Double Star of the Month in Hydra

• February 2019 - Double Star of the Month

  Epsilon Hya (08 46 46.51 +06 25 07.7) is one of the most observed double stars in the catalogue. According to the Washington Double Star Catalog (WDS) it has been measured 432 times since it was discovered by Wilhelm Struve in 1825. It seems to have escaped the attention of William Herschel although it would have been within the capability of his telescope.

  Since 1825 the companion, known as C, has moved 120 degrees in position angle with little change in separation. A measurement at Cambridge in 2017 showed it at 309 degrees and 2".93. It should be resolvable in 10-cm; the stars have visual magnitudes of 3.8 and 7.8.

  In 1860 Otto Struve, using the 15-inch refractor at Pulkovo suspected that the primary star was elongated, an impression he received again in 1864. In April 1888 Giovanni Schiaparelli, observing with the 15-inch refractor at Milan, noted a clear elongation and subsequent follow-up observations allowed him to say that the primary star was a close binary of short period.

  Eight revolutions have been traced out since discovery and the period of AB is close to 15.05 years. The stars are never wider than 0".27 and at the start of 2019 they are 0".22 apart and closing.

  A more distant star D (V = 12.5) at 210 degrees, 18" is also physical, and C is a spectroscopic binary of period 9.9 days meaning this is a quintuple system.

  The Struve pair is a fine sight on a good night - the stars are given as yellow and purple by Smyth but I saw them more as yellow and light blue.

  In the visually barren but telescopically interesting area between Sirius and Procyon there are a number of fine double stars and clusters.

  About 5 degrees west and a little north of 5 Pup (see the column for February 2018) is STF 1097 (07 27 56.66 -11 33 24.7), an easy 6.3 and 8.2 magnitude pair with colours of yellow and bluish.

  I came across it in Spring of last year and obtained 311 degrees and 20".8. I did not see the close companion to A that Dembowski had suspected in 1865, and Burnham confirmed nine years later with his 6-inch Clark. It should be visible in 20-cm although the low altitude of the star would have been a factor.

  BU 332, as it is known, is currently at 0".7 and may be closing; the stars are magnitudes 6.2 and 7.4. There are two faint comites. D is 9.7 at 157 degrees 23" (distance decreasing) and E is 12.4 at 43 degrees and 32".

  Espin noted that A varied between 6 and 6.8 with a period of 14 days, whilst Otero, more recently, suggests that it is the Burnham component which is likely varying by around 0.6 magnitude to produce the small observed variation in AB (0.13 magnitude) found by Hipparcos.

  Bob Argyle - Double Star Section Director

• March 2016 - Double Star of the Month

  STF 485/STF 484 (04 07 51.38 +62 19 45.4) are really just part of the open cluster NGC 1502 (H VII 47).

  

  For the small telescope user this is a pretty sight but its appears to be a real headache for the double star cataloguers and there is an extensive entry in the WDS catalogue notes trying to sort out which stars are which, and who discovered what.

  The brightest pair is STF 485 AE which consists of stars of mags 6.91 and 6.94 separated by 17". There are a further 5 comites of magnitude 10 or brighter within 140" of star A. Two of these, mags 9.63 and 9.81, form STF 484 at 336° and 22".8 (visible on the drawing to the east of the cluster centre). Recently, Andrè Debackére measured a new component in one of the pairs in this group (see DSSC 24 on this website).

  For the astrophysicist, the most interesting star is E. This turns out to be a multiple star consisting of a 2.69 day Algol system with a third star (but also a close binary) 0".1 distant circling the two every 54 years. The total mass in this system is more than 50 suns. The GCVS gives the name SZ Cam to the variable but ironically the distance derived from the dynamics of the 54-year system place the stars over 300 pc further than the accepted distance of 800 pcs to the cluster. There is little colour in this grouping as many of the components are hot early B stars.

  15 Hya (08 51 34.44 -07 10 38.0) is an object which I have not yet observed but which in terms of separation and magnitude difference looks like a tempting target.

  The star first entered the double star catalogues as H V 120 when William Herschel noted on 1782 Dec 28 that it appeared Extremely unequal with the brighter star white and the comes red whilst the separation was 43 arc seconds.

  The WDS gives magnitude 11.4 for the fainter star but relegates it to component C as in 1878 S. W. Burnham found that the primary was a close double (0".47) whose components he estimated had magnitudes 5.7 and 7.2. Since then the close pair has widened significantly and is now more than 1" apart so it should be visible in 15-cm in the UK; the seeing needs to be good as the declination counts against it.

  Burnham adds a further faint star of mag 12.1 at 55". AB is clearly a long period binary and the position angle has decreased to 121 degrees in 2003 although there do not appear to have been any measures since then.

  Bob Argyle - Double Star Section Director

  We have had an observation submitted for STF 485. If you have observed this double star – or the cluster it is in – please let us know.

• Double Star of the Month - April 2013

  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.

  Near the centre of the coarse Coma star cluster is STF1639 (12 24 26.81 +25 34 56.7) which forms an equilateral triangle with 12 and 13 Com and is the faintest of the three. It is a binary with a highly eccentric orbit (e = 0.93) and which is now resolvable in 10-cm aperture given a night of reasonable seeing. The stars are magnitudes 6.7 and 7.8 and appear to be dwarfs of spectral types A7 and F4. It was discovered by Struve and in 1827 the separation was 1".2. This decreased steadily until the 1890s when the pair was unresolvable in even the largest apertures. By the end of that decade it was again measureable and since then has increased in separation to 1".8 at the time of writing. The orbit currently in the USNO 6th Orbital Catalogue gives a period of 575 years which gives a distance of 0".09 for 1892 and the maximum is reached around 2175 when the stars will be 2".35 apart. The 10-cm telescope might also make out a distant third star of mag 11.5, 92" away in PA 160.

  Modern star atlases show the star N Hya (11 32 16.90 -29 15 39.7) firmly in Hydra and some 3 degrees north of the mag 3.5 xi Hya - itself a very unequal and wide double star with a 10.7 mag companion at 68", the distance of which is increasing. The WDS shows no measures of this pair since 1928. William Herschel found N Hya on 1783 Jan 10 and it is number 96 in his third category. He called it 17 Crateris and noted that both stars were reddish white. A few years later it appeared in Piazzi's Catalogue as 17 Hya. Modern telescopes tend to yellow tints - Hartung gives both yellow and Sissy Haas called them grapefruit colour. The stars are almost identical - both are spectral type F8V - and have changed little in separation and position angle over the last 200 years. The current position is 210° and 9".4. This is a nearby system which is certainly binary - the proper motion is significant.

  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