Double Star of the Month in Lynx

• March 2024 - Double Star of the Month

  In a sparse area of sky south of Ursa Major a faint triangle of naked-eye stars can be seen. They straddle the border between Lynx and Leo Minor. The north-eastern of these stars is 38 Lyncis (09 18 50.64 +36 48 09.3), a beautiful pair whose components are magnitudes 3.9 and 6.1 and whose position angle has reduced just 15 degrees in the 200 years since the pair was first measured. The separation is 2".6 so a moderate magnification needs to be used to separate the stars adequately.

  

  An orbit with a period of 2782.8335 years, published in 2019, needs to be regarded with some scepticism but the stars are certainly physical, as shown by the parallaxes which the Gaia satellite has provided. Speckle interferometry reveals that B is a very close pair with two observations only and a measured separation of 0".1 to 0".2, and the Washington Double Star catalog (WDS) indicates that the A component is a 2 day eclipsing binary, found by the TESS satellite.

  There are optical companions (C,D) at 79" and 173" respectively, both of magnitude 12.5, and a fainter component E (magnitude 14.7) which is 101" away, but which has the same large parallax and proper motion as the bright stars thus elevating the whole system to quintuple status.

  DUN 81 (09 54 17.66 -45 17 00.6) can be found in northern Vela. It is a bright and easy pair with components of magnitudes 5.8 and 8.2. When found in 1826 the PA was 239 degrees and separation 4".0. It is nearer to 5".6 today with no apparent change in position angle.

  

  These are distant stars but the parallax of each star suggests that they are related. The Gaia Data Release 3 (Gaia DR3) catalogue gives 1124 ± 6 and 1136 ± 16 light-years respectively for A and B, with similar proper motions. The primary is a B5 dwarf according to Simbad and whilst no spectral type can be found for B, its almost zero (B-V) colour suggests a hot, young star.

  Using a 200" search radius on the Gaia DR3 catalogue shows a number of stars around magnitude 20 at about the same distance as the bright pair although admittedly the errors in the parallaxes are large it may suggest that there is a loose cluster here.

  Bob Argyle - Double Star Section Director

• March 2017 - Double Star of the Month

  STF 1282 in Lynx (08 50 44.28 +35 04 15.4) is a neat pair which can be found 7 degrees north of iota Cnc and a little east. The components are magnitude 7.6 and 7.8 and in 2015 I found the relative position to be 282 degrees and 3".41.

  There has been little motion between the two stars since the pair was found by F. G. W. Struve. Also called S 582. Burnham records yellowish white and very white.

  A faint and distant star, mag 12.4 at 49", has a common proper motion with AB and is therefore likely to be a physical member of the system. It was found by the French observer G. Soulie. It, in turn, has a companion of magnitude 14.6 some 19" away which was found by Robo-AO, the first robotic adaptive optics system which is mounted a 60-inch telescope on Palomar Mountain.

  upsilon Carinae (09 47 06.12 -65 04 19.2) is a 3rd magnitude (actually V = 2.97) star which is brilliant white and is accompanied by a mag 6.0 companion which is also white.

  The separation between the two has barely changed since the early 19th century when the pairing was first noted by Rumker in Australia. It is almost certain that this is a binary system and both stars are massive, luminous and hot.

  The primary is an A8Ib supergiant with an absolute magnitude of -5 giving it a luminosity of about 9000 suns whilst the companion is somewhat earlier in spectral type (B3 or B4) and is about 600 times brighter than the Sun.

  Not surprisingly Sissy Haas includes it as a showcase pair and small telescopes should suffice in dividing the stars although the significant brightness difference might militate against using too small an aperture - the separation is currently 5".

  A paper written in 1986 speculates that the period of the pair might be close to 19,500 years, and gives the distance as 400 pc. In 2007 the Hipparcos satellite found 440 pc but with a formal error of 54 pc. E. J. Hartung notes the pair HJ 4252 (9.3, 9.5, 303 degrees, 12") about 5 arc mins south following.

  Bob Argyle - Double Star Section Director

• February 2017 - Double Star of the Month

  STF 958 Lyncis (06 48 12.23 +55 42 16.0) is a neat, bright pair in SW Lynx near the border with Auriga.

  Start by locating the bright trio of 12, 15 and 14 Lyn, all visual binaries of increasing difficulty, and move 2 degrees south of the beautiful triple 12. This brings you to 13 Lyn, an orange giant of magnitude 5.3. Move a further 1.5 degrees south and you will alight on STF 958.

  One of William Herschel's finds, it is doubtless a binary, and even although the movement since 1782 amounts a slight decrease only in the apparent separation, the WDS 6th Orbit catalogue contains two orbits computed for it. With magnitudes of 6.3 and 6.3 it is not clear which star is the brightest and even SIMBAD appears uncertain. The SB9 catalogue assigns spectroscopic duplicity to the A star and the period is given as 4.26 days. The WDS notes that the other component has variable proper motion, thus indicating that it is a physical quadruple system.

  Herschel called STF 958 A pretty double star and assigned colours of pale rose to both components. Struve, on the other hand, found both stars yellow, as did Webb in the 1850s, whilst Sissy Haas notes that both stars are khaki-white. A 11.2 magnitude star can be found 176" away in position angle 268 degrees.

  19 Pup (08 11 16.32 -12 55 37.3) is in a fairly sparse area of sky but it forms an approximate isoceles triangle with Sirius and Procyon.

  It is the bright star on the south edge of the galactic cluster NGC 2539 and not surprisingly it comes with a number of faint distant companions in the WDS catalogue.

  The primary is a G8 giant of magnitude 4.8 and small telescopes will easily show two distant companions, a magnitude 8.9 at 58" and a 9.3 a further 12" out from the primary.

  The WDS lists two more which are fainter but Burnham in 1899 found a very faint star at a distance of about 2" which became BU 1064 AB. It was also measured by Aitken but no further sightings seem to have been made since then. Van den Bos looked twice, in 1936 and 1939 without success. Steve Coe noted 19 Pup and recorded that it was a triple star with the primary yellow and the two brightest comites being white.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - February 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.

  15 Lyn (06 57 16.60 +58 25 23.0) is on the western edge of a coarse cluster of double stars which also includes the close binaries 4 and 14 Lyn. Discovered at Pulkova by Otto Struve at a distance of about 0".5 the two stars slowly widened reaching around 0".9 at the turn of the last century before the companion made a close (0".1) approach to the primary and is now slowly widening again. This is a good opportunity to see this pair which needs 20-cm on a good night because there is also a substantial difference in brightness of about a magnitude. Andreas Alzner's orbit from 2000 gives the ephemeris position of 232° 0".67 for 2012.0. It is missing from Sissy Haas' book which is a little odd given that 4 Lyn is included and is similarly difficult to resolve. A good night may also show the faint companion discovered by Burnham. This 12.5 mag star can be seen at 346° and 29" whilst a more distant 9.5 is some 187" away and the distance to AB is decreasing due to proper motion of the bright pair.

  Dun 39 (07 03 15.12 -59 10 41.1) is in Carina, lying about 3 degrees north preceding alpha Pictoris and forms a beautiful white pair which is easily resolvable in a small aperture. When discovered by Dunlop from Australia in 1826 the stars were separated by 2".8. The revised version of Hartung (1995) by Malin and Frew repeats the note in the original edition that it can just be seen with 7.5-cm aperture, but the two stars are closing and the last measure in 1997 put the separation at 1".4. In 2006 Graeme Jenkinson and Tin Napier-Nunn of the Astronomical Association of Queensland noted that x320 was needed on a 15-cm f/8 OG to see it clearly so it will be interesting to see if it is still accessible to 7.5-cm now. The pair is clearly binary and lies almost 500 light years away.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - March 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.

  STF1333 (09 18 25.97 +35 21 51.3) appears with a brief description in Chambers revision of Smyth's Celestial Cycle but not in the original publication. Both stars appear `very white' in this tome but a more recent observation by Sissy Haas calls them `pure lemon yellow' but she also notes that they are in a very low power field with the orange star alpha Lyncis and also close to the bright, unequal pair 38 Lyn. The primary is spectral type A8V and the secondary probably similar although the WDS gives no spectral type. Discovered by William Herschel in 1782, Struve measured it in 1827 and found 40° and 1".5. The pair has slowly widened to 1".9 with little change of angle, so it is likely to be a long period binary but the relatively slow movement makes it a fine test for a small telescope with the components of magnitude 6.6 and 6.7. The distance according to Hipparcos is 308 light years.

  Mu Velorum (10 46 50.36 -49 25 12.8) is a magnitude 2.8 yellow giant located in a relatively blank area of sky to the naked eye some 11° north of the eta Carina nebula. In 1880, Russell, in Sydney, found it to be double with the companion some 2.7 magnitudes fainter located at PA 55° and a separation of 2".8. The pair turned out to be binary and by 1949 it had closed to 0".2. The currently accepted period is 138 years so the system is now almost at the point in its orbit where it was discovered. The relatively large difference in magnitude makes this star a southern equivalent of zeta Herculis although the separation range is much greater thanks to an eccentricity of 0.84. It will be near widest separation for many years and thus easily accessible to small telescopes on nights of steady seeing. Hartung notes that both stars are yellow, the companion being a G2 dwarf and therefore almost identical to the Sun. A is also a spectroscopic binary.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - February 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.

  STF1338 (09 20 59.4 +38 11 17.9) is a beautiful binary about 1.5 degrees north of the 4th magnitude star 38 Lyncis which is also a pretty pair. Its almost equal components are given as mags 6.72 and 7.08 in the WDS which also notes a third faint star (mag 11.4) some 144" away in PA 166 degs. Hartung finds the colours both bright yellow whilst Smyth and Chambers in the Cycle of Celestial Objects second edition of 1881 give both stars to be white. The WDS perhaps favours the latter colours listing the spectral types as F2V and F4V. Orbital motion is slow and from the discovery position of 121 degrees, 1".76 the pair has advanced to 303 degrees, 1".01 in 2009 according to the 303 year period given in the USNO 6th Orbit Catalogue. This pair is 42 parsecs distant according to Hipparcos and is an easy object in a small telescope at all times. It reached a maximum distance of 1".65 in 1865 and closest approach will be about 0".96 in 2044.

  Gamma Volantis (or Piscis Volantis) (07 08 44.82 -70 29 57.1) is a showcase pair according to Sissy Haas in her book and an observation by Ross Gould with 35-cm records the colours as deep yellow and dull yellow. With magnitudes 3.86 and 5.43 this is clearly one of the sky's most spectacular pairs but Smyth and Chambers merely note: 'A double star'. Hartung gives bright golden and pale yellow for the stars whose spectral types are KOIII and F2V. The discoverer was Dunlop and it is number 42 in his catalogue. Since 1826 there has been little motion in either PA or separation - possibly a slight closing but at 14".4 this is a pair for the small telescope or stabilized binoculars. The bright star is some 53 parsecs away.

  Bob Argyle - Double Star Section Director