April 2026 - Double Star of the Month

STF 1527 (11 18 59.91 +14 16 06.9) is a fine binary star located about 1.5 degrees south-east of theta Leo, and also just over a degree north of the galaxies M65 and M66.

In 1822 it was found at 10 degrees and 3".7 but it was closing, and by the end of 1990s it was well below 1 arc-second. I was able to measure it in 1998 with the Cambridge 8-inch at 0".9 and I have not managed to resolve it since. The stars approached to one-third of an arc-second in 2010 and at present are opening again with the 415 year orbit of Tokovinin predicting 325 degrees and 0".76 for mid-2026, making it once more a target for the Cooke refractor.

The stars have magnitudes 7.0 and 8.0 and are almost exactly 100 light-years away according to an earlier Gaia catalogue. When Gaia DR4 comes out at the end of this year we should get much more certainty in the value of the distance to each component.

Another fine sight is DUN 159 in Centaurus (14 22 37.52 -58 27 33.0). It can be found 3 degrees north of the midpoint of the line joining alpha and beta Centauri.

Ross Gould, observing from Australia with a 175-mm refractor finds that the colours are bright yellow and dull yellow and notes that the field contains some faint pairs and star groups. I measured both AB and a nearby magnitude 10.7 star (D) with the Johannesburg refractor in 2016. The bright pair has magnitudes of 5.0 and 7.6 and the PA was 157 degrees and 9".1, down from 161 degrees and 12".8 found by Dunlop in 1826. Star D is at 9 degrees and 43".6.

A has no data in Gaia DR3 presumably because it is a known spectroscopic binary but the B component lies at a distance of 250 light-years. The The Washington Double Star Catalog gives an additional component C also magnitude 10.7 at 254 degrees and 17".5 but this was not seen and neither this star nor C appear to be in the Gaia catalogue.

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.

March 2026 - Double Star of the Month

Sir William Herschel decribed 12 Lyncis (06 46 14.15 +59 26 30.1) when he observed it in 1780, and called it a curious treble star. He recorded colours of white and white inclining to rose colour for the two closest components, whilst the more distant 3rd star appeared to him to be pale red. In 1831 F. G. W. Struve catalogued 12 Lyn as STF 948 and measured separations of 1".5 between stars A and B and 8".7 between A and C.

Since then the close pair has moved through more than 80 degrees of position angle with only a slight increase in separation. During the same interval star C moved through 7 degrees of position angle. AB are at postion angle (PA) 64 degrees and 1".9 for the current date. This is a physical triple system with the period of AB given as 733 years in an orbit calculated by Izmailov in 2019. Measurements with the Cambridge 8-inch Cooke showed that the PA decreased by 8 degrees between 1995 and 2017.

A drawing by Jeremy Perez shows colours of yellow for A and C with B being bluish. These can be compared to the estimates by Reverend Webb who on three occasions between 1832 and 1852 noted the hues as white, ruddy and bluish, but in 1854 noted C to be ruddy. Sissy Haas record colours of yellow, yellow and bluish silver in her book Double Stars For Small Telescopes, and labels it a showcase system which no-one would argue with.

Five-and-a-half degrees south of Beta (β) Carinae (V = 1.7) lies HJ 4206 (09 17 25.21 -74 53 39.8). It was found by John Herschel during sweep 547 with the 20-foot reflector at Feldhausen on Feb. 22, 1835. He decribed the system as delicate and beautiful and noted that the primary was accompanied by a magnitude 13 at 5" and a magnitude 11 at 40".

The Washington Double Star Catalog gives the magnitudes of B and C as 9.6 and 10.3, so not only considerably brighter but also reversing the order of brightness. The corresponding positions are 344 degrees and 7".1, and 354 degrees and 46".

In late 1894, Robert Innes, using a 6.25-inch Cooke refractor borrowed from Walter Gale, noted that the primary was a very close pair (I 12). In fact, he registered only an elongation on the first occasion he looked at it, and then the star was single in good seeing a month or so later. Recent measures put the stars 0".3 apart with little apparent motion. The magnitudes are 5.3 and 6.6.

Bob Argyle - Double Star Section Director

February 2026 - Double Star of the Month

About 10 degrees east of beta Aurigae lies a coarse cluster of mostly naked eye stars all bearing the label ψ (psi). The Cambridge Double Star Atlas (2nd edition) shows that ψ², ψ⁵ and ψ⁷ are all double but they do not appear in the catalogue, presumably because they are all too wide. Nevertheless, 56 Aur = ψ⁵ Aur (06 46 44.34 +44 34 37.3), is worth looking at because it has a fine colour contrast.

It was found by William Herschel in 1782 when the position angle (PA) and separation were 17 degrees and 53". and he noted colours of white and pale red. By 2022 the values of PA and separation were 44 degrees and 28" due mostly to a proper motion, largely in declination, of the A star. In 1970 I found the colours to be yellow-orange and blue with a 21-cm mirror.

Pi Puppis (07 17 08.56 -37 05 51.0) is a wide, unequal double star found by James Dunlop at Parramatta in New South Wales. It is about 8 degrees south of the blue supergiant eta CMa (V = 2.5). The primary is a very bright (V = 2.9) K4 giant whilst there is a V = 7.9 companion at 213 degrees and 69" with this separation slowly decreasing with time.

Gaia DR3 gives distances of 560 and 970 light-years respectively for A and B but the parallax measured for has a 20% error and this is because the Hipparcos mission in the early 1990s found that A itself was a double star with a V = 6.9 companion 0".7 distant. Since discovery this star has moved just 3 degrees but it does not appear in DR3.

Pi Puppis lies in a cluster called Collinder 135 which has six members brighter than magnitude 6. It might have had a common origin with a nearby cluster UBC 7. The centres of the clusters are currently 24 pc apart but the stellar motions indicate they would have been closer in the past.

Bob Argyle - Double Star Section Director

January 2026 - Double Star of the Month

The Hyades moving group contains stars which are not immediately proximate to the open cluster on the sky. The most famous is perhaps Capella, a bright spectroscopic binary with a 104 day period which is orbited by a binary pair of red dwarfs. Another member is ENG 22 (05 41 20.33 +53 28 52.7) which can be found 2.5 degrees WSW of delta Aurigae (V = 3.7).

It was catalogued by Rudolf Engelmann but recent measures by the Gaia satellite have revealed the parallax and proper motion of both components. The pair consists of stars of V magnitudes 6.0 and 8.9 which lie at a mean distance of 40.0 light-years and which are moving across the sky at 0".5 per year. The separation and position angle have remained fixed at 98" and 72 degrees so this is probably a binary system. The primary is a K dwarf and the companion has spectral type M0.

Adhara = epsilon CMa (06 58 37.55 -28 58 19.5), at V = 1.5, is the second brightest star in the constellation, but 4 million years ago, it was the brightest star in the sky reaching magnitude -4.

In the nineteenth century a visual companion was seen at the Cape Observatory, some six magnitudes fainter and 7".5 distant in position angle 167 degrees. Ernst Hartung noted that the the pair can be seen in 75-mm aperture, and that the primary is brilliant white whilst the companion is deep yellow.

The brightness of the primary is probably the reason why the error in the parallax found by Hipparcos is significantly larger than usual. Gaia has no results for this star but it is possible that the brightest stars in the sky will be investigated towards the end of the project. The satellite was switched off on March 27, 2025 and work on the Gaia DR4 catalogue is now in progress and the results are expected towards the end of next year.

Bob Argyle - Double Star Section Director