Double Star of the Month in Gemini

• 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

• February 2024 - Double Star of the Month

  S 548 (07 27 40.54 +22 08 29.3) was noted by William Herschel in 1782 and catalogued by him as H V 66. He noted that the stars were very unequal and the larger one was pale red and the smaller one was dusky. The distance was noted as 34" 39'". Little has changed in the relative positions since then with the Washington Double Star catalog (WDS) giving 277 degrees and 35".3 for 2019.

  

  The pair can be found two degrees following delta Gem (see this column for Feb 13). The catalogue magnitudes are 7.0 and 8.9. In 1892 Thomas Espin added a faint companion of magnitude 12.4 at 24 degrees and 11".9. The three stars are unrelated. A lies 1800 light-years away, B is 940 and C is 8350 light-years distant.

  In southern Puppis right on the border with Vela is HJ 4093 (08 26 17.74 -39 03 32.3). This is a fine pair for the small aperture, it is triple in apertures of 40-cm or more, whilst the spectrograph reveals that component A is an Algol system called NO Pup.

  

  Stars A and B (magnitudes 6.5 and 7.1) are currently at PA 122 degrees and 15".0 having almost doubled their separation since discovery by John Herschel. Using the 26.5-inch refractor at Johannesburg, Willem van den Bos found that B is a close double (Ba,Bb), with stars of magnitude 7.9 and 8.1, currently 0".2 apart and a binary with a period of 103 years. In addition the WDS notes that Andrei Tokovinin finds a close component (D) only 5".4 from A but with a K magnitude of 17.3, but this star does not appear in the Multiple Star Catalogue.

  Examining Gaia DR3 with a field of radius 100" shows two stars of magnitudes 12.2 and 13.1 respectively 64" and 66" from A which share the same parallax, so perhaps this is a small cluster. There is, however, no data for the B component and the significant change in the AB distance since discovery may indicate that B is unrelated.

  Bob Argyle - Double Star Section Director

• February 2023 - Double Star of the Month

  One and a half degrees just east of the fine but unequal binary pair delta Gem (see this column for February 2013) is the pair STF 1108 (07 32 50.63 +22 53 15). The star is displayed on page 8 of the second edition of MacEvoy and Tirion's fine Double Star Atlas (CUP) but without a label. Having checked back over my file of measures I note that this pair does not appear in it, nor does it appear in my earlier visual survey carried out with a 21-cm in the late 1960s.

  

  The Washington Double Star catalogue gives the visual magnitudes as 6.6 and 8.2 whilst Gaia DR3 notes G mags of 6.3 and 8.8. There has been little motion since the pair was found by F. G. W. Struve. DR3 notes that the parallaxes are the same within the errors (and indicate a distance to the pair of 643 light-years) although these are some ten times larger than might be expected for stars of this brightness.

  Half a degree south is the unequal, wide pair 63 Gem and about a degree to the south-west is STF 1081 (7.7, 8.5, 204 degrees, 1".9, slowly increasing)

  2 Pup = STF 1138 (07 45 29.14 -14 41 25.7) lies in a string of naked eye stars which stretch about 30 minutes of RA along the line of south declination 15 degrees. Other objects in this area include M46 and M47 and the binary STF 1104 (see this column for February 2020). STF 1138 is about a degree due east of M46 and is a beautiful, easy pair for the small aperture.

  

  During a visual survey carried out in the late 1960s I found colours of yellow and lilac whereas Admiral Smyth records hues of silvery white and pale white for `2 Argo Navis'. The stars of magnitudes 6.0 and 6.7 were separated by 16".7 at PA 340 degrees when I measured them in 2015, having closed from 17".4 when measured by William Herschel in 1782.

  A third star of magnitude 10.6 lies 100" away in PA 229 and is some five times further away than the stars in the pair. They both have very precisely determined parallaxes thanks to Gaia DR3 which puts them 279 light-years away with an uncertainty of about 1 light-year.

  Bob Argyle - Double Star Section Director

• February 2020 - Double Star of the Month

  Starting at the fine pair 38 Gem (see this column for Feb. 2016) and moving 2 degrees due East, brings the observer on the pair STF 1007 (07 00 37.52 +12 43 24.2) and, a further 20 arc-mins East, upon HJ 3288.

  The brighter and wider of the two is STF 1007 which was left out of Lewis' treatise on the Dorpat pairs because it was too wide (the writer found 28 degrees, 67".4 in 2014). In fact, Burnham noted two fainter and closer companions on March 16, 1873 with his 6-inch refractor, neither of which could be seen in the 8-inch Thorrowgood with the micrometer field illumination on. C is 11.4 at 300 degrees, 15" and D is 10.0 at 244 degrees, 22" whilst Burnham called them magnitudes 14 and 12 respectively. The Washington Double Star Catalog (WDS) notes that D was found to be a close lunar occultation double.

  HJ 3288 is a pair with magnitudes 7.3 and 8.7 and the writer found 217 degrees, 38" in 2013.

  Originally found as a close bright pair by F. G. W. Struve, STF 1104 (07 29 21.91 - 14 49 53.40) turns out to be a physical quintuple system. The AB pair has magnitudes of 6.4 and 7.6 and at discovery was found at 292 degrees, 2".4. At 2017 it was 38 degrees, 1".8 and a preliminary orbit was computed by A. A. Tokovinin in 2014 who found a period of 729 years. This predicts a minimum separation of 1".7 around 2045 so the pair is always within range of 10-cm.

  In the 1880s two further stars were noted - an 11.8 at 20" (C) and a 13.2 at 72" (D). Since then D has been rapidly left behind by the considerable proper motion of AB, which is 0".3 per year. C, however, is keeping pace and is clearly physical. Dr. Tokovinin also found that C was a close pair of dwarf stars separated by 0".1 and also noted that a star 1072" away which was noted by Luyten and labelled LP 722-24, is also moving through space with a similar proper motion and distance.

  The group is 120 light years from us.

  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

• February 2016 - Double Star of the Month

  38 Gem (06 54 38,63 +13 10 40.1) can easily be swept up since it directly follows the 3.4 mag xi Gem by a little over 2 degrees.

  The current orbital period, 1898 years, as determined by Brian Mason in 2014, is clearly very uncertain but the position for 2016.0 is 143° and 7".31 in close agreement with measures by the writer late last year. The stars are of visual magnitude 4.8 and 7.8 so the quadrant in which B lies is certainly the second whilst Sissy Haas puts it in the 4th.

  Admiral Smyth gives light yellow and purple, but E. J. Hartung sees yellowish and pale-orange, whilst to Sissy Haas the colours appear lemon-white and greyish.

  A third, much fainter star C of mag. 11.3 can be seen at a distance of 119" whilst Andrei Tokovinin noticed a 15.0 mag dot at 151". The primary, a dwarf star of spectral class F0 is 96 light years away.

  STF1121 (07 36 35.71 -14 29 00.3) is not a double or multiple star - rather it forms the bright core of the open cluster M47 in Puppis.

  This cluster was discovered by the Italian astronomer Giovanni Hodierna sometime before 1654. As well as finding a dozen or so deep-sky objects before Messier catalogued them, Hodierna also compiled a small list of double stars.

  The WDS contains 26 entries to cover this system and its large array of comites, but the small telescope user will easily be able to see AB (6.9, 7.3 at 300° and 6".5), whilst amongst the more obvious comites D is mag 9.5 at 72" (distance increasing), E is 9.9 at 70" (distance decreasing) and G is 7.7 at 82".

  It is perhaps best seen with a pair of large binoculars. A report on the Cloudy Nights website for 2004 notes that the AB pair can be split easily with Celestron 25 x 100s. M47 and nearby M46 can be swept up in a wide-field telescope by moving 20 degrees due south of Procyon.

  Bob Argyle - Double Star Section Director

• February 2015 - Double Star of the Month

  In mid-February, Gemini passes the meridian at 10pm and offers an excellent opportunity to view some of the fine double and binary stars in this constellation. One of the less-well known systems is 20 Gem (06 32 18.52 +17 47 03.4) which is eaily found about 3 degrees north preceding the bright star gamma Gem. Wil Tirion's Cambridge Star Atlas shows another double star about a degree south of 20. This is STT143 (6.2, 10.4, 103°, 7".6 with a fainter third component at 345° and 47"). Also known as STF924, 20 Gem offers a beautiful pair of stars which Webb noted as topaz yellow and cerulean blue, but as these are the exact hues noted by Smyth in the Bedford catalogue it may be assumed that Webb was merely repeating the Bedford colours. More recently Sissy Haas finds both stars to be gloss-white.

  There is little relative motion between the stars and the current situation is that the PA is 211° and the separation 20".2. Hipparcos found the determination of the parallax of star A difficult and finds a distance of 262 light years with an error of 30%. This is no doubt due to the motion induced by the duplicity of A confirmed by both occultation observations and also directly as a spectroscopic binary.

  Looking about 1° slightly north preceding the V = 4.4 star q Puppis, one will alight on the wide pairs DUN 67 (08 13 58.31 -36 19 20.2) and DUN 68 about 1.5 arc mins south and west of it. DUN 67 consists of stars of magnitude 5.0 and 6.0 which are currently separated by 66 arc seconds and PA 174°. The separation is slowly decreasing. Hartung chooses not to include them is his book 'Astronomical Objects for Southern Telescopes' but Gould with 175-mm finds both stars to be pale yellow and notes the existence of several fainter pairs in the area. The components of DUN 68 are both mag 7.3 but form a wider pair than its neighbour. The PA is 25° and the separation is 125" and increasing.

  The whole region is fine with the large scattered star cluster NGC 2546 about 1.5° further south adding further reason to take in the area with a pair of binoculars as well. Both pairs are at the same distance from us within the stated errors of the Hipparcos parallaxes.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - February 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.

  It was near delta Gem (07 20 07.39 +21 58 56.4) that Pluto was first seen on a plate taken by Clyde Tombaugh at Lowell Observatory in February 1930. By a strange coincidence, Herschel found the mag 8.2 companion to delta on March 13 1781, the very same night that he discovered Uranus and later recorded the system as H 2 27. The primary star is a late A9 giant, of visual magnitude 3.55, and at a distance of 60 light years according to Hipparcos. To see the companion comfortably requires 15-cm and a night of reasonable seeing. Some disagreement attaches to the colour of the secondary. It is given as K3V in the WDS and E. Hartung noted it as reddish, as it did T W Webb but some years before he had recorded the hue of the star as purplish. The primary is also a single-lined spectroscopic binary and has also been seen double at lunar occultation’s but attempts to resolve it directly using speckle methods have so far failed.

  BU 332 (07 27 51.66 -11 33 24.7) is a multiple star which appears coincident with NGC2396 on Map 8 in Norton, close to the point where Monoceros, Canis Major and Puppis meet. Just after completing the notes on this system I read the Sky and Telescope for February 2013 and found that it featured in Sue French's column (page 57) under the name STF 1097. AC has mags 6.2, 8.7 and the components seem relatively fixed at 313° and 20". In 1865 Baron Dembowski suspected that A was double and it was later confirmed by Burnham using his 6-inch Clark refractor. Star B is magnitude 7.35 and there has been very little motion - amounting to 7° retrograde in PA, and the stars are possibly closing up. Small telescope users should be able to see the more distant D (V = 9.7) at 157o, 23". A 12.7 mag star at 32" will need at least 10-cm. Hartung notes that it is a beautiful field and that the close pair is deep yellow and white.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - February 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.

  With Gemini high in the sky in the early Spring sky, a number of binaries are on display for the small telescope user. One of the more difficult is STF1037 (07 12 49.08 +27 13 30.2), a pair of yellow stars which are locked in a highly eccentric orbit. They present a good test object because at present the separation is 1".00 according to the 116 year orbit which is given in the USNO 6th orbital catalogue. The magnitudes are listed in the WDS as 7.2, 7.3 and with the 8-inch refractor at Cambridge this pair has always been more difficult to see and measure clearly than the parameters would suggest. It is a good time to observe STF1037 - the pair is closing up again and will reach well below 0".1 in about 30 years time, and it not be this wide again until around 2063. In the 19th century, the German observer Madler was convinced that B was double again and the volume by Lewis on the Struve stars does show a loop in the apparent motion of B but no convincing evidence for a third component has come to light. Madler, and Dembowski failed to see the faint star C (V~13) found by Otto Struve. It is located at 78°, 14" but may be variable.

  Puppis is a glorious constellation for the double star aficionado and one of the best objects is k Puppis (07 38 49.88 -26 48 14.0), a third magnitude star some 8° east of delta CMa. Discovered by William Herschel (H III 27) the stars are both hot blue dwarfs of spectral type B6 and might be expected to appear white in the eyepiece. Malin and Frew, in their revision of Hartung's book thought so but noted that Hartung himself had them as pale yellow. From his observatory in Victoria, Australia this object would have passed almost overhead. Haas also calls them white but gives the star name as kappa. There has been some angular motion since 1800 - star B has moved about 8° retrograde and can now be found at 318° and 9".9. Burnham added a faint, distant star, mag 13.7 at 7" from A, but as this has not been measured since 1927, it would appear to be a difficult object.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - February 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.

  This month the two binary systems in question both have late-type giant stars as primaries.

  In 1882, using the 12-inch refractor on Mount Hamilton, Burnham found that eta Gem (06 15 52.70 +22 30 24.6) was double and it eventually became number 1008 in his catalogue. In several cases where he discovered very faint and close companions, Burnham tended to underestimate the brightness of the companion. When, for instance, he found alpha UMa to be double (BU 1077) he gave the magnitudes as 2.0 and 11.1 whilst the modern values for A and B, as found by Hipparcos are 1.95 and 4.87. In the case of eta Gem, he estimated A and B to be 3.0 and 8.8 - again Hipparcos notes that B is considerably less faint than Burnham's estimate and gives V equivalent magnitudes of 3.3 and 6.0 with the companion at a distance of 1".1. Since then the position angle has reduced by about 40 degrees and the separation has increased to 1".5. Even in 1961, Hartung was able to see it with 10.5-cm aperture and these days it is somewhat wider than that. The primary, spectral type M3.5I-II is orange but no colour estimate of B is noted.

  In the 1880s, astronomers at Sydney Observatory were busy looking at the double stars, including re-observation of those of John Herschel using an 11.4-inch refractor. Under the directorship of H. C. Russell, R. P. Sellors was one of the observers. The WDS shows 24 pairs under his name, the first and brightest of which is beta Phoenicis - a bright, close binary. The second brightest pair is SLR 8, located in Vela at (08 32 04.97 -53 12 43.1). Consisting of stars of magnitudes 6.13 and 7.08 this pair was separated by 0".4 in 1892, widened to about 1" in 1925 and is now closing again. An aperture of 15-cm should show it but there have been no measures since 1991 - an indication of how the southern pairs continue to be neglected. The colours are orange-yellow and whitish, reflecting the spectral types of KOIII and A3. This system is 227 parsces distant according to the revised Hipparcos parallax.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - February 2007

  In this new 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 constellation of Gemini is well up in the northern sky during the middle of the month with the famous pair of stars Castor and Pollux at the eastern end of the group. The two stars are well contrasted with Castor showing pure white, as befits its membership of the class of A stars. Pollux, on the other hand, is orange, in reality a cool giant star and is actually the brighter of the two visually, prompting suggestions that one of the stars has changed in output in recent times.

  For the small telescope user, Castor (7 34 35.9 +31 53 18) is of real interest. The star is a brilliant binary with two white components of magnitudes 2.0 and 2.9 currently separated by 4.4 arc seconds in position angle 59 degrees, making it easily visible in a 60-cm telescope. It may have been first resolved by Cassini in 1678 but it was certainly noted by Bradley in 1718. Since then the position angle has decreased by almost 300 degrees, with closest approach around 1965 when the separation was 1.8 arc seconds. Some 72 arc seconds to the south-east is a star of magnitude 9, known as Castor C. This star revolves around Castor AB in a period of many thousands of years. The remarkable fact about the Castor system is that all three visible stars are spectroscopic binaries, making the Castor system a rare example of a sextuple star.

  Sirius (06 45 08.9 -16 42 58, mags -1.5, 8.5). The brightest star in the sky is also one of the nearest, located 8.7 light years away. The details of the discovery of the white dwarf companion are well-established. Bessel first noted that the proper motion of Sirius was not linear but the predicted companion was not seen until January 1862 when Alvan Clark was testing the 18.5-inch objective for Dearborn Observatory. Uniquely, Peters calculated an orbit for the Sirius system 11 years before the star was first seen. His value for the period, 50.01 years is very close to the currently accepted value.

  There is much speculation about the smallest aperture required to see the Pup. It depends crucially on several factors - the separation of B from A, the quality of the atmosphere and the quality of the telescope optics. When B is near periastron it cannot be seen in any telescope. Between 1890 and 1897 when the separation was less than 4 arc seconds, there were no sightings recorded.

  A recent observation of Sirius B was reported by Ralph Aguirre of the Sacramento Valleys Active Astronomers in March 2006. At a separation of 7.3 arc seconds B was seen with a 130-mm Takahashi refractor at x140 but he found it was better seen at x220, a point which earlier observers seem to agree about. This year the writer plans to use a hexagonal diaphragm on the 8-inch refractor at Cambridge in an attempt to get his first glance of this elusive object.

  Bob Argyle - Double Star Section Director