Double Star of the Month in Canes Venatici

• 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

• April 2019 - Double Star of the Month

  This month's pairs consist of a very easy pair in the southern hemisphere, and a much more difficult one in the north.

  When I used the 28-inch refractor at Herstmonceux in 1970, I observed STF 1606, an orbital pair in Canes Venatici (12 10 47.34 +39 53 29.5), which was found at a separation of 0".41. The pair then closed to 0".29 in 1990 and is now opening.

  For 2019.0 the position angle and separation will be 141 degrees and 0".6, so it should be just resolvable in 20-cm and I will look forward to seeing this pair as double for the first time in 49 years.

  It sits in a little group of three Struve pairs which also includes STF 1622 (see the column for April 2012) and STF 1624. The group is four degrees preceding and slightly south of beta CVn. STF 1606 is also practically coincident with NGC 4145, abarred spiral galaxy of V=11.3.

  In 2011, Shaya and Olling published a paper in Astronomical Journal in which they identified over 800 very wide pairs which they concluded were physically connected. From that list, number 588 (SHY 588) (12 02 39.44 -10 42 48.9) is a pair of stars with V magnitudes of 7.5 and 8.6. The current separation is 331" at PA 115 degrees.

  Gaia DR2 indicates that the brighter star has a distance of 177.6 light-years whilst the fainter is 186.8 light-years distant. The proper motions are similar but the difference in distance is supiciously large for them to constitute a binary, although Shaya and Olling used the Hipparcos data which actually suggests that the two stars are further apart in distance than does DR2. The telling factor may be the quoted error on the parallax of A which is seven times that on the parallax of B and suggests higher multiplicity.

  For the binocular user this is an easy pair and the field is enlivened by a V=8.5 star some 19 degrees and 262" distant from A. DR2 indicates that this is more than three times more distant than the SHY pair and therefore unrelated.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - April 2012

  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.

  2 CVn = STF1622 (12 16 07.55 +40 39 36.6) is a beautiful pair with components of contrasting hues which sits near the western border of Canes Venatici with Ursa Major and about 5 degrees following beta CVn. The primary star, an early M giant is accompanied by a late F dwarf and many writers have commented upon the colours to be seen here: - Webb called them very gold and blue, Dembowski thought them yellow and azure, Franks made them orange and blue and more recently Sissy Haas recalled brick red and silvery sapphire. The stars are magnitudes 5.9 and 8.7 and are currently separated by about 11".4 which makes them an easy target even for the small telescope.

  mu Crucis (12 54 35.66 -57 10 40.4) is simply one of the most beautiful doubles in the sky. A pair whose components of visual magnitude 3.9 and 5.0 share common proper motion and distance, this system belongs with the Scorpio OB2 association of young hot stars, and Hipparcos places both stars about 412 light years away. Shatsky and Tokovinin used the ADONIS near infrared adaptive optics system on the ESO 3.6-metre reflector to search for faint, close companions and they found two objects within 5" of component B. No magnitudes or proper motions are available so it is too early to say if these are physically connected but the two bright stars certainly form a very long period binary. Mu Crucis was found by Dunlop in 1826. The spectral types are both B and Hartung record them as both white whereas Richard Jaworski sees a tinge of yellow in the fainter star. The current separation of 35" and the brightness of the stars almost makes this a southern equivalent of Albireo but without the colours.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - April 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.

  The two stars in this month's column both have early A type primary stars and are each located around 125 light years from Earth but as objects for observation, one can be seen in stabilized binoculars, the other needs at least a 30-cm telescope.

  alpha CVn (12 56 01.67 +38 19 06.2) is the leader of the northern constellation of the Hunting Dogs and sits in an desolate part of the sky to the naked eye, below Ursa Major and above the faint coarse grouping of stars which form Coma Berenices. It was found by William Herschel and the magnitudes are 2.8 and 5.5. Hipparcos, which has measured both components, has had trouble with the distance to star B but even though the error in the trig. parallax is some 30% the distance still agrees with that of star A within the mutual errors. The clincher here is the proper motion of each star - around 0".2 annually and in the same direction. The separation has reduced from 22" in 1777 to around 19".1 today - slow enough to be used by the writer as a standard for micrometer calibration. Both stars are brilliant white and A is the prototype of the alpha2 CVn variables - it possesses lines of rare earths in its spectrum and the amplitude of variability is some 0.14 mag in a period of 5.5 days. The WDS states that both stars are spectroscopic binaries but neither appears in the 9th catalogue of SB orbits.

  gamma Cen (12 41 31.20 -48 57 35.6) can be found by extending the line between alpha and gamma Crucis by about the same distance again. In 1847 John Herschel recorded his observation of the star in sweep 553 - "A star 4m. which I am very much inclined to believe close double, but could not verify it owing to bad definition. Tried 320 but it will not bear that power". He noted that there was indisputable evidence of rapid orbital motion (5°.4 in just over one year to 1836.28). The stars then closed rapidly to about 0".13 over the next 11 years and reached their closest point again in 1933. With a period of 84.5 years these two almost identical A stars are closing rapidly again and at the time of writing can be found at 311°, 0".26 - wait another year and the distance will be 0".18.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - April 2009

  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.

  25 CVn = STF1768 (13 37 27.70 +36 17 41.4) is a beautiful, unequal pair situated about 11 degrees south of M51 in Ursa Major. It was missed by the elder Herschel and found by Struve at Dorpat in 1827. The eccentric nature of its apparent orbit(e = 0.80) was such that it disappeared from view to all observers between 1859 and 1876. The current orbit by Soderhjelm gives a period of 228 years and predicts a separation of 0.2 arc seconds for 1864. The star is currently just starting another run into periastron so it is well placed for observers with small telescopes. The magnitudes are 5.0 and 7.0 and the position for 2009 is 97° 1".7. The revised Hipparcos parallax is 16.45 mas putting it at a distance of 61 parsecs. The primary is an A7 subgiant and there are few colour estimates in the literature. Webb makes them white and blue, as does Chambers is his revision of Smyth. The original Bedford catalogue does not contain an entry for this star.

  beta Hya = HJ 4478 (11 52 54.56 -33 54 29.3) was found by the younger Herschel during one of his sweeps at the Cape in 1834. The star is given as both beta Crateris and beta Hydrae in Herschel's 1847 volume and beta Crateris in Chambers version of Smyths Bedford catalogue from 1881. This pair is now a challenge for the 20-cm telescope and it likely that it will be many years before it opens up again so it is worth making an effort to see it if possible whilst it is well-placed in the sky for the southern observer. Hartung gives the colours of both stars as pale yellow but the WDS lists the spectrum of the primary as B9III. The separation has decreased from 1".7 at discovery to about 0".7 now and the position angle is increasing. magnitudes are 4.7 and 5.5 and the distance is 95 parsecs.

  Bob Argyle - Double Star Section Director