Double Star of the Month in Hercules

• 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

• 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

• 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

• July 2014 - Double Star of the Month

  zeta Her (16 41 17.46 +31 36 07.0) is part of the Keystone of Hercules, the south preceding component of the four stars in the pattern. Only 35 light years from us, it has long been known to be a close and difficult pair, but at the time of writing it is opening up and will soon be as easy to resolve as it gets. The ephemeris for the 34.5 year period orbit shows that in summer 2014 the stars are at 140° and 1".20. The difficulty comes with the large difference in magnitude - in the visual the components are magnitudes 2.95 and 5.40. The writer has followed this pair since 1990 and has been able to measure it each year since apart from 2001 - 2004 inclusive when it was too difficult for the 20-cm Cooke refractor at Cambridge. Over many years there have been suggestions of a sub-period due to one of the stars being a close, unresolved pair, and in 1983 a third component was detected in the infra-red but since then no confirming observations have been made and at present it is assumed that zeta is a simple binary star. The primary star is of spectral type early G and sometimes appears orange to observers with the companion appearing green by contrast.

  eta Oph (17 10 22.66 -15 43 30.5) was one of S W Burnham's later discoveries and is also known as BU 1118. This bright, twin pair of white early A stars of visual magnitudes 3.05 and 3.27 was separated by 0".4 at discovery in 1889. Like zeta Her above this pair is now close to its maximum separation and is actually starting to close. In summer 2014 it will be found at 232° and 0".57. This needs 30-cm on a night of very good seeing as it is low from the UK. The orbital motion accelerates rapidly as periastron approaches in 2024 at which time the stars are 0".006 apart and moving at 15 degrees per DAY. The period of this highly inclined and very eccentric (e = 0.95) orbit is 88 years. There are two faint comites of magnitude 11.2 and 12.4 both about 100" distant. Eta (combined magnitude 2.4) can be found about 15 degrees north following Antares.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - July 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.

  With the summer constellations now becoming dominant, the two pairs selected this month are to be found in the constellations of Hercules and Ophiuchus.

  STF2194 (17 41 05.50 +24 30 43.2) sits in the preceding edge of a glorious 1 degree field which includes the bright stars 83 and 84 Herculis. The pretty pair itself is an optical system and since 1783 it has widened gradually from 14".3 to 16".3. The main stars are magnitudes 6.5 and 9.3 and the current position angle is near 7°. Webb noted them as orange and blue in 1850, using his 3.7-inch refractor whilst Franks early in the last century found them to be yellow and lilac. The primary star is a K0 giant which is also a spectroscopic binary. About 169" away in PA 163° is a third star, nominally the same brightness in the visual as B.

  STF2173 (17 30 23.78 -01 03 46.2) is a visual binary with a period of 46.4 years. Its highly inclined apparent orbit means there are two opportunities for small apertures to see the two stars before orbital motion takes them beyond the range of most amateur telescopes. The writer measured this pair in 1992 and 1997 with the Cambridge 8-inch Cooke telescope when the separation was respectively 1".1 and 0".7. The two stars are now separating reaching a separation of 0".8 in the south-eastern quadrant in 2012, they then close rapidly and open out again, reaching 1".1 in the north-west in 2037. Both stars are yellow and close to V = 6.0 so this pair forms a beautiful test object at present for 15-cm aperture. Whilst in this area, check out another target some 4 degrees slightly preceding this pair - 41 Oph, a close unequal binary which is one of R. G. Aitken's last discoveries. It needs at least 15-cm on a good night.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - July 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.

  Alpha Herculis or Rasalgethi (17 14 38.86 +14 23 24.9) is a red supergiant star (M5Ib/II) and the leader of Hercules. It has long been known as a visual binary and is particular attractive to small telescope users because of the contrast offered between the primary and its G5 giant companion some 4".6 distant. The primary is usually seen as orange/red but the companion has variously been reported as white (Hartung), bluish-turquoise (Haas) and sage green (Perez). The primary is a semi-regular pulsating variable whose brightness varies from about magnitude 3.1 to 3.9, and the main period of oscillation appears to be 1343 days. The companion is about 5.4. The motion is orbital but extremely slow and the period is thought to be about 3600 years, but this is complicated by the fact that each component is double again. The primary has been resolved occasionally by speckle interferometry at the 0".02 level whilst the secondary has been known as a spectroscopic binary for many years; the period is 51.6 days. There is also another companion to A with a period of 10 years which does not appear to be the speckle companion so alpha is at least quintuple. For the larger telescope user two faint companions mag. 11.1 at 79", and mag. 15.5 at 19" appear to be merely optical.

  Located in Ara, just on the border with Scorpius, HJ 4949 AB (17 26 51.98 -45 50 34.7) is the primary, of the wide pair DUN 216 (5.6, 7.1) which was found by Dunlop in 1826 at 30O and 33".1. By 1999 the pair had widened to 102" thanks to the proper motion of C. AB is a beautiful pair barely 2" apart which has moved about 20 degrees retrograde since being found by John Herschel in 1834. The stars, of magnitudes 5.6 and 6.3, are both late B in spectral type and hence appear white and afford "a beautiful contrast with the surrounding starry field" according to Hartung. Hipparcos places them at distance of 613 light years.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - August 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.

  100 Her (18 07 49.6 +26 06 04) is a bright and wide pair of white stars in eastern Hercules, about 15 degrees south preceding Vega. It forms a fine sight for the small telescope. The original separation derived by Herschel in 1777 showed the stars to be 17".0 apart whilst a more recent measure by the author in 1995 indicates that the stars have closed to 14".2 with very little change in angle.

  These stars caused the Hipparcos satellite some difficulty as the errors in parallax, even in the revised version, are some ten times worse than might be expected but they still show that the parallaxes of the two stars are the same within the (large) errors and thus indicate that the two stars are probably physically related. In 1985 the CHARA team from Georgia State

  University discovered that component A was a close binary. Subsequently, the period turned out to be less than 16 years. Perhaps the same situation might apply to star B, as an explanation for the large error in parallax. Both stars are noted as white by several observers.

  HJ 5014 (18 06 49.9 -43 25 30) was another product of John Herschel's fecund search for new double stars at the Cape of Good Hope. Unfortunately there were few observations in the following 50 years when the pair moved through almost 180 degrees of position angle, widening considerably as it did so. Wierzbinski produced some orbital elements in 1958 with a period of 191.2 years. It was clear around the beginning of this century that the real period was much longer and Andreas Alzner produced an orbit in 2002 increasing the period to 450 years.

  This is a beautiful pair of white stars (both A5 dwarfs) each of visual magnitude 5.7 which can be well seen in 10-cm aperture. The position for 2010.0 is 2.4 degrees and 1".72, and it will continue to widen until 2170 or so.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - July 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.

  Both systems featured in this month's column are close to the Sun but vary considerably in difficulty of observation. One is a real test for a medium aperture whilst the other can be seen in a small telescope.

  Mu Herculis = STF2220 (17 46 27.72 +27 43 21.0) was found by William Herschel in 1781 and is a wide and very unequal pair of magnitudes 3.42 and 9.78. The current position angle and separation is 248 degrees and 34".9 values which have increased only marginally since Struve measured the pair in 1831. The large proper motion of A and the small change between A and B over time mean that the two stars form a physical system. In 1854 Alvan Clark found that the companion was double. It turns out to have a period of 43.2 years and is a severe test of resolution and light gathering power. The components are mags 10.2 and 10.7 and the separation varies between 0".5 and 1".5. At the time of writing the stars are 1".1 apart and will close until 2018 when a separation of about 0".6 is reached. This is a good opportunity to see this system as a triple star. The revised parallax from Hipparcos is 120.33 mas putting the group at a distance of about 27 light years. The spectral type of A is G5IV and that of the close pair appears to be dwarf M, not withstanding the fact that Chambers in his revision of Smyth's Bedford Catalogue, assigns to it a colour of cerulean blue.

  As a nearby solar-like star, mu Her A is a good candidate for hosting a planetary system and in 1994 two independent series of radial velocities were taken. What they showed was a slow drift which indicated a possible period of 30 years. In 1998, a star of V magnitude 12.7 was seen 1.4 arc seconds from A using adaptive optics on the 100-inch reflector at Mount Wilson. This object is close to the sub-stellar mass limit and further observations will be needed to establish its physical connection to A. The WDS lists another star, mag 11.5 at 256", but it far from clear that this also belongs to the system.

  About 4 degrees north preceding the 3rd magnitude star alpha Arae is BSO 13 (17 19 02.95 -46 38 11.4). Picked up as early as 1824 in the mural circle at Paramatta (Sydney) it is a similar system to eta Cas - a nearby, long period, unequal pair with a G-type primary and M secondary, in this case G8V and M0V. The stars, which are only 28.7 light years distant, are now moving slowly apart in their 693 year orbit and are currently at 257 degrees and 9".9 so they are visible in 75-mm aperture with ease, although larger apertures will show the colours to greater effect. Hartung notes deep yellow and orange. The WDS gives magnitudes of 5.61 and 8.88 but van den Bos in a series of measures in Johannesburg in the 1920s consistently estimated the magnitude differences as 2.5 or so, so there may be some real variation in the brightness of the companion.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - August 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.

  95 Herculis (18 01 30.40 +21 35 44.5) is a double star much beloved of the Victorian observers, due to the suspicion that the colours of the components changed over a period of years. Hartung records pale and deep yellow, a conclusion agreed with by Frew and Malin in their revision of his book. Chambers, in his revised version of Smyth's `Celestial Cycle' notes that a friend of Smyth, a Mr. Higgens of Bedford, claimed that the intensity of the green and red colours of A and B varied from time to time and that the green star recovered its hue first. `On this statement being submitted to Sir G. B. Airy he did not view it with favour'.

  It seems likely that this pair is binary - the significant proper motion in dec of A (0.039 arc seconds per year) would have carried it 9 arc seconds away from B over 230 years, whilst the separation has reduced from 9".0 in 1777 to 6".3 in 2007 with a small decrease in position angle. The revised Hipparcos parallax puts the A5 giant primary at 123 parsecs. This is one of the finest pairs in the northern sky for small telescopes.

  The Australian amateur Walter Gale has his name on three stars in the WDS catalogue. Gle 3 was described in August 2007 notes and Gle 2 is xi Pavonis, (18 23 13.62 -61 29 38.1) a bright yellow giant K4 star. The revised Hipparcos parallax puts the star at 143 parsecs and the relative position of the companion has changed from 140°, 4".0 in 1894 to 156°, 3".4 in 1988. Hartung gives the colour of the companion, some 3.7 magnitudes fainter than the V=4.4 primary, as white.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - July 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.

  The double stars selected for this month have a number of common features. Both are relatively close-by, both have short periods, and both are visible in small to medium-sized telescopes at the present time.

  zeta Herculis (16 41 17.48 +31 36 06.8) is a famous binary found by the elder Herschel on 1782 Jul 18. Although the separations at the closest and widest points in the apparent orbit are respectively 0".6 and 1".6, the large difference in magnitude, and the glare of the 3rd magnitude primary makes the system a difficult one to measure accurately.

  In his 1906 volume, Thomas Lewis devotes five pages to the orbit of this pair which he found could only be explained if the orbital period was changing with time. This seemed to imply the presence of a third component but no such star has been found and there is no evidence from the extensive radial velocity history of this star. Recent direct measurements of the diameter of star A, a G7 giant, show that it is some 2.5 times the diameter of the Sun. The distance from Hipparcos measurements is 35.2 light years and the V magnitudes are 2.9 and 5.6. The pair, whose orbit has a period of 34.45 years, is now widening and in 2007.5 the position will be 200°.2, 1".08.

  MLO 4 = BU 416 = R 298 (17 18 56.36 -34 59 22.5) is just visible to the naked-eye on a clear night and can be found sitting some 3 degrees north-west of lambda Sco in the tail of the Scorpion. It was found to be double by Burnham with the 6-inch Clark in 1876 and it appears as BU 416 in his 1906 General Catalogue. His estimates of the magnitudes were 6.0 and 8.5. The modern measures of the magnitude difference by Hipparcos are nearer 0.9. It was also found independently by Russell using the 11-inch refractor at Sydney. According to R. T. A. Innes however, the pair was `first noted at the Melbourne Observatory in 1867', hence its catalogue name.

  The distance to MLO 4 is 22.7 light-years, although, strangely, the quoted Hipparcos parallax error is almost 12%, with the annual proper motion exceeding 1 arc second. The period of the binary is 42.15 years, according to Soderhjelm in 1999 and for 2007.5 the position is 206°.0, 1".47.

  The orange hues of each component betray the late spectral types; they are dwarf stars of class K3 and K5. There is some evidence that a more distant member of the system (31") is a M-type star. It has similar proper motion to AB so is physically connected.

  Bob Argyle - Double Star Section Director