Double Star of the Month in Orion

• January 2021 - Double Star of the Month

  

  32 Orionis (05 30 47.06 +05 56 53.3) is easily found. It follows, and is slightly south of, Bellatrix (gamma Ori) by about 2 degrees. It was picked up by William Herschel on Jan 20, 1782 and he noted that the stars were Considerably unequal and The distance or black division between the two stars with 278 is about ¼ diameter of L(arge star)…. Herschel noted that the position angle was 232 degrees 10'. During the following century, measurements of 32 Ori showed the stars closing down to a distance of 0".3 before slowly increasing again to the current distance of 1".3.

  Thomas Lewis in his 1906 volume on the Struve stars thought the motion was explained by the proper motions of the two stars whilst van den Bosin 1962 also thought that the stars were not associated. The USNO Orbit catalogue gives a period of 614 years and predicts that the stars will widen until about 2100 before closing again. Gaia EDR3, unfortunately, does not help since it contains only an observation of the brighter component. The stars have V magnitudes of 4.4 and 5.8 and should be nicely seen in 10-cm aperture.

  Six degrees due south of 32 Ori, and just a little below the celestial equator is Mintaka, or delta Orionis (05 32 00.40 -00 17 56.7, V = 2.4), the most westerly of the three Belt stars.

  For the small telescope, the magnitude 6.8 companion (actually component C) located 52" away in PA0 degrees is an easy object to see. This star is itself a spectroscopic binary but it seems to be unassociated with its much brighter neighbour. Gaia EDR3 gives the distance of C as 1245 light-years and whilst the position of the A component is in the catalogue, there is no information on either parallax or proper motion. Hipparcos in 1997, however, found a distance of around 690 light-years, so it seems almost certain the stars are unassociated.

  The bright star is a close triple system. In 1978, Wulff Heintz using the 24-inch Sproul refractor, found a close visual companion at a distance of 0".2 which has a visual magnitude of 3.8; a preliminary orbit gives a period of 346 years. These two stars are very bright and hot, late O-type giant stars. In addition A is also an Algol-type eclipsing system. In 1877, Sherburne Burnham found a magnitude 14 companion (B) at 229 degrees and 33".

  Bob Argyle - Double Star Section Director

• January 2017 - Double Star of the Month

  The two pairs selected for this month are ostensibly relatively, bright and wide pairs; easy for the small aperture, or even binoculars. In fact, both are more complex.

  The northern object is S 503 in Orion (05 56 03.43 +13 55 29.7). The brightest stars are mags 6.7 and 8.4 and they are currently 76" apart in position angle 322 degrees. The WDS however lists 6 companions, most are field stars but at least one is travelling through space with S 503.

  Component C (mag 11.6 at 310° and 55".3) and D (mag 8.2 at 334° and 299") both also found by South are optical and should be visible in 10-cm aperture. Willem Luyten found a 13 magnitude star at 205° and 47" (E) which has the same space motion as A and which the WDS notes is a white dwarf.

  In late 2015, Marcel Fay, who has been publishing his CCD astrometry with a 283mm reflector in El Observador de Estrellas Dobles found two further stars of around magnitude 13 (F and G) at distances of 32" and 9" from A. The status of these is as yet unclear but further measures will show whether they are co-moving with A.

  In 2011 Shaya and Olling published a list of widely separated pairs of stars with common proper motions. This list includes pairs such as Capella and 50 Per, and gamma and tau Persei which have almost 100% probability of being physically related.

  Number 185 on this list is a wide pair of apparently unrelated stars in Columba: SHY 185 (06 36 54.07 -36 05 18.4). Star A is a G1 dwarf of visual mag 6.4. Some 288" away in PA 129 is a mag 7.3 star (B) of spectral type G0V. Both stars are close binaries of short period.

  A is RST 4816 which has a period of 14 years and is currently at 99° and 0".1 according to the orbit, whilst B was found by W. S. Finsen (FIN 19) and is predicted to be 339° and 0".3 at present. The Hipparcos parallaxes for A and B agree well within the quoted errors and the proper motions are similar.

  There are two other double stars to be found close by. One is UC 1454 (7.2 and 12, 181°, 33"), on the southern edge of SHY 185 and about 40 arc minutes south of the group is the fine pair BU 755 whose stars of mags 5.9 and 6.9 are now 1".5 apart and a faint third component which is mag 11.5 can be see 21" away (HJ 3875).

  Bob Argyle - Double Star Section Director

• January 2014 - Double Star of the Month

  The two stars to be highlighted this month are both systems of higher multiplicity although the closest visual components are at the limit of amateur instrumentation.

  2 Cam (04 39 58.03 +53 28 23.7) is a Struve pair (No 566) which has somehow eluded me for more than 40 years. I first saw it in 2012 with the Cambridge 8-inch Cooke and the companion was quite well seen and measurable. Since it was first measured in 1828 at 312° and 1".5, the two stars have slowly closed and moved in a retrograde manner and last year I obtained 169.7° and 0".89. A preliminary orbit gives a period of 425 years for this unequal (5.6 and 7.5) pair of stars. In 1901, using the Yerkes 40-inch OG, Burnham noted that the primary was a close pair in itself with the new component (V = 7.4) being found at 317° and 0".2. This turned out to be a rapid binary and the currently accepted period is 26.89 years, whilst the separation of the stars never exceeds 0".3. Dembowski adds another pair in the field (D 4) and it can be found about 4 minutes preceding - mags 9.0, 10.3 and separation 5".8. Whilst in this area look at 1 Cam = STF 550, a fine bright pair.

  eta Orionis (05 24 28.62 -02 23 49.7) This fine, bright pair of white stars is a good test for the 20-cm aperture. It was first resolved, when separated by 0".9, by W. R. Dawes who noted 'This close and beautiful object was discovered by me on Jan 15, 1848, with an aperture of only 4.25-in which I happened to be using on my 6.33-in refractor'. It is one of 13 pairs in the WDS under his discovery code. The current separation is around 1".8 and the stars have been slowly separating since discovery. Notwithstanding the fact that they are similarly hot, young and massive stars, the WDS notes that A and B form an optical system; the more distant mag 9.4 star at 114" is also believed to be unconnected with the bright stars. DA 5 was not known to Smyth but Webb notes colours of white and purplish (this may be the observation of Dawes) whilst others see only two white components. The A star is actually a massive triple system. Many years ago it was found to be an eclipsing spectroscopic binary of 7.88 days period with both stars possessing about 12 solar masses. More recently, speckle interferometer observations by H. A. McAlister and colleagues found a third companion with a period of 9.9 years, whose mass is about 1.5 solar.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - January 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.

  Rigel = beta Orionis (05 14 32.27 -08 12 05.9) is perhaps the most intrinsically luminous star in a double star system which can be seen in a small telescope. Whilst the measured luminosity of other bright stars such as Canopus have varied wildly over many years, the advent of the Hipparcos mission has been able to pin down the geometric distance to these objects much more accurately. The current trigonometric parallax for Rigel from Hipparcos is 3.78 milli-arcseconds with an uncertainty of about 10%. This translates to a luminosity of 48,000 times that of the Sun. The faint companion star whose estimated magnitude has also fluctuated over the years might be dominated by the light of the dazzling primary star 9 arc seconds distant but it is an equally interesting object in its own right. In 1871, Burnham, using his 6-inch Clark refractor, suspected an elongation of this star, and having then examined it with the 18.5-inch OG at Dearborn was convinced that there was a `real and measurable' elongation. Around 1900, Aitken, Hussey and Barnard using the 36-inch refractor at Lick all recorded separations in the 0.09 to 0.16 arc second range for BC. Occasional sightings have been reported since then, including as late as 2005 but no orbit exists and in fact the reality of the companion to B must still regard as unproven. Rigel B is known to be a spectroscopic binary but this cannot be BC. It needs a good night to see Rigel B clearly in the glare of the primary. The magnitude is near to 6.8 rather than the 10.4 first assigned to it by Burnham.

  19 Cam (05 22 33.53 +79 13 52.1) is a mag 5.1 late F dwarf star some 68 light years distant. A companion star of mag. 9.2 was first noted, apparently by Piazzi, in the compilation of his Palermo catalogue and later catalogued by Struve as STF 634.This is a good example of an optical double. In 1825, South found star B at 346° and 37", by late 1926 it was at 62°, 9" and by 2008 it had reached 133°, 27". Both stars have significant proper motions (0.18 and 0.16 arc seconds per annum) but almost in opposite directions on the sky. This is an attractive pair for small telescopes although rather difficult to find being in a sparse area near the north celestial pole - the colours were given as light yellow and pale blue by Smyth.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - January 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.

  The two binary stars being highlighted this month have in common long orbital periods but differ in other respects. Whilst 14 Ori has a relatively circular, face-on orbit, the orbit of HJ 3683 is both highly inclined and highly eccentric.

  14 Ori = STT98 (05 07 52.87 +08 29 54.9) This is one of Otto Struve's discoveries with the 15-inch refractor at Pulkova. The stars are magnitude 5.76 and 6.67 and both appear white to the writer but Hartung saw them as pale and deep yellow. The primary is of spectral type Am. The orbital period of 198 years sees the stars range in separation from 0.7 to 1.1 arc seconds so they can be seen with small to moderate apertures on most occasions, although the quoted magnitude difference of 0.9 always seems a little optimistic and good seeing is essential to see them well from the latitude of the UK. The writer found the pair at 305°.9, 0".90 in late 2006. Hartung points out that the fainter pair STF643 some 6 arc minutes south has the same proper motion.

  HJ 3683 (04 40 17.72 -58 56 39.6) was picked up by John Herschel in sweep 518 with his 20-foot reflector and he noted the pair as `very fine' and noted them as equally bright on two occasions whilst the WDS gives the magnitudes as 7.33 and 7.45. At the time of discovery the separation was about 3.5 arc seconds but the pair began to close and when Innes observed it in 1922, the star was single. At the last measurement recorded in the WDS for 2002 the pair appeared close to its discovery position. This is a system very similar to gamma Virginis but with an even more eccentric orbit (e = 0.95) and longer period (326 years). At periastron in 1918 the angular separation was 0.03 arc seconds and the angular velocity 1 degree per day. This pair of G dwarfs is 31.2 parsecs distant according to the revised parallax calculated by Floor van Leeuwen in his book `Hipparcos, the New Reduction of the Raw Data' issued by Springer (2007).

  Bob Argyle - Double Star Section Director

• Double Star of the Month - January 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.

  With Orion as well-placed as it gets in northern latitudes, it is worth taking a look at lambda Ori (05 35 08.9 +09 56 03) in the head of the Hunter. In binoculars, lambda forms a coarse triangle with phi 1 and phi 2 Ori. In larger telescopes and those operating in the infra-red, there appears to be a cluster of about a dozen B stars and a number of low-mass stars. Lambda Ori makes its own H II region by ionising the surrounding cool gas which appears to be in the form of an expanding ring. It may be that this is the site of an ancient supernova, some 300,000 years ago because the small proper motion of lambda itself does not project to the centre of the ring and that it may have been given a kick by a putative binary partner after that star went supernova. The young neutron star Geminga is also though to have been in that area at the time of the explosion. For the small telescope user, lambda is a fine pair with the 3.5 and 5.5 mag stars both brilliant white, reflecting their spectral types of 09.5II and B0.5V. Never less several observers have seen colour including Webb who thought they were yellowish and purple, whilst Olcott considered them yellow and red. The separation of 4".4 is virtually unchanged since records began.

  In the sprawling southern constellation of Puppis, there are many fine pairs for the small and medium telescope. A particularly noteworthy quadruple system can be found in Dunlop (Delta) 30 (06 29 49.1 -50 14 20). First listed by Dunlop in 1826 this unequally bright pair of stars is given by Hartung as yellow and reddish. The WDS lists the magnitudes as 5.97 and 7.98 and the separation is currently 12 arc seconds, making it an easy object. In later surveys, first Russell, and then the Harvard College expeditions to Arequipa in Peru found that both components were close visual doubles. The brighter component is known as R65 and has a period of 52.9 years. It is currently 0".7 apart and closing. The fainter component of the wide pair is known as HDO 195 and has a period of 101 years - it is also closing and in 2007 its separation is just below 0".4. Both systems should be resolvable in a 30-cm telescope.

  Bob Argyle - Double Star Section Director