April 2024 - Double Star of the Month

STF 1645 (12 28 04.45 +44 47 39.5) is a fine pair in Canes Venatici found 3.5 degrees WNW of beta CVn (chara) and also 3/4 degrees north of the galaxy NGC 4449.

T. W. Webb, in Celestial Objects for Common Telescopes reports an observation by Bird, a Victorian amateur astronomer, as a lovely pair as I ever saw. Webb also calls them yellow which chimes with the spectral types of F9V and KV given in the Washington Double Star catalog (WDS). In 1972 using a 10-in (25-cm) Newtonian I made the colours yellow and blue, whilst Sissy Haas using a 60-mm refractor finds both are peach-white.

The stars have visual magnitudes of 7.5 and 8.1 and they are currently at 156 degrees and 9".8, but the stars were 16" apart when found by William Herschel in 1791. Nevertheless they both appear to be 147.7 light-years from the Earth and are moving almost directly south on the celestial sphere at the rate of 0".2 per year.

Two degrees north of iota Librae is a coarse triangle of stars. The most north-westerly of this trio is SHJ 195 (15 14 28.13 -18 25 42.7) which again, despite the catalogue name, was discovered by William Herschel.

Again this is a pair of F5 stars whose visual magnitudes are given as 7.5 and 8.1 in the WDS, which Admiral Smyth described as white and bluish. Although low in the Cambridge sky, I made a pair of measures in 2009 with the result 140.3 degrees, 45".59.

Gaia EDR3 records parallaxes that are the same within the quoted errors, although the error of the bright star's parallax is some ten times larger than the typical value for that magnitude. The proper motions are also very similar - the stars are moving at 0".1 per year.

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 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

February 2024 - Double Star of the Month

S 548 (07 27 40.54 +22 08 29.3) was noted by William Herschel in 1782 and catalogued by him as H V 66. He noted that the stars were very unequal and the larger one was pale red and the smaller one was dusky. The distance was noted as 34" 39'". Little has changed in the relative positions since then with the Washington Double Star catalog (WDS) giving 277 degrees and 35".3 for 2019.

The pair can be found two degrees following delta Gem (see this column for Feb 13). The catalogue magnitudes are 7.0 and 8.9. In 1892 Thomas Espin added a faint companion of magnitude 12.4 at 24 degrees and 11".9. The three stars are unrelated. A lies 1800 light-years away, B is 940 and C is 8350 light-years distant.

In southern Puppis right on the border with Vela is HJ 4093 (08 26 17.74 -39 03 32.3). This is a fine pair for the small aperture, it is triple in apertures of 40-cm or more, whilst the spectrograph reveals that component A is an Algol system called NO Pup.

Stars A and B (magnitudes 6.5 and 7.1) are currently at PA 122 degrees and 15".0 having almost doubled their separation since discovery by John Herschel. Using the 26.5-inch refractor at Johannesburg, Willem van den Bos found that B is a close double (Ba,Bb), with stars of magnitude 7.9 and 8.1, currently 0".2 apart and a binary with a period of 103 years. In addition the WDS notes that Andrei Tokovinin finds a close component (D) only 5".4 from A but with a K magnitude of 17.3, but this star does not appear in the Multiple Star Catalogue.

Examining Gaia DR3 with a field of radius 100" shows two stars of magnitudes 12.2 and 13.1 respectively 64" and 66" from A which share the same parallax, so perhaps this is a small cluster. There is, however, no data for the B component and the significant change in the AB distance since discovery may indicate that B is unrelated.

Bob Argyle - Double Star Section Director

January 2024 - Double Star of the Month

Admiral Smyth's Bedford Catalogue contains an entry for 124 Tauri, and from his description of it as a coarse quadruple star of which components B and C are 6".2 apart in PA 318 degrees this appears to agree with Struve's catalogue entry of STF 755 (05 39 09.17 +23 17 17.7).

According to the Washington Double Star catalog (WDS) A is magnitude 7.8 at 31 degrees and 149". This is HD 37387, a K2 giant star which appears close to the reflection nebula GN 05.36.2.The fourth star, D, is 50 degrees and 95" from A and has V = 11.1.

There is no label for 124 Tauri in the the Cambridge Double Star Atlas (2nd edition) but the star appears about 30' W of a small right-angled triangle of 6th magnitude stars which in turn sits 2 degrees NNE of zeta Tauri. Smyth happened upon it in 1835 whilst looking for Comet Halley. He gives the colours as garnet (A), pale blue-white (B,C) and bluish (D).

Some 3.5 degrees WSW of epsilon CMa is a triangle of 5th and 6th magnitude stars, the brightest of which is 10 CMa. In the same field, north following is HJ 3891 (06 45 31.20, -30 56 56.3), a double star discovered by John Herschel from the Cape of Good Hope.

The primary is a B2III star of V = 5.7, and is accompanied by a 8.2 magnitude star at 223 degrees and 5".0. There has been little change since 1838. The stars have similar parallaxes and the mean distance to the system is 1968 light-years. The primary star is also called HP CMa.

Just 90 arc-minutes to the SE is a wide pair swept up by the elder Herschel in 1782. H V 108 has stars of similar brightness (5.8 and 7.7) but they are separated by 43" in PA 66 degrees. The A component has a particularly close and faint companion just 0".6 away which was discovered from Robert Rossiter in South Africa, whilst the WDS notes that the B star has a variable radial velocity. Andrei Tokovinin regards this as a physical quadruple - the brighter stars appear in Gaia DR3 with respective distances of 634 and 623 light-years.

Bob Argyle - Double Star Section Director