found in 1826 and since then the separation has reduced by 10" although the proper motions are almost identical. The spectral types suggest colours of pale yellow and pale orange. The parallaxes are similar but that of the brighter component has a very large error which may mean that there is another component close in.
Move the telescope 4 degrees due east of DUN 236 and you will alight on MLO 6 (see this column for Sept 2012), a fine and unequally bright pair.
Bob Argyle - Double Star Section Director
August 2025 - Double Star of the Month
About 3 degrees north of zeta Aql (V = 3.0) star atlases show a close pair of stars of around 6th magnitude which are aligned almost E-W. This is STTA 177 (19 12 34,45 +16 50 47.2). The stars were 121" apart in 1874 but are closing and are now near 97", still making them a good target for binoculars.
The A star is of interest as it is both a close visual binary but it is also an eclipsing binary of the Algol type with a period of 3.5 days. When S. W. Burnham was prowling the skies looking for new pairs with his 6-inch Clark refractor one of the stars he alighted on was this one. He found a pair of stars with magnitudes 7.1 and 8.0 separated by only 0".5. There has been little motion since then. I managed to measure them once in 2012 with the Cambridge 8-inch when the measure gave 0".7 but more recent measures put them at 0".6. There are two fainter and unrelated stars in the field.
Corona Australis sits below Sagittarius and on the sky it occupies a rectangle with sides of about 15 degrees in RA and 7 degrees in Dec. About 4 degrees east of the centre of this box and 1 degree south-east of mu CrA (V = 5.2) lies HJ 5066 (18 50 58.65 -41 03 45.7). It is marked as double in the second edition of The Cambridge Double Star Atlas but it is not labelled. The primary is a magnitude 6.5 B giant and its companion of magnitude 9.2 lies 10" away in PA 85 degrees.
Move another degree SSE and you will arrive at HDO 291, 6.1, 11.4, 341 degrees, 37" which is separating fairly rapidly due to the difference in proper motion between the two stars, the primary of which is a K giant. In one of his last discoveries, Robert Innes in 1926, using the then new Johanneburg 26.5-inch refractor, found B to a close equal pair (10.2, 12.2, 292 degrees, 0".6). A recent version of the Washington Double Star catalog (June 2025) records this discovery observation as the only observation.
Bob Argyle - Double Star Section Director
July 2025 - Double Star of the Month
Zeta Lyrae (18 44 46.36 +37 36 18.4) is the third star in the tight, naked-eye triangle that includes Vega and the Double-Double (epsilon). It appears in many double star catalogues, including those of Herschel, the Struves and South and Herschel but according to the Washington Double Star catalog (WDS) it was found by Francesco Bianchini (1662—1729) with the date 1737 which presumably refers to a book published posthumously called Astronomicae et geographicae observationes selectas... written in Latin, but available in full on the internet.
The brightest component, zeta1 is magnitude 4.34, and is a spectroscopic binary, whilst the fainter component, zeta2, is 5.62. The stars lie 157.9±0.2 light-years away according to Gaia DR3. An observation with the Cambridge 8-inch in 2010 gave 149 and 45".4 but there is very little relative motion between the two stars.
Zeta Lyrae is resolvable in binoculars and both stars appear white. There are four faint field stars ranging from mags 13.3 to 15.7 which lie between 22" and 78", two of which were found by Burnham with the 18.5-inch at Dearborn and the faintest was detected in the Lick 36-inch.
WNO 6 (18 28 57.36 -26 34 55.5) lies 1.5 degrees south of the 2.8 magnitude lambda Sagittarii, the orange-hued K1 giant star at the top of the 'Teapot' lid of Sagittarius. It is a fine pair for the small aperture with stars of magnitudes 6.7 and 8.0 separated by 42" with a position angle of 182 degrees. Gaia DR3 places them at significantly different distances, the A component has a distance of 639 light-years away but it has a large formal error of 24 light-years. B is 472 light-years away.
It makes a tight triangle with two other stars the most north-westerly of which is BU 133, a rather difficult pair requiring at least 25-cm as the stars have passed 0".6 and are still closing, having been 1".8 apart when discovered by Burnham using his 6-inch Clark refractor. The magnitudes of 6.6 and 8.5 add to the difficulty of resolution.
Bob Argyle - Double Star Section Director
June 2025 - Double Star of the Month
Two degrees of tau Her (V = 3.9) is STF 2063 (16 31 47.73 +45 35 53.8). This is a rather wide but also rather unequal pair with the two stars shining at magnitudes 5.7 and 8.7 respectively.
There has been very little motion since William Herschel noted them on 1782 Nov 8. and the pair entered his catalogue as H 4 62 and which he called very or extremely unequal noting colours of white and 'br' although it is unclear what this actually is. Bright red? Bluish-red? Brown?
I checked with Bruce MacEvoy's excellent complete catalogue of the Herschel double stars and he does not comment on the colours. I observed the pair with a 21-cm reflector at x96 and found colours of white and blue.
The primary has a spectral type of A1V, and the Gaia DR3 catalogue reveals the stars are at the same distance (230 light-years) and have very similar proper motions. The companion lie at PA 195 degrees and the stars are currently separated by 16".3.
A rather similar pair can be found in the southern hemisphere in the constellation of Ara. HJ 4978 (17 50 28.40 -53 36 44.7) is a little more unequal (5.7 and 9.2) and somewhat closer (12".3 and PA 268 degrees). The system, noted as nu1 Arae by John Herschel, lies 1.5 degrees due east of the globular cluster NGC 6397.
The SIMBAD catalogue notes that the primary star is an Algol variable called V539 Ara, it has a period of 3.17 days and the eclipse depth is 0.53 magnitude, but SIMBAD gives no reference to the star as a wide visual double.
Whilst in the area, check out the pair R 303 which is one degree SW of HJ 4978. The stars are magnitudes 7.8 and 9.0 with a separation of 3".6 and a PA of 108 degrees.
Bob Argyle - Double Star Section Director
May 2025 - Double Star of the Month
Lambda Bootis is the prototype of a class of stars with unusual chemical composition - the abundance of metals is unusually low and they also tend to be accompanied by accretion disks or planetary systems. Two degrees NE of lambda is STF1843 (14 24 38.91 +47 49 50.0) is a wide and rather unequal pair which lie 316 light-years away.
The Washington Double Star catalog (WDS) gives magnitudes of 7.7 and 9.2 and a recent measure puts the companion at 186 degrees and 19".8 with the separation widening with time. A third star of magnitude 9.3 can be seen 101" away in PA 63 degrees but the parallax given by Gaia DR3 indicates that it is more than 800 light-years away. There is, however, another member of the system - a magnitude 17.8 star found by Andrei Tokovinin. An observation which I made with a 10-inch reflector at x96 showed colours of yellow and lilac.
In eastern Virgo lies phi Virginis (14 28 12.22 -02 13 40.6) which presents a greater challenge to the observer with a small telescope. It is the magnitude of the companion and its relatively close separation to the A star which means that a 15-cm telescope is recommended. The two stars are given catalogue magnitudes 4.0 and 10.0 and the current separation is 5".2, although when first measured in 1827 they were only 4" apart.
The pair was measured with the Cambridge 8-inch in late 2015 and given that the micrometer uses a red LED to illuminate the field and hence shows the wires in outline I would have said that the B star must be brighter than 10. Both stars lie at 121 light-years distance and are moving through space with almost identical proper motions of 0".139 per year. The relatively large error on the parallax of A as measured by Gaia chimes with the comment in the WDS that A may be a spectroscopic binary.
Bob Argyle - Double Star Section Director
April 2025 - Double Star of the Month
With Arcturus riding high in the Spring sky, STF 1825 (14 16 32.84 +20 07 18.7) can be easily found as it is just 1.5 degrees north and a little west of the bright, orange giant star. The components have magnitudes 6.5 and 8.4 and the current separation is 4".4 so it is a relatively easy object for the small telescope.
The very slow orbital motion is reflected in the fact that my two sets of measures made in 1993 and 2019 show a difference in position angle of just 2 degrees. This is a relatively nearby system as the parallax determined by Gaia DR3 shows, the distance is 108 light-years and both stars are moving across the sky at more than 0.1 arc-second per year.
In Centaurus, near the border with Lupus, and about 9 degrees north and slightly west of Hadar (beta Centauri, V = 0.6) is HJ 4651 (14 09 35.03 -51 30 16.8), a wide and unequal pair of unassociated stars. The magnitudes are 6.0 and 8.9 and the current separation is 65". The primary star is the eclipsing binary V869 Cen and is almost four times nearer to the Sun than its fainter companion.
Of more direct interest but certainly a more difficult object is SLR 19 which is to be found 1.5 degrees north and a little preceeding HJ 4651. It is an almost equally bright pair which was first observed by R. P. Sellors in Australia using the 11.5-inch refractor at Sydney Observatory. The motion to date amounts to 45 degrees and an orbit by Izmailov gives a period of 415 years which predicts a closest separation of 0".6 in 2054. The predicted position for 2026.0 is 337 degrees and 0".9 and the magnitudes are 7.1 and 7.4.
Bob Argyle - Double Star Section Director
March 2025 - Double Star of the Month
The spectral type of the primary of STF 1689 (12 55 30.32 +11 29 46.4) is given as M4III in the Washington Double Star catalog (WDS). One might expect to see a deep-orange, or red star but observations of the pair in the literature seem to indicate that the primary colour reflects an earlier spectral type.
T. W. Webb notes yellowish, whilst Admiral Smyth recorded pale white. All three observers noted that the companion was blue or bluish. More recently, I recorded orange and blue using a 10-inch reflector at x96 in 1970. Sissy Haas gives peach-white, and a note on TheSkySearchers website records colours of orange and bluish.
Gaia has observed both components and the stars are unrelated. The primary is 960 light-years away whilst B is 474 light-years distant. The components are slowly separating due to the different proper motions and in 2019 the relative position was 222 degrees and 30".3. The pair can be found 2 degrees NNW of epsilon Virginis.
R 213 (13 07 24.30 -59 51 37.8) was discovered in 1874 by Henry Chamberlain Russell. Russell was the Government Astronomer at Sydney Observatory from 1870 until 1905. He was using a 11.5-inch refractor made by Schröder to carry out a survey of some of the John Herschel double stars which the latter discovered at the Cape. It was one of the most difficult of his discoveries being found at 207 degrees, 0."3. His description of the stars was a very beautiful double star - both alike and orange coloured
.
When I observed the pair in 2016 using the 67-cm refractor at Johannesburg, the PA was very similar but the distance has increased to almost 0".8. With WDS magnitudes of 6.6 and 7.0, the pair should be divided in 15-cm. R 213 lies in Centaurus close tothe border with Crux and can be swept up by moving 2.5 degrees due east from beta Crucis (Mimosa), which is DUN 125 - magnitudes 1.3, 7.2, 23 degrees, 373" - a fine binocular double.
Bob Argyle - Double Star Section Director
February 2025 - Double Star of the Month
Castor (07 34 35.86 +31 53 13.8) has already been discussed in this series (Feb 2007) but since then considerable advances have been made in our knowledge of this famous sextuple system.
It is eighty years since systematic radial velocity measurements of the two bright components, both of which are spectroscopic binaries, were made. Recent observations using a 1.5-metre telescope and an echelle spectrograph have refined the periods to within 4 seconds for AaAb (period = 9.2 days) and 0.1 second for BaBb (period = 2.9 days).
Combining these data with new and direct images of the spectroscopic pairs using ground-based interferometry has produced masses for the four main stars to remarkable accuracy. They are 2.37 suns for Aa, 0.39 suns for Ab, 1.79 suns for Ba and 0.39 suns for Bb, the errors in each case being below 0.02 sun.
The work was described by Dr. Guillermo Torres and collaborators in a 2022 paper which appeared in Astrophysical Journal. The paper also gives the elements of an orbit for AB with a period of 459 years.
The pair was measured in May 2024 with the Cambridge 8-inch Cooke, and a mean of three nights gave 50.6 degrees and 5".76 as the stars continue to widen.
DUN 38 (07 03 57.32 -43 36 28.9) is a physical quadruple star which is located in southern Puppis about 4 degrees east of the 3.2 magnitude star nu Puppis. Three of the four components can be easily seen in a small telescope.
This is a bright and pretty pair easily seen in small apertures with the components A and B being magnitude 5.6 and 6.7 and spectral types G1V and K1V, which suggest colours of deep yellow and orange which is indeed what is observed by Ross Gould using 175-mm. He also notes that star C is bright orange.
The current separation is 21" and the position angle is 125 degrees. Component C is 185" away in PA 335 degrees. A and B are both 55.6 light-years away, and whilst the C star is somewhat more distant it shares the large proper motion with A and B and is considered to be a physical companion. C is a rapid, close and unequally bright binary with a period of about 4.1 years.
Bob Argyle - Double Star Section Director
January 2025 - Double Star of the Month
About five degrees east of the line joining the two stars which form the 'horns' of the constellation of Taurus, the Bull is a right-angled triangle of 5th magnitude stars. These are 132, 136 and 139 Tauri. If you acquire 132 Tau in the field then about 20 arc-minutes north-east is the wide pair STTA66 (05 47 56.13 +24 41 12). This is an entry in the catalogue of wide pairs which Otto Struve compiled during his survey at Pulkovo in the 1840s.
The stars, which are relatively bright - 7.0 and 7.7 - and are separated by 94" in position angle 166 degrees. Rather surprisingly, they appear to be at the same distance as determined by the Gaia satellite and a search around them reveals a star of magnitude 19 also at that distance although the quoted error is substantial.
132 Tauri itself is recorded in the Washington Double Star catalog (WDS) as a very close visual binary, although only observed on one occasion in 1979. During the Hipparcos survey another component was recorded. The two stars were given as 5.0 and 9.1 at 230 degrees and 3".8. And again the only observation appears to be the first one and such a pair should be visible in 25-cm (the WDS gives magnitudes 5.0, 9.1, 230 degrees, 3".8). It certainly it should have been detected in the recent Gaia observations and does not appear there.
Looking through the earlier entries in this column which occupy the southern sky I find that I have been remiss in not including one of the finest of John Herschel's pairs - HJ 3945 or 145 CMa (07 16 36.84 -23 18 56) known quite widely, especially in the United States, as the 'Winter Albireo'. The stars are not quite bright as those in beta Cygni but form a spectacular pair for the small telescope.
The primary is a K3 supergiant and its companion an F0 dwarf. John Herschel notes Orange and green. Fine contrast of colours
. Modern observers also attest to the strong contrast of colours.
This is certainly an optical pair - star B is 346 light years away whilst star A is 2,600. The stars can be found 3 degrees east of the magnitude 3.0 blue supergiant omicron 2 CMa. The 6th magnitude star HIP 35578 which is more than 3 degrees to the south and has almost identical proper motion to B has been formally identified as a physical companion (C). However Gaia DR3 gives its distance as 296 light-years - about 50 light-years closer than B.
Bob Argyle - Double Star Section Director
