Double Star of the Month in Sagittarius

• May 2023 - Double Star of the Month

  STF 1850 (14 28 33.239 +28 17 25.9) is a wide and fairly bright pair which can be found 3.5 degrees WNW of the spectacular pair epsilon Boötis also known as Pulcherrima - 'most beautiful' (see the column for May 2020).

  

  The stars have changed little in relative position since they were catalogued by F. G. W. Struve and they are currently separated by 25" in PA 263 degrees. Although both are given spectral class A1V by the Washington Double Star catalog (WDS), the stars are magnitudes 7.1 and 7.6. They are rather distant, and Gaia DR3 gives the mean distance of the two as 873 light-years with an uncertainty of 1% in each case.

  SHJ 263 (18 17 51.13 -18 47 54.6) sits in the yellow area marked Star Cloud M24 in The Cambridge Double Star Atlas, although it is not labelled. It is probably more accurate to define SHJ 263 as a foreground grouping of stars.

  The main components are 6.8 and 9.3, separated by 54" in PA 11 degrees. The WDS lists six other companions of magnitude 12 and 13 within 30" of A and B. The only entry in Gaia DR3 for this group is that of the primary star which is given a distance of 1038 light-years whereas the distance of the star cloud as a whole is given as 10,000 light-years in the on-line literature.

  

  Twenty minutes NE is SHJ 264 (18 18 43.26 -18 37 10.8) and the bright pair is called AC. These stars are magnitudes 6.9 and 7.6 with a separation of 17" in PA 51 degrees.

  S. W. Burnham has added a couple of more difficult tests for 25-cm aperture. A is a close pair, the companion (B) is magnitude 8.2 and only 0".5 distant from A and has moved from PA 155 degrees in 1878 to a current value of 136 degrees. Some years before this, Burnham had noted a 13.1 magnitude star (D) 6" away from C whilst Philip Fox added a 10.8 at 145" from A.

  The primary star is a B0Ib supergiant and Gaia DR3 gives distances of 1826 and 4672 light-years respectively for A and B.

  Bob Argyle - Double Star Section Director

• September 2022 - Double Star of the Month

  I noted, rather surprisingly, that this column had yet to deal with gamma Delphini (20 46 39.50 +16 07 27.5) so this is a good opportunity to do so. One of the finest pairs in the sky because of its glorious colours, gamma has been known since the time of James Bradley (1755).

  

  The stars are magnitudes 4.4 and 5.0 and they are currently 8".8 apart in position angle 264 degrees. Although there has been little motion between the two since Bradley's time, an orbit with a period of 3249 years has been calculated which predicts a closest approach of 0".8 in around 2300. The apparent orbit as plotted in the USNO 6th orbital catalogue shows little, if any, sign of orbital curvature but the stars have the same parallax, if only just.

  The results of radial velocity observations of both stars with the intention of finding exoplanets have not confirmed any.

  Smyth observed the pair on many occasions and found colours of yellow and light emerald, whilst I made them golden yellow and blue-green when I first observed them during a youthful survey of double stars in 1968. Thirty arc-minutes south preceding gamma is the pair STF 2725, also a long-period binary. It has stars of magnitude 7.5 and 8.2 at 12 degrees and 6".2.

  Right on the border between Sagittarius and Microscopium, R 321 (20 26 52.95 -37 24 10.5) is a fine binary which is well worth sweeping for.

  

  Found by H. C. Russell at Sydney Observatory, this pair has a period of 186.4 years and the motion of the stars is in decreasing position angle (retrograde). In 1880, when first observed, the stars according to Russell were 1".1 apart , then by 1950 they had closed up to 0".26 before later opening out. At the present time they are 1".6 apart, with widest separation occurring around 2029. The stars have visual magnitudes of 6.2 and 7.9 and are orange in hue - each is an early-K dwarf. They should be comfortably divided with 10 or 12-cm.

  Whilst in the area, move about 3 degrees SSW, and you will find DUN 230 (20 17 49.68 -40 11 05.1) which is an easy object for the smallest aperture. The two components are both of late-F spectral type and whose visual magnitudes are 7.4 and 7.7; the stars are 9".7 apart in PA 117 degrees, and lie 257 light-years away.

  Bob Argyle - Double Star Section Director

• August 2022 - Double Star of the Month

  In the column for August 2019, I described 16 Cygni, a bright and easy double foundabout 6 degrees north of delta Cygni. Starting with 16 Cygni, and moving west about 5 degrees a coarse triple of 6th magnitude stars will appear in the finder field. The easternmost of these is STF 2486 (19 12 05.03 +49 51 20.7) which consists of stars with V = 6.5 and 6.7 which are currently 7".1 apart in position angle 202 degrees.

  

  This is a relatively nearby (82.5 light-years) binary pair whose period has been found to be 1459 years but clearly this is a preliminary value, since the position angle has decreased but 21 degrees since 1819. The proper motion of the stars is more than 0".6 per year, so they are rapidly leaving behind star D (V = 11.1) at 196", but rapidly approaching star C which is V = 13.2 at 27".

  Sir William Herschel's third and last catalogue of double star discoveries includes many pairs which are low in the sky from the UK, and two of them were found close to omicron Sagittarii.

  

  The first of these, H N 126 (19 04 20.28 -21 31 53.7), is the more difficult of the two. It is about 15 arc-minutes NW of omicron and consists of stars with magnitudes 7.9 and 8.1 which are currently 1".3 apart in PA 183 degrees. This binary has almost completed 270 degrees of its apparent orbit since discovery, has an orbit of about 502 years, and lies 168 light-years distant.

  About 1.25 degrees due south of omicron is H N 129 (19 04 14.20 -22 53 47.5), significantly easier at 309 degrees and 8".3, but with two components which are considerably unequal (V = 6.9 and 9.2).

  Bob Argyle - Double Star Section Director

• August 2021 - Double Star of the Month

  I have been following STT358 in Hercules (18 35 33.22 +16 58 32.5) for almost 50 years. In 1970, with the 28-inch refractor at Herstmonceux, a measure gave the values of 168 degrees, 1".7. In 2018 using the Cambridge 8-inch Cooke, the separation had remained virtually unchanged and the position angle reduced to 149 degrees. This fine binary has a period of 380 years according to Wulff Heintz in 1995 and it is currently at 142 degrees and 1".5. The pair will remain within range of 15-cm or so for decades to come, only reducing to 1".3 by 2100 before widening again.

  

  Gaia EDR3 shows that both stars have the same trigonometrical parallax to within the stated errors (±0.1 light-year in each case) and the mean distance of the system is 111.2 light-years. The pair can be found 3 degrees WSW of 111 Her. In 2007 I. N. Reid found a magnitude 12.6 star 349 degrees and 35" from A which is 0.3 light-year further out. This turns out to be an M3 dwarf.

  One of the brightest triples in the sky is β Sgr which sits in the extreme south-west corner of the constellation and is unfortunately not visible from the UK. The two brightest components, called β¹ (19 22 38.30 -44 27 32, V = 4.0) and β² (19 23 13.14 -44 47 59.2, V = 4.3) are separated by 21' on the sky and thus easily visible to the naked-eye.

  

  β¹ has a magnitude 7.1 companion at a distance of 28" and position angle 76 degrees, and was first noted as double by James Dunlop in 1826. Ernst Hartung thought these stars physically connected and indeed Gaia EDR3 seems to indicate that they lie at a similar distance, but the parallax for A has a very large quoted error, which may be partly due to the star's brightness. Hartung also noted the colours of the components of β¹ as bright pale yellow and ashy-white, while Ross Gould found them to be white and yellowish, more in line with the spectral types of B9V and A5V.

  Draw a line from β¹ through β², extend it by about 1.5 degrees and you will come upon I 116, a fine triple star which will be well seen with 20-cm. The close pair has magnitudes of 8.6 and 9.4 and they are currently at 24 degrees and 2".7, whilst 16" from A is another 8.6 magnitude. star at PA 190 degrees. The discoverer, Robert Innes, re-visited the system some years later and added a magnitude 13 star only 2".4 from C. This star was last measured in 1960.

  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 2017 - Double Star of the Month

  The northern star (STF 1575, 11 51 57.57 +08 49 48.0) is one of the few doubles to be found in Virgo north of the equator. It is a William Herschel discovery (H 4 49) and since 1782 there has been little change in separation which is currently 30".4.

  Hipparcos demonstrates that both stars have similar parallaxes and proper motions and Shaya and Olling in their 2011 paper have indicated that there is a near 100% chance of the stars being physical.

  The magnitudes are 7.4 and 7.9 and the pair can be found a little more than 5 degrees due south of Denebola (beta Leo) and it forms the vertex of an isoceles triangle of stars which include 4 and 6 Virginis.

  Thomas Lewis (1906) incorrectly calls this pair H III 51 and notes both components are white, which are the colours derived by F. G. W. Struve. In fact both stars have spectral type K0. Richard Harshaw using 20-cm finds orange and white.

  About 1.3 degrees below the bright star Kaus Australis (lambda Sgr) there is a triangle of 5th and 6th magnitude stars. Each of these is a visual pair which can be seen in 15-cm. At the peak of the triangle is WNO 6 (18 28 57.76 -26 34 55.0) with magnitudes of 6.7 and 8.0 and which appears to be unchanged in over 100 years. The writer found 182.0 degrees and 41".83 in September 2016.

  About 15 arcminutes preceding are the other two members of this triangle.

  The northernmost star is BU 133 found by the great Americam observer on 1873, July 6 using his famous 6-inch Clark refractor. He estimated that the stars were both 7.5 magnitude and indeed all the early estimates show no difference in brightness between the stars and yet the WDS gives 6.6 and 8.5. The writer recently measured it (September 2016) and found 233.2 degrees and 0".77 which would make the pair a stiff test in 15-cm.

  The remaining star is magnitude 6.7 and has a magnitude 8.7 companion at 54" and PA 135 degrees. This component was mentioned by Burnham and subsequently ignored.

  Bob Argyle - Double Star Section Director

• July 2016 - Double Star of the Month

  39 Dra (17 57 12.56 -30 22 24.80) can be found about 4 degrees following and slightly north of xi Dra in the 'head' of Draco. It is also known as b Dra.

  The binocular user will see a pair of mag 5.1 and 8.0 stars separated by about 89". Any aperture larger than 6-cm will show the companion to A which William Herschel found in 1780 and allocated to his class 1 i.e. between 0 and 2". Since that time the pair has widened with increasing position angle and in 1993 was found at by the writer at 351° and 3".9. The AB stars are magnitudes 5.1 and 8.1 so it requires good seeing to see B clearly.

  This is a quintuple system since all 3 bright stars are physically connected and both A and B are spectroscopic binaries. Surprisingly, although the motion in AB amounts to just 25 degrees in 230 years, a highly speculative orbit with a period of 3962 years appears in the USNO Sixth Orbit Catalogue.

  In October 2011, John Nanson found the primary was a distinct yellow watered down by a weak touch of white, I could see a slight tinge of blue in "B" and "C" was just weakly white. There is much more information on this system on the Starsplitters website.

  PZ 6 (17 59 05.28 -30 15 10.8) is a beautiful pair which lies about 1.5 degrees due west of gamma Sagittarii. In the 1st edition of the Cambridge Double Star Atlas it is given as PZ 4 but is corrected to PZ 6 in the second edition.

  An observation of this pair on the evening of 5 September 2013 using the Johannesburg refractor showed the colours of the stars to be deep yellow and lilac.

  The WDS gives spectral types M1Ib and G8II so this is a rare pairing of a supergiant and a giant. There is little motion between the two stars and the primary star sits a little over 1,000 light years away but with an uncertainty in the distance of 30%.

  The stars are magnitudes 5.4 and 6.0, and the current separation of 5".7 appears to indicate that the stars have been slowly closing since the first measure in 1826. A 13.2 mag star at 25" was measured by the writer in September 2013.

  By moving the telescope another 1.5 degrees further west, and crossing the border from Sagittarius into Scorpio, the observer will come across PZ 5 - another bright wide pair easily resolvable in a small aperture. The stars are mags 6.7 and 8.2 and both white - the spectral types are A3 and B9. Note, however, that PZ 5 is not labelled in CDSA 2 whilst it is in the first edition. The position in 2013 as determined by the writer was 104° and 5".7. On the same night a mag 11 companion was also noted at a distance of 49".

  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

• Double Star of the Month - August 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.

  Those who have read last month's version of this column will have noted that the northern object was 26 Dra, one of Burnham's very unequal pairs. This month's selection is another from that stable, BU 648 (18 58 01.47 +32 54 05.8) can be found in Lyra, in the same low power field as gamma, and 20 minutes or so north-west of the bright star. Burnham assigned magnitudes of 6 and 9.5 to this discovery which was only separated by 0".6 at the time. The WDS gives magnitudes of 5.3 and 8.0 and it can certainly be seen with 20-cm on a good night. At the time of writing, the separation is 1".23 and the position angle 247° at mid-2013. This star has also recently been in the news because it is accompanied by a planetary object whose mass depends on which of the two stars it is orbiting. This is not known at present but if, for instance, it accompanies star B, then the mass is estimated at 1.5 Jupiters. This discovery is unusual because it was made astrometrically, rather than by radial velocity measurement, using the PHASES (Palomar High-precision Astrometric Search for Exoplanet Systems) part of the Palomar Testbed Interferometer instrument. There are 4 distant comites ranging between magnitudes 11.0 and 12.6.

  H N 126 (19 04 21.53 -21 31 53.7) belongs to William Herschel's last double star catalogue which was published in 1823, a year after his death. In this case the N stands for 'New'. This 'small yellow binary' says E. J. Hartung, can be found in a 'field profusely sown with stars' and it is very close to omicron Sagittarii. Burnham's Celestial Handbook also attributes the label HU 261 to the system but it has now reverted back to its original discoverer. This is a pretty pair of long period and the two components are moving slowly retrograde. At this time the position is 186°, 1".25. The writer has not observed this pair from the UK but it should be well seen in 20-cm provided the seeing and transparency are sufficiently good at this low declination.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - August 2012

  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.

  The two pairs selected this month are almost copies of one another. They have the same separation, and position angle, similar magnitudes and even the colours are close.

  Eta Lyrae (19 13 45.49 +39 08 45.5) appears as H 4 2 in the great astronomer's first list of double stars published in 1782, but it was first listed by Christian Mayer in 1779. The primary is a luminous object more than 1,000 light years away. The companion appears to be an F star and with the primary being an early B subgiant, one might have expected colours to be white and yellow when in fact Hartung records yellow and ashy. The WDS gives magnitudes of 4.38 and 8.58 with current distance and PA being 28".5 and 79°. A fainter and more distant star (mag 11.4) can be seen at a distance of 161". Along with theta, also a wide, unequal pair, it follows Vega by about 7 degrees.

  Beta1 Sgr (19 22 38.29 -44 27 32.1) is not only not the second brightest star in Sagittarius but it is actually about 12th in the list. The naked eye can make out both beta 1 and its close neighbour beta 2 (mag 4.27) some 20 arc mins away. Dunlop listed beta 1 as a telescopic double in 1826 and and since that time the position angle has increased slowly to 76° and the separation decreased slightly to 28".7. Hartung gives the colours as pale yellow and ashy white - the spectral types are actually B8V and F0V perhaps reversing what one might expect from the colours. However, Ross Gould using a 175-mm refractor disagrees with this judgement and gives the colours as white and yellowish. The stars are magnitudes 3.98 and 7.21 and again this is an optical system. Beta1 lies in the far south-western part of Sagittarius, close to the border with Corona Australis and Telescopium.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - September 2008

  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.

  About 2.5 degrees north of the Dumbbell nebula M27 and a little proceeding is 16 Vul = STT 395 (20 02 01.37 +24 56 16.3) , one of Otto Struve's discoveries at Pulkova and when first found in 1843 it was a difficult pair at 89 dgs and 0".5. In the intervening period is had almost doubled in separation and the position angle has increased to 125 degrees so that a 15-cm telescope should resolve this beautiful pair. Hartung notes that both stars are yellow. This is a relatively distant system - the Hipparcos revised parallax is 14.55 mas with an uncertainty of 0.50 mas, putting it at a distance of 69 parsecs. It is certainly a binary system of long period as the significant proper motion testifies.

  The spectral types of 21 Sgr (18 25 21.04 -20 32 29.8) are given as A + K2III in the WDS and the magnitude difference is 2.4 in the visual. It is difficult to reconcile the apparent colours seen in this beautiful pair which are orange and greenish according to Hartung with these spectral types. The writer also recently viewed this pair in the 26.5-inch refractor at Johannesburg and the similarity with Antares was immediately apparent - the companion is definitely greenish. The Hipparcos B-V value of +1.3 certainly suggests that the light of the system is dominated by the K giant star and that the spectral types in the catalogue should be reversed. The revised parallax puts the primary at 126 parsecs. There is slow retrograde motion with the discovery position by Jacob from 1846 showing 297 dgs and 1".8 whilst a recent measure put the companion at 280 dgs and 1".7.

  Bob Argyle - Double Star Section Director