Double Star of the Month in Lupus

• June 2022 - Double Star of the Month

  Gamma Herculis (16 21 55.24 +19 09 10.9) appears in William Herschel's first double star catalogue as H V 19. Gamma is easily found as it sits just 3 degrees SW of the most south-westerly star, beta, in the Keystone of Hercules.

  

  It is a late-A giant star of magnitude 3.8 and in 1780 Herschel found a magnitude 10 companion at 251 degrees and 41".8. By 2013 the position angle had reduced to 226 degrees and the separation was 43".3. This change is due entirely to the difference in proper motions. The Gaia satellite finds the primary star is 193 light-years away whilst the companion is more than 1,770 light-years distant. An additional optical companion, of V = 13.3, can be found at 298 degrees and 82" from B.

  The Washington Double Star Catalog (WDS) states that A is a spectroscopic binary, but it does not appear in the Ninth Catalogue of Spectroscopic Binaries (SB9), compiled by the late Dr. Dmitri Pourbaix. Additionally an infra-red survey has revealed an object 8" from A which has a K magnitude of 8, and thus a V magnitude of perhaps 10-11. Whether it is connected to A is not known.

  Eta Lupi (16 00 07.33 -38 23 48.1) is a hot, early B-type star of magnitude 3.4 which lies in Lupus, 16 degrees due south of delta Sco, the middle of the three bright stars in the Scorpion's head. Eta is part of the Upper Centaurus - Lupus association and thus has a similar distance (460 light-years) and proper motion to a group of stars in the neighbourhood.

  

  The WDS shows three companions, two of which have similar parallaxes to eta, and all of which appear white in the eyepiece. The B component is magnitude 7.5 at a distance of 14" and position angle 19 degrees, a relative position which has changed little since the pair were first noted by Rumker from Parramatta. C is magnitude 9.4 at 115" and PA 248 degrees. This star was also noted by Rumker who appears to have estimated a distance of 1 arc-minute but this may be in error (possibly a misprint for 2 arc-minutes?) as the star moves through space together with the brighter components. The star listed in the WDS as ANT 2 AD (the designation refers to Rainer Anton who measured the previously uncatalogued component D in 2007) does not appear to belong to the association and is almost four times more distant. It can be found at 293 degrees and 136".

  Searching the Gaia EDR3 catalogue within a radius of 200 arc-seconds from eta shows a magnitude 14 star which also moves with, and is equally distant from us, as A, B and C, and which can be found at 290 degrees and 118".

  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

• 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

• June 2014 - Double Star of the Month

  STF1883 (14 48 53.22 +05 57 15.9) is in Virgo near the northern border with Boötes. In `Celestial Objects' however it appears in Boötes. In Thomas Lewis' work on the Struve stars he gives the magnitudes of both stars as 7.0. The WDS however lists 7.02 and 8.95. The current value for B must be somewhere between these limits as the writer has resolved this pair with the 8-inch Cooke at Cambridge which would have been much more difficult if B was near magnitude 9. The WDS notes than one star is variable - this is presumably B and the amplitude must be considerable if the stars were deemed to be equally bright a century ago. The star has only the designation SV ZI1089 and does not appear to be a fully-fledged IBVS variable star. STF1883 is a binary star of period 216 years which has a highly inclined and eccentric orbit. Near 1".2 at discovery in 1830, it closed to around 0".25 in the early 1930's before widening to 1" where it is today. This value will not increase very much before the stars begin to close again in about 30 years time.

  HJ 4788 is d Lupi (15 35 53.25 -44 57 30.0) which can be found as one of triangle of naked-eye stars some three degrees following epsilon Lupi. This pretty pair consists of pale and deep yellow components in a well-occupied field (according to E. J. Hartung). The stars are mags 4.60 and 6.51 and the separation has slowly decreased from 3".1 in 1836 to 1".9 in 2013 whilst the position angle has increased from 349° to 13° over the same time interval. The primary is an early B-type dwarf which is 428 light years away and the WDS notes it is a spectroscopic binary.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - June 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.

  Situated in the north of Boötes, STT 298 (15 36 02.59 +39 48 08.7) is one of the more rapid binary systems found by Otto Struve at Pulkovo and it now embarking on its fourth orbit since discovery. Look for the naked-eye pair nu1 and nu2 Boötis some 6° following beta Boötis and STT 298 can be found just south preceding phi Boötis. The star is nearby (the distance is 73 light years) so the orbit is relatively large in angular terms. The stars are almost near peristron and at 182°, 1".18 in mid-2013 they offer a good opportunity to see a pair with a period of only 55.6 years. The system moves across the sky at almost 0".5 per year and is accompanied at a distance of 121" by star C which is mag 7.8 and also a K dwarf. For the telescopic observer there are two fainter but unrelated stars of mags 12.1 and 13.9.

  DUN 178 (15 11 34.82 -45 16 39.0) is an orange KOIII giant star of mag 6.3 accompanied at a distance of 30".6 by a mag 7.3 white star according to Richard Jaworski using a 100-mm aperture in Australia. Both these stars appear in the Hipparcos catalogue but do not appear to be connected in any way. A is 510 light years away whilst B is 400 light years distant. At discovery in 1826 the pair were separated by 40" so the change is purely due to different proper motions. This pair is located in the heart of Lupus, a constellation rich in visual double stars, and can be found in the same low-power field as lambda Lupi. In 1929 Willem van den Bos, using the 26.5-inch refractor in Johannesburg, found a companion of mag 9.6 some 1".1 distant from A. There has been little change in the position of this star in the intervening 80 years and it offers a challenge to a 30-cm telescope in a good location.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - May 2011

  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.

  STT 288 (14 53 23.35 +15 42 18.7) can be found about 3.5 degrees due south of xi Boötis as a star just below the usual limit of naked-eye visibility. The components are magnitudes 6.9 and 7.5 and revolve around one another in about 313 years. About 100 years ago, the Greenwich Observatory observer William Bowyer considered the two stars were optical in nature. To him they appeared to be steadily increasing in separation since discovery in 1845 when Otto Struve found them separated by 0".45. A few years after Bowyer's observation, the stars began to slowly close again. The orbit is fairly eccentric and the stars will continue to close to 0".51 in about 50 years time. Hipparcos places them at a distance of 155 light years. In the meantime at PA 160°, 1".06 they form an excellent test for the 10-cm aperture.

  Lupus is full of fine pairs and this column regularly features stars in that southern constellation. This month is the turn of Kappa Lupi = Dunlop 177 (15 11 56.07 -48 44 16.2), a beautiful pair of pale yellow stars according to E J Hartung. More recently Richard Jaworski notes them as yellow-white and plain white whilst the WDS gives the spectral types as B9.5V and A5V whilst the brightnesses are listed as 3.8 and 5.8. To the small aperture this is one of the easiest and brightest pairs in the sky - the current separation is 26".2 and this has decreased about 3" since 1826. With very similar proper motions, it is highly likely that the stars form a binary system, but Hipparcos has had some trouble in deciding the distance of the fainter component.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - May 2009

  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.

  At this time of year the constellation of Lupus is nearing the meridian in southern latitudes and offers a number of beautiful and sometimes difficult pairs for the small telescope user. Pi Lupi (15 05 08.16 -47 03 04.3) was found by John Herschel in South Africa and since that time (1836) it has been slowly widening so that it is now within range of a 75-mm aperture. It seems to have given early observers no little difficulty and values for the observed separations vary wildly which in a bright equal pair is difficult to explain. The current position is 67°, 1".6 and as this value of separation has not changed significantly for about 30 years, we may conclude that this is a binary in a highly inclined orbit which is close to apastron.

  The WDS gives the magnitudes as 4.56 and 4.60 and the negative value of (B-V) for both stars confirm that these are hot blue dwarf stars probably around spectral type B5. The revised parallax is 7.33 mas putting them at a distance of about 445 light years. A subsidiary note in the WDS says that both stars are spectroscopic binaries.

  Eta CrB (15 23 12.23 +30 17 17.7) is one of William Herschel's most important discoveries and one of the shortest period visual binaries visible in a small telescope. A recent orbit gives the period as 41.556 years with an error of 5 days so the pair has made five revolutions since discovery. The tilt of the apparent orbit means that the pair will stay rather close for a number of years. From the current separation of 0".56 the stars widen to 0".67 in 2014 before closing to 0".38 in 2020 and then widening to 1".0 in 2032.

  Eta CrB is also a relatively nearby system with a revised Hipparcos parallax of 55.72 mas which equates to 58.5 light years. The WDS gives the magnitudes as 5.64 and 5.95 and spectral types as FOV and GOV so that the stars appear yellow to the visual observer. There are three comites listed in the WDS, the first two, called C and D are field stars, but star E is a physically connected L8 dwarf some196 arc seconds distant from AB (equivalent to 3600 Astronomical units) and apparent visual magnitude 17. Imaging this star would be an interesting project for the well-equipped CCD astronomer.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - May 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.

  Both binaries in this months notes had orbits calculated for them about 20 years ago by Wulff Heintz, a great visual observer who died on June 10, 2006 after more than 50 years of observational activity. The orbits remain in the catalogue.

  Located in the western part of Boötes, STF1785 (13 49 0.28 +26 58 48.5) is a rather faint (mags 7.4 and 8.1) but attractive pair of orange K-type dwarf stars, discovered by Sir James South in 1823. The system is located fairly near the Sun at a distance of about 45 light years. The predicted position for mid-2008 is 179o.9, 3".13 but recent measures by the writer seem to indicate that the position angle is about 3 degrees smaller than this possibly suggesting that the 155.75 year period is a little short.

  Lupus is a bright constellation located halfway between the head of Scorpio and alpha and beta Centauri and it contains some beautiful pairs for telescopes of all apertures. Gamma Lupi, (15 35 08.46 -41 10 00.1) a brilliant white binary whose primary is a distant B subgiant has a highly inclined orbit of 190 years period and at times of closest approach the two

  components are only 0.07 arc seconds apart as happened in 1930. The stars of apparent magnitude 2.95 and 4.45 are currently almost at maximum separation (0".83 in 2013) so see them whilst you can. Fortunately this pair was near widest separation when found by John Herschel from the Cape in 1835.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - June 2007

  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.

  sigma CrB (16 14 41.04 +33 51 31.8) was discovered by William Herschel on 1780 August 7 and appears as the third entry in his class I pairs (separation 0 - 4"). The components passed within about 1".2 seconds of each other in 1830 and have been widening ever since. The maximum distance according to the 889 year orbit of Scardia occurs in 2245 at 9". Victorian observers suspected changes of colour and magnitude. Webb, in `The Intellectual Observer' for 1863 (page 134) offers the following summary: `Creamy-white and smalt-blue. There is much discrepancy about the smaller star. Struve calls it ``certainly not blue'', and differing very little from the other, 1825; Struve, white 1836.69; Dembowski, yellow, ashy, and doubtful blue, 1854 to 1857; Secchi, sometimes blue, sometimes yellow, 1855 to 1857. I fancied it, with a 3.7-inch object-glass, at one time ruddy, at another bluish, from 1850 to 1855, apparently changing even while being looked at; a versatility of hue which I have remarked on other stars similarly circumstanced, and which may possibly depend upon equal sensitiveness to colour in different conditions of the retina; during a short glimpse with 5.5-inches, 1862.57, the companion seemed bluish; at the same time I thought, as I had done in former years, that there was more than 0.5 mag of difference. Struve gave more than 1 mag. Secchi's discordances are considerable, ranging between 0.5 and 2 mags, from 1855 to 1857; but the honesty of that excellent observer, in recording every temporary impression, must be allowed for'. Small telescopes will show a fainter third star which does not belong to the system. In 1829 it was 44" distant from the main pair and last year measures put the distance at 90", due to the motion of nearby sigma (the Hipparcos parallax yields a distance of 21.68 parcsecs).

  mu Lupi (15 18 32.05 -47 52 30.7) This beautiful triple system can be well-seen in apertures of 10-cm and above. Strangely John Herschel does not mention the third component is any of his observations made with the 7-foot and 20-foot telescopes from Feldhausen, yet star C is mag 7.1 compared with the 5.1 and 5.2 of A and B. The close pair is clearly binary but the period is several hundred years since the motion in the last 180 years is 40 degrees retrograde whilst separation has halved to its current value of about 1".0. Hartung thought the distant star reddish. This star is also a physical member of the system since Hipparcos assigns it the same parallax as AB - 11.22 mas - corresponding to a distance of 90 parcsecs.

  Bob Argyle - Double Star Section Director