Double Star of the Month Archive 2018

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 2018 - Double Star of the Month

In Cassiopeia, about 3 degrees west of the magnitude 2.2 star beta Cas (the westernmost of the five in the well-known 'W') is tau Cas. Move a further 3 degrees west and you will alight on SHJ 355 (23 30 01.92 +58 32 56.1).

There are nine components in the Washington Double Star Catalog (WDS), most of which are visible in a 6-inch, and a drawing by John Nanson on the Star Splitters website using that aperture, shows the halo of faint stars around the brightest member of the group which is magnitude 4.9. This B3 star is a well-known eclipsing binary of the Algol type (AR Cas) with a primary dip of 0.14 V magnitudes and a period of 6.06 days.

The small aperture will have no problem in picking out the C component at 269 degrees and 75". Larger apertures may see that both A and C are close, unequal doubles. AB is one of Otto Struve's discoveries (STT 496) and B is some 4.4 magnitudes fainter than A yet now only 0".8 distant. At least 30-cm will probably be needed for this. 20-cm may suffice to show the companion to C discovered by W. R. Dawes in 1841. This pair (DA 2, CD) are magnitudes 7.2 and 9.0, at 213 degrees and 1".3.

John Herschel discovered that delta Sculptoris (23 48 55.48 -28 07 48.1) was double before his journey to South Africa.

The primary is magnitude 4.6 and lies about 12 degrees east of Fomalhaut. He estimated the distance to the magnitude 9.4 companion as 80", but Burnham in his 1906 catalogue suggests that this was a little large.

It appears in that volume because in 1881 Burnham added a close and faint companion to delta (BU 1013) using the 36-inch refractor at Lick. This star now known as B is only 3".4 away and is magnitude 11.6.

Since 1881 there has been but 11 degrees of direct motion between the two stars which are clearly physical, because delta is moving though space at more than 0".1 per year. In fact the distant C, (297 degrees, 74") also possesses the same transverse motion as AB. Gaia DR2 tells us that delta is 139.2 light years away, with an uncertainty of less than 0.1 light year, whilst C is 144.25 +/- 0.01 light years distant.

Bob Argyle - Double Star Section Director

September 2018 - Double Star of the Month

STF 2735 (20 55 40.64 +04 31 57.70) is a pretty pair discovered by William Herschel in 1782. It is 1 degree WNW of 1 Equulei which is a bright triple, although the primary pair is now beyond all but the ground-based arrays.

F. G. W. Struve noted the stars were yellow and ash, whilst Smyth in the Bedford Catalogue noted orange tint and purple. More recently John Nanson on the Star-Splitters blog, using a 5-inch f/15 refractor at x191 thought the primary was white with a weak but noticeable gold-yellow tinge.

The stars form a very long period binary and have moved only 8 degrees in almost 200 years. Gaia DR2 puts them at a distance of 351 light-years. Webb noted that the stars were magnitudes 6.2 and 7.5 but the Washington Double Star Catalog (WDS) reduces the difference in magnitude to 1.0. John Nanson suspected that they were more unequal than that and indeed Gaia DR2 gives G magnitudes (similar to V) of 6.1 and 7.4. Using the Cambridge 8-inch refractor the writer found 283.6 degrees and 2".21 in 2011.

Three degrees west of the nearby dwarf Epsilon Indi is a coarse binocular triple which seems to have evaded Dunlop and been first pointed out by W. S. Jacob and which sits in the WDS catalogue as JC 25 (21 43 59.16 -57 19 30.4).

The two brightest stars sit 152" apart in PA 4 degrees and have both very similar and quite significant annual proper motions (115 mas in RA, -53 mas in Dec) and parallaxes (22.54 mas and 22.49 mas, respectively for A and B) such that they are almost certainly physical. The third star, C, is 187" away in PA 214, is magnitude 7.5 and is unconnected. Gaia DR2 puts it 600 light-years away.

Bob Argyle - Double Star Section Director

August 2018 - Double Star of the Month

Embedded in the Milky Way in Cygnus, about 2 degrees East and slightly south of 22 Cyg, is HJ 1470 (20 03 39.5 +38 19 38.3) a deep-red star which lies at a distance of 1630 light years (with an error of about 30 light years) according to the latest results from the Gaia mission (DR2). Simbad gives the spectral type as M0III and the star is about 225 times as bright as the Sun.

John Herschel noted a distant companion of magnitude 9.3. The Cloudy Nights website contains drawings of HJ 1470 and three nearby pairs which together form an arc of stars about 22' across and known as Chaple's Arc or the Fairy Ring. The other pairs are considerably less impressive.

An observer in the US using an 8-inch at x53 noted that the primary was strong yellow-orange/reddish and greyish-blue. At the beginning of 2005, I measured the pair with the Cambridge 8-inch. The result was 340 degrees and 28".6.

Browsing though Sissy Haas' excellent descriptive guide to visual double stars, I came across the pairs S 715 (19 17 39.96 -15 58 01.7) and S 716 (19 18 05.55 -15 57 13.4) which can be found in Sagittarius.

The brighter pair is S 715 where the two components have magnitudes of 7.1 and 7.9 and they are currently at 17 degrees and 8".4. Just 6 arc minutes preceding and 1 arc minute north is S 716 with magnitudes 8.4 and 8.6 at 194 degrees and 5".0.

Gaia has observed both pairs; each appears physically connected but the components of S715 are 480 light-years away whilst the stars in S 716 are both 1030 light-years distant.

S 716 is also known as Stone 46. Ormond Stone (1847-1933) was Director of Cincinnati Observatory where he found a number of pairs using the 11-inch refractor, in this case about 40 years after South first noted it.

I measured S 715 in 2016 with the Johannesburg telescope, but S 716 was not noticed, although it should have been clear in the 6-inch finder.

Bob Argyle - Double Star Section Director

July 2018 - Double Star of the Month

On a straight line between alpha Oph and 93 Her, but two-thirds of the way towards 93, and therefore just in Hercules, is STT 338 (17 51 58.46 +15 19 34.9) a neat close pair which was discovered by Otto Struve at Pulkova.

At the time of discovery the stars of magnitudes 7.2 and 7.4 were separated by only 0".6 in position angle (PA) 223 degrees. Since then orbital motion has taken them almost 60 degrees retrograde in angle and the separation has increased to 0".8.

Sissy Haas, recalling T. W. Webb, describes them as them gold and green white, but in fact the latter term was abbreviated in Webb to mean Greenwich. A recent orbit by Dr. Jean-Louis Prieur and colleagues assigns a period of 1276 years.

In a small rectangular area of about 20 x 9 degrees, just below the Teapot of Sagittarius, is the constellation of Corona Australis. It has a number of attractive double stars two of which HJ 5014 (August 2009) and gamma CrA (August 2010) have already been described in this column.

Kappa CrA (18 33 23.13 -38 43 33.6) is a fine pair which was noted by James Dunlop and is number 222 in his catalogue. The stars are magnitudes 5.9 and 6.2 and the current PA and separation are 358 degrees and 21".5.

Dunlop's 1826 separation of 30" must be an error, as the stars appear to have common proper motion and Gaia DR2 also indicates that they are both around 695 light-years away. Not connected however, are two fainter and more distant stars, a 13.1 magnitude at 202 degrees and 33", and an 11.6 magnitude at 247 degrees separated by 96".

About 2 degrees following is lambda CrA (COO 227), a pair of stars of magnitudes 5.1 and 10.0 at PA 213 degrees and separated by 30". It is, nevertheless, a physical pair and DR2 gives distances of 205 and 200 light-years respectively, with similar proper motions. A third star (mag. 9.9) is at 51 degrees and 43".

Bob Argyle - Double Star Section Director

June 2018 - Double Star of the Month

Easily found four degrees due west of alpha CrB, STF 1932 (15 18 20.19 +26 50 24.7) is a visual binary whose period is 203 years, so the current aspect of the stars closely resembles that at the time of discovery.

The separation around the cycle ranges from 0".6 to 1".6 and at the moment the stars are as widely separated as they get (2018.5, 266 degrees, 1".62). The magnitudes are nearly equal (7.3 and 7.4) and both stars are F-class giving a yellowish aspect to the observed colours.

Whilst in the area look at eta CrB (STF 1937), four degrees NNE which is now a test for 25-cm. The position angle is currently increasing 20 degrees per year and by the middle of 2018 will be at 246 degrees and 0".42.

Beta Scorpii (16 05 26.23 -19 48 19.4) skirts the southern horizon during the short northern summer nights. It was first seen as double by Benedetto Castelli in 1627 and was later catalogued by William Herschel as H 3 7. With the two bright components of magnitude 2.6 and 4.5 separated by 13".7 and 20 degrees the pair is not difficult even low down.

A century after Herschel, S. W. Burnham noticed a close and very faint companion to A about an arc second away and of magnitude 10. More modern measures show that this system has closed in considerably and that the estimated period is 610 years.

Slipher found that A was a spectroscopic binary and a lunar occultation observation of A in 1976 indicated another component at a distance of 0".1, but no further observations of this pair have been forthcoming. McAlister found that C was also a close binary with a period of 39 years and a separation of about 0".1. Take into account the distant magnitude 7.5 at 519" and 30 degrees from A and this is a physical sextuple star.

Bob Argyle - Double Star Section Director

May 2018 - Double Star of the Month

Not far from Arcturus in the Spring sky is pi Boötis (14 40 43.56 +16 24 05.9) a beautiful pair of white stars found by Christian Mayer and later called H III 8 by William Herschel and STF1864 by F. G. W. Struve. The stars are magnitudes 4.9 and 5.8 and have shown little motion since discovery. Smyth, Webb, Sissy Haas and me all find the both stars are white and the spectral type are B9 and A6.

The primary star, at least, is over 300 light years away but the quoted error on the Hipparcos parallax is significantly large and is probably not affected by the presence of the visual secondary. In 1984 the primary was found to be a spectroscopic binary and the Washington Double Star (WDS) Catalog notes that the secondary is also a spectroscopic binary. In 2015 I found the stars at 113 degrees and 5".4 apart.

The upcoming Gaia catalogue may well settle the issue of whether the visual pair is physical, and if it is then we have here a quadruple system. A third star of mag 10.6, first noted in 1881 is 163 degrees and 127" distant.

k (not kappa) Lupi (15 25 20.21 -38 44 01.0) is a magnitude 4.6 star located in central Lupus about 2 degrees north of delta.

It was observed by James Dunlop who noted a couple of distant 9th magnitude companions. Dunlop's original paper reads for entry number 183: A star of the 6th mag with two stars of the 10th and the measured separations are 12 and 15". It's possible that Dunlop meant 120 and 150" as the latest WDS positions (for 2016 and 1999, respectively) are 203 degrees and 93" for AB and 134 degrees and 149" for AD.

In 1896 Robert Innes, observing from Cape Town using a 7-inch refractor, found that the B component was an almost equal double (I 87) at a distance of 1".4 since which time the position angle has reduced by 40 degrees to 207 degrees and the separation is now just below 1". The relative faintness of the two stars means that this is now a stiff test for 25-cm aperture. Innes also added a magnitude 11.5 at 17 degrees and 42".

Bob Argyle - Double Star Section Director

April 2018 - Double Star of the Month

Just 6 degrees north of Denebola (beta Leo) sits 93 Leo (11 47 59.23 +20 13 08.2) a rather undistinguished white star of magnitude 4.6.

It caught the attention of William Herschel in 1782 who called it H VI 80. Some 40 years later it came to the attention of F. G. W. Struve during the Dorpat survey but the companion star at magnitude 9.0 and distance 77" did not impress him enough to put in the main catalogue, so it was relegated to one of the two Appendix Catalogues of essentially wide pairs.

In 1900, the primary was found to be a spectroscopic binary by Campbell and Wright at Lick Observatory where four spectra of the star showed a range of 38 kms per second.

More recently 93 Leo has been shown to be an RS CVn star, a variable type which involves chromospheric activity on one component of a pair of giant stars, in this case the spectral types are F8III and A6III.

The period was shown to be almost 72 days and then the Mark III interferometer (which was based at Mount Wilson Observatory but closed in 1992) resolved the two stars and defined a visual orbit of great precision even though the separation varied from only 5 to 8 milliseconds of arc.

Hipparcos showed the primary star to be 232 light years away but the companion has recently been observed by Gaia which shows that it is at the same distance and moving through space with the same proper motion as A making this a physical triple system.

Smyth neglects to include in his catalogue. Webb notes a curiously similar pair in the field. This is SHJ 130 (7.5, 10.0, 30 degrees, 71") and is about 13 arc-minutes south-west of 93 Leo.

STF 1604 (12 09 28.54 -11 51 25.4) lies in a barren area of sky on a line between beta Virginis and gamma Corvi and about 6 degrees along the line from the latter star.

There are three components for the small telescope, although the B star is rather faint and is of late spectral type.

AB is magnitude 6.9 and 10 at 89 degrees, separated by 9" (2016). Both components have a significant and similar proper motion and have been approaching the third unconnected star C (magnitude 8.1) since 1831 when the distance AC was 58 arc-seconds. Closest approach was 9".6 in 1983, and I measured the distance at 10".8 in 2017.

The stars were in Virgo when Webb wrote his book but have now sneaked over the border into northern Corvus.

Bob Argyle - Double Star Section Director

March 2018 - Double Star of the Month

Tucked away is an obscure part of Ursa Major is the red dwarf binary STF 1321 (09 14 22.79 +52 41 11.8). Containing stars of magnitudes 7.8 and 7.9 it sits about half a degree west of the centroid of a triangle of stars formed by 15, 18 and θ UMa.

Although discovered by Struve almost 200 years ago the orbital motion has amounted to only 50 degrees or so and the projected period is 975 years. Both stars are a distinct yellow colour and currently separated by 16".8 in position angle 99 degrees.

This is one of the nearest stellar systems to the Sun. Hipparcos measured both stars and came up with a distance of 19 ± 0.6 light years for the A component but most recently an interim measurement from Gaia gives a distance for the B star of 20.52 ± 0.05 light years.

The Hipparcos parallax error is exceptionally large and implies there may be underlying structure. Both stars have been suspected of being spectroscopic binaries but this was disproved, at least at the 0.1 km per second level by Morbey and Griffin. Further searches for faint companions have so far revealed nothing. It will be interesting to see what Gaia finds for the A star - the fact that the parallax has not yet been published may be revealing in itself.

The pair move across the sky at more than 2.5 arc-seconds per year and is fast approaching two faint companions found by Ball (mag. 11.9) and Espin (mag. 14.5).

BSO 18 in Vela (08 42 25.41 -53 06 50.5) contains two stars bright enough to be in the HR catalogue, the primary HR 3467 of magnitude 4.8 and HR 3466 of magnitude 5.6, 76 arc-seconds distance in position angle 311 degrees, so this is a fine pair of white stars for the binocular user.

It is easy to find as it is 25 arc-minutes south following the bright star o Velorum (mag. 3.6) which is itself embedded in the galactic cluster IC 2391, so the whole area is a spectacular telescopic view.

B in turn has a magnitude 9.9 star (D) at 266 degrees and 60 arc-seconds. The bright components of BSO 18 share the proper motion of the cluster and are both at a distance of about 500 light years.

In 1929 W. H. van den Bos found a close companion to the B component some 2.5 magnitudes fainter and 0.5 arc-seconds away. Since then the distance has increased only slightly and the position angle has increased by 40 degrees to 153 degrees. This pair (B 1625 BC) was last measured in 1991 and poses a challenge for 30cm.

Bob Argyle - Double Star Section Director

February 2018 - Double Star of the Month

Lambda Geminorum (07 18 05.61+16 32 25.7) sits in the ecliptic zone which means it can be occasionally occulted by the Moon. High speed photoelectric measurements of the star's brightness during occultation show that there is another star close in. The star is also known as a spectroscopic binary which may be the same object.

For the visual observer the challenge is to see the faint companion discovered by F. G. W. Struve. The WDS give its magnitude as 10.7 and I can honestly claim never to have seen it. John Nanson, however, finds it slightly easier than delta and kappa Gem, which I don't, so clearly I will have to take another look this Spring.

Lambda is only 101 light years distant and the position angle and separation, currently 36 degrees and 9".3 have changed little since the 1830s. As lambda has a significant proper motion then it seems that the faint star is travelling with it through space.

The duplicity of 5 Pup (07 47 56.71 -12 11 33.8) is also down to Struve, and it is known as STF 1146. During the 19th century the components, of magnitudes 5.7 and 7.3, changed very slowly relative to each other but by 2016 the pair were about 1 arc second apart.

This is a highly-inclined long-period system, like STF 1527, and the current orbit predicts a period of 1331 years and a close approach of 0".7 by 2044. At present it is well separated in 15-20cm but the difference in magnitude and low altitude in the UK sky makes the task of resolving it a little trickier. I have only been able to make two measures in the last five years.

The surrounding area of sky is very rich. Move 3 degrees south and then swing west by 4 or 5 degrees and you will encounter more bright Struve pairs as well as M46 and M47.

Bob Argyle - Double Star Section Director

January 2018 - Double Star of the Month

7 Cam (04 57 17.2 +53 45 07.5) is a fairly unprepossessing system in the F G. W. Struve Dorpat catalogue and appears as number 610 in that list.

The stars are magnitudes 4.9 and 11.3 with the current position 242° and 26 arc-seconds, and there is no evidence that the stars are in anyway connected. It is 7 degrees north of Capella and about 4 degrees preceding, and is part of a group of 4th and 5th mag stars which also contain the splendid pairs 1 and 2 Cam.

In 1864, Baron Ercole Dembowski discovered that A was an unequal, close pair, the new component being magnitude 7.9 at 307° and 1".2. Since then the stars have closed in, but are now slowly widening, although by 2020 the separation will still only be 0".62 in PA 196°.

A recent orbit with a period of 2733 years has since been replaced in 2014 by a linear ephemeris by Drummond, although this has now, in turn, been updated by Hartkopf in 2017.

The two recent linear solutions give 315° 0".88 and 196° 0".62 respectively for epoch 2020 but it is difficult to understand why there is such confusion about the quadrant in which the companion is located as there is such a large difference in magnitude. Suffice to say it will take sustantial aperture to see Dembowksi's companion but visual observations will help to confirm which of the two predictions is right.

With Canis Major skimming along the southern horizon at present there is a brief window of opportunity to delve into its treasures.

Mu (μ) CMa (06 56 06.59 -14 02 34) is a pretty pair but rather close and unequal and tends to be more difficult than it really is due to its low altitude from the UK.

The stars are magnitudes 5.3 and 7.1 with a current separation of 3".1, showing a very slow closing since discovery almost 200 years ago. Struve gave colours of orange and reddish although the Washington Double Star Catalog (WDS) gives spectral types of G5III and A2.

By the way, don't forget to take a look at Sirius, just 3.5 degrees south-west of μ CMa - the companion is now fully 10 arc-seconds distant.

Bob Argyle - Double Star Section Director