Double Star of the Month Archive 2021

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.

June 2021 - Double Star of the Month

About three degrees north of zeta UMi (magnitude 4.3) is a group of naked eye stars, the most southerly of which is the binary pi1 UMi = STF 1989.

Image of a finder chart for the double star STF 1972 in Ursa Minor
A finder chart for the double star STF 1972 in Ursa Minor created with Cartes du Ciel.

The brightest star in the group is pi2 (magnitudes 6.6, 7.3) which is also a pretty, wide pair STF 1972 (15 29 11.19 +80 26 55.0). (This pair is listed in the notes of the second edition of the Cambridge Double Star Atlas (CDSA2) but not identified with its Struve number on page 1 of the Atlas). CDSA2 notes doubles 6' south-following and 10' north. William Herschel found the pair in 1782 when the separation was 26", whilst a recent measure in 2018 gives 32".

This is actually a physical quadruple system as both bright stars are 71.1 light-years away and each is also a spectroscopic binary. A recent deep imaging investigation, looking for exoplanets has also noted two twentieth magnitude objects within a few arc-seconds of B but the single epoch of observations does not indicate whether they are co-moving or connected to B. The relatively large proper motions of the bright stars has increased the distance to a 11.4 magnitude field star by 55" since 1911.

The southerly objects chose for this month's column are just south of the celestial equator, in the constellation Virgo. In 1781 William Herschel noted a wide unequal pair which appears as H 6 51 (14 57 29.32 -00 11 05.74) in his second list of double stars, published in 1784. He records the position as in Monte Maeneli Heveliana. In the Hevelius star atlas, Bootes appears to be standing on ground called Mons Maeneli or Mount Maenelus which was a constellation created by Hevelius in 1687.

Image of a finder chart for the double stars H 6 51 and BU 348 in Virgo
A finder chart for the double stars H 6 51 and BU 348 in Virgo created with Cartes du Ciel.

The Washington Double Star Catalog (WDS) catalogue gives V magnitudes of 5.6 and 10.4 with position angle 224 degrees and separation 86". The primary has a strong orange hue, a consequence of its spectral type of K1III. The SIMBAD catalogue calls the bright star 1 Ser and rather surprisingly Gaia EDR3 shows that both stars have the same parallax within the quoted errors, showing that they are 322 light-years away.

Nearby (just over one degree due east) is the much more difficult BU 348 (2 Ser - 15 01 48.92 -00 08 24.9) which requires 30-cm to resolve clearly, although S. W. Burnham discovered it with his 6-inch Clark refractor. Here the stars are magnitudes 6.1 and 7.5 and the current separation is only 0".5. If this is not challenge enough, try and see the 14.5 magnitude star C, 37" distant, which was found by Burnham in 1899. The close pair has a parallax of 3.252 mas corresponding to 1002 light-years.

Bob Argyle - Double Star Section Director

May 2021 - Double Star of the Month

1 Bootis (13 40 40.50 +19 57 20.4) is a star just about visible to the naked eye on a good night. It can be found about 4 degrees WNW of eta Bootis. It is not an easy double star for the small aperture and I have not actually observed it, even thought the primary is a naked-eye object.

Image of a finder chart for the double star 1 Bootis in Bootes
A finder chart for the double star 1 Bootis in Bootes created with Cartes du Ciel.

The Washington Double Star Catalog (WDS) gives the magnitudes as 5.8 and 9.6 with the current position angle and separation being 133 degrees and 4.4 arc-seconds. A 15-cm aperture should give a good view of the stars, the primary being white - its spectral class is A1V. There is a faint field star, magnnitude 12.2, 88 arc-seconds away whilst the 7.4 magnitude star at 208 arc-seconds has a similar parallax and proper motion as the stars of the binary. According to Gaia EDR3 the three lie at a distance of 310 light-years.

For the first time in this series, (as far as I know, and deliberately at least!) a binary star is going to be included a second time. This is because it is an exceptional object, about which much has been learned recently so it seems a good time to take another look at alpha Centauri (14 39 36.50 -60 50 02.3).

Image of a finder chart for the double star alpha Centauri in Centaurus
A finder chart for the double star alpha Centauri in Centaurus created with Cartes du Ciel.

In May 2007, alpha Cen was at PA 235 degrees and 8".4. This month it can be found at 354 degrees and 6".35, although it is now widening, but only as far as 10".4 in 2020, and then it closes to 1".7 in 2038. Extensive direct imaging has been done of both the stars in the bright binary and the physically connected Proxima Centauri.

To date there definitely seems to be one planet orbiting Proxima and recent observations suggest there is a second one. Both bright components have been suspected of having planetary companions and a recent paper purports to show a candidate close to the A component.

The latest data from the Gaia mission - data release EDR3 - includes a measurement of Proxima Centauri. The satellite finds the star to be 4.2465 light-years away with a quoted error of 0.0003 light-years. The main stars are far too bright to have been considered for data reduction at present though it is hoped that this might be done towards the end of the present mission.

Alpha Centauri is always a 'goto' object for the observer fortunate enough to be south of latitude +20 degrees or so. I have seen the stars in the 67-cm refractor at Johannesburg, and they appear like car headlights - dazzlingly bright.

Bob Argyle - Double Star Section Director

April 2021 - Double Star of the Month

35 Sex (10 43 20.91 +04 44 51.6) lies south of the main body of the constellation of Leo and about 5 degrees SSE of the magnitude 3.9 star rho Leonis. It is a fine pair with the principal stars shining at magnitudes 6.2 and 7.1 and the current separation of 6".8 puts the pair within range of small apertures.

Image of a finder chart for the double star 35 Sextantis in Sextans
A finder chart for the double star 35 Sextantis in Sextans created with Cartes du Ciel.

The angular motion in 2 centuries has amounted to about 2 degrees with the stars closing in very slowly. There seems to be no doubt that this is a binary pair - the Gaia EDR3 parallax data show that they are at 687 and 718 light-years respectively each with a formal error of about 22 light-years. Each star appears to be an early K giant which would imply that they should appear orange or deep yellow. Webb found colours of yellow and blue and later yellow and ruddy and noted that there was Much difference as to colours.

Just over 5.5 arc-minutes away from AB in PA 210 degrees is a star of magnitude 8.1, which is the long period binary A 2769 - 8.4, 9.4, 207 degrees, 0".5.

Nestling within the bounds of the five main bright stars of Crux, CPO12 (12 28 16.88 -61 45 55.6) can be found 1.5 degrees N. of alpha Crucis (Acrux) and 20 arc-minutes east of the open cluster NGC 4349. The stars are V magnitudes 7.3 and 8.2 and with a current separation of 2".1 they can be easily seen in 75 to 100-mm aperture.

Image of a finder chart for the double star CPO 12 in Crux
A finder chart for the double star CPO 12 in Crux created with Cartes du Ciel.

Since the pair was discovered in 1880 the position angle has reduced from 271 degrees to 183 degrees and a preliminary orbit by Dr. Andrei Tokovinin suggests that the period is about 690 years. In 1939, Robert Rossiter, using the 27-inch refractor at Bloemfontein in South Africa, noted that the B star was a close double. The Washington Double Star Catalog (WDS) gives a magnitude of 8.8 for each component. This is also a binary system but the stars are about ten times closer than A-B. The period is 28 years and the stars are currently 0".18 apart. By 2030 they will have reached their maximum separation of 0".24. There is another, much fainter companion - a star of magnitude 13.7 at 331 degrees and 7".9.

Acrux, of course, is a magnificent pair - see the column for April 2007 for more details.

Bob Argyle - Double Star Section Director

March 2021 - Double Star of the Month

STF 1426 (10 20 32.32 +06 25 47.6) is a long period binary located about six degrees south-south-east of Regulus. More specifically it is about 45' preceding the 6.1 magnitude star 43 Leo.

Image of a finder chart for the double star STF 1426 in Leo
A finder chart for the double star STF 1426 in Leo created with Cartes du Ciel.

It is also a triple with the C component (magnitude 9.4) having been first found by Herschel in 1782. The stars are magnitudes 8.0 and 8.3 and the current position is 317 degrees and 0".9, so 15-cm is needed to see them clearly divided. The stars have moved prograde by 60 degrees and widened from 0".6 since discovery. The C star is a relatively easy object for the small aperture and is currently at 9 degrees and 8", and Asaph Hall added a magnitude 12.6 with the Washington 26-inch which is currently at 41 degrees, 39".

The catalogue orbit for AB gives a period of 2209 years and predicts that the stars will now slowly close reaching 0".6 around 2175. One degree south-east is the wide and unequal optical pair SHJ 115, the primary of which has the significant proper motion of 0".25 per year and a distance of 190.6 light-years.

The pair chosen for the southern part of this column offers a real challenge to observers. BU 208 (08 39 07.90 -22 39 42.8) was one of the pairs which S. W. Burnham found with his famed 6-inch Clark refractor.

Image of a finder chart for the double star BU 208 in Pyxis
A finder chart for the double star BU 208 in Pyxis created with Cartes du Ciel.

When discovered the pair was 1".3 apart but later obserations saw the stars close to 0".25 and the period was recently found to be 123 years. The stars have magnitudes 5.4 and 6.8 and in early 2021 they can be found at 77 degrees and 0".34, but they are closing so any plan to resolve them would benefit from an early attempt!

Gaia EDR3 shows a parallax for the system as a whole which converts to a distance of 64 light-years, and its considerable annual proper motion of half an arc-second per year is continuing to widen the distance to a field star of magnitude 11.4 currently 121" away.

In addition to reports of the spectroscopic binary nature of the A star, the Washington Double Star Catalog (WDS) also contains a note on another component which has an H magnitude, of 13.7 at a distance of 7".5. It was noted whilst using the Subaru telescope on Mauna Kea to look for debris disks and planetary bodies. The authors noted it was near the edge of the detector and they do not give much weight to it.

Bob Argyle - Double Star Section Director

February 2021 - Double Star of the Month

In December 1968 I was observing with friends in the back garden of a house in Newcastle-upon-Tyne using a 12-inch reflector. We looked at a number of double stars that night, including phi2 Cancri = STF 1223 (08 26 47.08 +26 56 07.8) and 24 Cancri = STF 1224 (08 26 39.82 +24 32 03.7). These two pairs form a kind of wide double-double and can be found in northern Cancer.

Image of a finder chart for the double stars STF 1223 and STF 1224 in Cancer
A finder chart for the double stars STF 1223 and STF 1224 in Cancer created with Cartes du Ciel.

Starting with the beautiful pair iota Cancri, move about 4 degrees SE to find phi2. This is a pair of magnitude 6.9 and 7.5 stars separated by 5".7 and currently at PA 53 degrees. With the 12-inch I noted that the stars appeared white and lilac at a power of x208. Gaia indicates that these stars are at the same distance from us (347 light-years).

STF 1224 is the brighter of the two pairs with the components having V = 5.2 and 6.2. The separation is 5".2 and the position angle 219 degrees. I recorded both stars as being blue-white. Robert Aitken found that the B component was a very close binary of short period. The BC pair revolves in just 21.8 years and the separation stays close to 0".15 throughout the cycle. The Gaia EDR3 catalogue gives a parallax for the A component of 14.429 mas giving a distance of 226 light-years.

The magnitude 2.5 star pi Puppis (07 17 08.56 -37 05 50.9) lies in a rich area of the Miky Way which is part of the Vela-Puppis star forming region. It is surrounded by a number of naked-eye stars and the open cluster Collinder 135.

Image of a finder chart for the double star pi Puppis
A finder chart for the double star pi Puppis created with Cartes du Ciel.

Pi, which is distinctly red, is a close, very unequal double star which was discovered by Hipparcos in 1991 but whose nature does not appear to have been confirmed since. Along with a magnitude 7.9 star some 67" distant in PA 213 degrees it also forms the pair DUN 43.

To the north of pi are the bright stars upsilon 1 (V = 4.7) and upsilon 2 Puppis (V = 5.1) which are 4 arc minutes apart. Both these bright stars are variables. Upsilon 1 is also known as NV Puppis whilst upsilon 2 is NW Puppis. A V = 8.8 star lies at 119" and 215 degrees from B whilst William Jacob, observing from India, discovered a fainter companion (V = 9.1) to C at 3".1 and 209 degrees; this pair is now known as JC 10. There is another open cluster nearby called UBC7 and its possibly binary relation with Collinder 135 has been discussed.

Bob Argyle - Double Star Section Director

January 2021 - Double Star of the Month

Image of a finder chart for the double stars 32 Ori and delta Ori in Orion
A finder chart for the double stars 32 Ori and delta Ori in Orion created with Cartes du Ciel.

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