May 2023 - Double Star of the Month

STF 1850 (14 28 33.239 +28 17 25.9) is a wide and fairly bright pair which can be found 3.5 degrees WNW of the spectacular pair epsilon Bootis also known as Pulcherrima - 'most beautiful' (see the column for May 2020).

The stars have changed little in relative position since they were catalogued by F. G. W. Struve and they are currently separated by 25" in PA 263 degrees. Although both are given spectral class A1V by the Washington Double Star catalog (WDS), the stars are magnitudes 7.1 and 7.6. They are rather distant, and Gaia DR3 gives the mean distance of the two as 873 light-years with an uncertainty of 1% in each case.

SHJ 263 (18 17 51.13 -18 47 54.6) sits in the yellow area marked Star Cloud M24 in The Cambridge Double Star Atlas, although it is not labelled. It is probably more accurate to define SHJ 263 as a foreground grouping of stars.

The main components are 6.8 and 9.3, separated by 54" in PA 11 degrees. The WDS lists six other companions of magnitude 12 and 13 within 30" of A and B. The only entry in Gaia DR3 for this group is that of the primary star which is given a distance of 1038 light-years whereas the distance of the star cloud as a whole is given as 10,000 light-years in the on-line literature.

Twenty minutes NE is SHJ 264 (18 18 43.26 -18 37 10.8) and the bright pair is called AC. These stars are magnitudes 6.9 and 7.6 with a separation of 17" in PA 51 degrees.

S. W. Burnham has added a couple of more difficult tests for 25-cm aperture. A is a close pair, the companion (B) is magnitude 8.2 and only 0".5 distant from A and has moved from PA 155 degrees in 1878 to a current value of 136 degrees. Some years before this, Burnham had noted a 13.1 magnitude star (D) 6" away from C whilst Philip Fox added a 10.8 at 145" from A.

The primary star is a B0Ib supergiant and Gaia DR3 gives distances of 1826 and 4672 light-years respectively for A and B.

Bob Argyle - Double Star Section Director

If you'd like to try out the Clear Skies Observing Guides (CSOG), you can download observing guide for the current Double Stars of the Month without the need to register. CSOG are not associated with the Webb Deep-Sky Society but the work of Victor van Wulfen.

April 2023 - Double Star of the Month

In the constellation Coma Berenices, in addition to the famous cluster of galaxies, there is also a grouping of naked-eye stars about 13 degrees north of the galaxy cluster. Taking the pairs 12 and 13 Comae and 14 and 16 as the vertices of an isosceles triangle, then the third vertex is occupied by STF 1633 (12 20 41.33 +27 03 16.4) a beautiful pair described by Webb as Very pretty. Solitary. Observing it with a 21-cm reflector I found the colours to be yellow and delicate light blue.

Of the four bright stars in the 'triangle' only 12 appears to be in the double star catalogue. Also known as SHJ 143, this is a wide, unequal pair in 15-cm (magnitudes 4.9, 8.9, 167 degrees, 65") which should also show the more elusive 11.8 magnitude star at 57 degrees, 37" to A.

Just over a degree south of the Sombrero galaxy (M104), and over the border into Corvus is STF 1669 (12 41 16.22 -13 00 53.6) a beautiful visual double which turns out to be at least a physical quadruple star under the gaze of the spectroscope.

The AB components are both magnitude 5.9 and the current position and angle and separation is 314 degrees and 5".3. This shows a change of +15 degrees and -1".6 since being measured by Struve in 1828.

A third star (C) of magnitude 10.3 is 46" away in position angle 228 degrees. Strangely, Gaia DR3 records similar values for the trigonometrical parallax of stars A and C (equivalent to a distance of 266 light-years) whilst the B component is significantly nearer us (235 light-years) but is certainly regarded as physical by Dr. Andrei Tokovinin is his Multiple Star Catalog (MSC). The MSC notes that A has a period of 44.5 days whilst that of B is 3.15 days.

Bob Argyle - Double Star Section Director

March 2023 - Double Star of the Month

At the extreme eastern edge of Ursa Major, where it projects slightly into Leo Minor, there is a triangle of three naked-eye stars - 42 and 43 Lyn and a brighter but unlabelled star of Vmag 4.8 which turns out to be HD 82741. About a degree south of 42 Lyn is STF 1374 (09 41 21.88 +38 57 01.9) which is a long period visual binary.

Since being measured by F. G. W. Struve (275 degrees, 3".3) the pair has closed slightly with increasing angle and is now to be found at 312 degrees, 2".8. This motion is apparently sufficient to allow the calculation of an orbit with a period of 1815 years.

This is a relatively unequal pair with magnitudes of 7.3 and 8.7 given in the Washington Double Star (WDS) catalogue. In the neighbourhood is STF1369 (7.0, 8.0, 150 degrees, 25".1) which is just north of HD 82741. Another star of magnitude 8.4 lies 116" distant and the A component is the close pair COU 2084 with a period of 50 years and a current separation of 0".1.

SEE 115 (09 37 12.65 -53 40 06.6) lies in southern Vela near the border with Carina. It sits neatly at the centroid of the isosceles triangle formed by φ, L and κ Vel.

It was discovered by T. J. J. See, but in the 125 years which has elapsed since then the position angle has only increased by 8 degrees whist the separation remains at 0".7. This is a good test for a 20-cm aperture as the stars are magnitudes 6.1 and 6.3.

Although it has an entry in Gaia DR3, there is no information on either parallax or proper motion even though a relatively wide pair such as this might be expected to have been resolved satisfactorily by this stage of the satellite mission.

Just half a degree WNW is the open cluster NGC 2925 some 10'x10' in size and discovered by Sir John Herschel.

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

January 2023 - Double Star of the Month

STF 845 lies in Auriga (06 11 36.59 +48 42 39.6) and is also known as 41 Aurigae and it is also H 3 82, observed by William Herschel in November 1782 and recorded by him as a pretty double star. It is a beautiful pair for the small telescope. The writer first saw it in 1969 using the 12-inch reflector of a friend. At x208 the colours were noted as yellow and lilac. The Washington Double Star (WDS) catalogue gives the spectral types as A1V and A6V.

James Dunlop found the 30th entry in his catalogue in 1826. It is located in Pictor at 06 29 40.03 -50 14 20.7 some 2.5 degrees NNE of Canopus. It offers a fine sight to the small telescope user.

The stars are magnitudes 6.0 and 8.0 and at present they are separated by 11".5, being 14" apart when observed by Dunlop. The position angle has hardly changed between these two epochs and is currently 312 degrees.

In 1871, Russell, observing with an 11-inch refractor at Sydney Observatory, found the primary star to be a close double. R 65 has a period of 111 years according to Docobo and Ling in 2021. The stars are almost equally bright but according to the orbit, which is extremely eccentric, the separation never exceeds 0".7 and, at times, drops to 0".012 which was the case in mid-2021. By 2027 the stars will be at least 0".4 apart.

In the 1890s Harvard Observatory was site testing in the Peruvian Andes and was using a 13-inch refractor. This telescope discovered several hundred new double stars amongst which was the fainter companion of DUN 30. This has also turned out to be a binary pair of period 101 years, giving a predicted position of 224 degrees, 0".44 in early 2023. The stars have visual magnitudes 8.0 and 8.7.

Bob Argyle - Double Star Section Director