Double Star of the Month in Pegasus

• September 2023 - Double Star of the Month

  Four degrees north and slightly east of beta Pegasi is the Herschel pair H N 11 (23 07 27.73 +32 49 3.3) but now known in modern atlases and handbooks as STF 2978. It was found by Herschel on Sep 6th, 1784 and included in his third and last double star compilation. This list was not published until 1821 when it appeared in the first volume of Monthly Notices of the RAS. This is an easy and pretty pair with components having magnitudes 6.4 and 7.5.

  

  Over the last two centuries there has been but little motion in position angle and separation. A recent measure in 2018 put the stars at 145 degrees and 8".3. Gaia DR3 gives precise distances for the two stars, 450.31 and 453.31 for A and B respectively. Star A is an Algol variable with an amplitude of 0.22 magnitudes and is known as V343 Peg.

  HWE 91 in Piscis Austrinus (22 55 56.89 -32 32 2.9) was also independently discovered by two astronomers. The name represents Herbert Howe, astronomer at Cincinnati Observatory who used an 11-inch Clark refractor on a survey for new double stars. In autumn 1877, using a magnification of x230, he examined the star delta PsA and found a very faint and relatively close companion and estimated the magnitudes as 5.0 and 10.5. Two years later and unaware of Howe's finding, S. W. Burnham came across the star using his 6-inch Clark refractor but observing this time from Mount Hamilton in California. He called it BU 772 but later conceded that Howe had priority of discovery.

  

  The current position is 250 degrees and 4".9 but there is little motion in this system. The stars are equally distant from us within the errors of the parallaxes and they have similar proper motions. Gaia DR3 also indicates that the stars have G magnitudes of 3.94 and 9.80. Hwe 91 is easily found lying as it does just 3 degrees south of Fomalhaut.

  Less than one degree WSW from Hwe 91 is HJ 5367 (also gamma PsA) which presents a similar test. Here the stars are magnitudes 4.5 and 8.2 and they are currently 4".1 apart in PA 255 although this has increased from 2".5 when found by John Herschel in 1836.

  Bob Argyle - Double Star Section Director

• October 2020 - Double Star of the Month

  In this column exactly 10 years ago, the fine binary star 72 Peg was described. It remains a severe test for 20-cm aperture but as the stars are almost equally bright at V=6, so separating the components, which are now 0".59 apart, is made somewhat easier.

  About 2 degrees to the north-east is another Burnham discovery, BU 858 (23 41 17.6 +32 33 40), somewhat wider than BU 720 (BU 858 is 0".8 at present) but with components whose V magnitudes are 7.8 and 8.8 this is an equally challenging pair.

  

  Since discovery in 1881 the companion has moved almost 60 degrees in position angle, but there is no orbit as yet. A magnitude 12.9 star at 23" is being left behind by AB and was also found by Burnham (BU 389) but earlier in his career and with the famous 6-inch refractor. Gaia DR2 does not give data on A and B but quotes a parallax for both stars of 6.66 mas, equivalent to 490 light-years. The faint star is much more distant and clearly optical.

  The second edition of the Cambridge Double Star Atlas notes that tau Aqr is preceded, about 45 minutes to the south-west, by a somewhat fainter but still naked-eye star called 69 Aqr (22 47 42.7 -14 03 23). The SIMBAD catalogue also lists alternative names for tau and 69 as tau 2 and tau 1. Tau is unmistakeable - it a magnitude 4.0 star with a distinctly reddish hue - an MO giant in fact.

  

  The closer pair, of which 69 Aqr is the brighter component, are separated by about 20" but this distance has been closing since 1781 when William Herschel found the stars 35".8 apart. Measures I made in 1994 and 2005 show the distance decreasing over that time interval. This is purely down to difference in proper motions as the stars are at significantly different distances. The pair is also known as STF 2943. The WDS notes that A is also a spectroscopic binary and a putative faint and distant companion found in 1918 does not exist. Herschel noted colours of reddish-white and dusky. A is a hot blue dwarf of spectral type B9 and the companion has a somewhat later spectral type, but Simbad does not elaborate.

  Bob Argyle - Double Star Section Director

• September 2015 - Double Star of the Month

  Nestling in the region about 4 degrees north following the upper left-hand corner of the Square of Pegasus, AC 1 (HIP1669) (00 20 54.10 +32 58 40.9) seems to have been a little neglected by the double star community. It doesn't even have a note in the WDS catalogue but things may change now that Henry Zirm has published an orbit for it.

  It was discovered by Alvan Clark using a 7.125-inch object glass of his own make in October 1856, and published in MNRAS a year later with additional notes by W. R. Dawes. At the time of discovery it was 0".4 apart and Dawes was of the opinion that it escaped the attention of F. G. W. Struve at Pulkovo because it was too close at that time.

  This view has been borne out - the stars are now easily measurable with a 20-cm refractor. The magnitudes are 7.3 and 8.3. Hartung notes that the stars are 'deep-yellow' and also points out the presence of a orange-red star some 4'.5 SW (HR 59 - spectral type K5III). Zirm's orbit with a projected period of 525 years is clearly provisional as the stars are close to maximum separation and only 12 degrees has been described in the apparent orbit. The author found 289°s; and 1".9 in autumn 2014.

  Psi 1, 2, and 3 Aquarii are three bright stars of visual magnitude 4.2, 4.4 and 5.0 respectively. They can be found about 3 degrees east of the centre of the line joining alpha Peg to Fomalhaut, in a region rather low down from the UK but filled with interesting visual doubles.

  Some 4 degrees directly below psi 3 is 94 Aquarii (23 19 06.51 -13 27 30.4) a fine, wide pair which is worth seeking out. Its mag 5.3 and 7.0 stars appeared yellow and orange to Hartung who was able to observe them close to the zenith whilst the Reverend Webb noted a reddish glare in A whilst B appeared greenish.

  The proper motion of the pair amounts to more than 0".3 per year so the fact that the change recorded in separation over 200 years amounts to only 2 arc seconds tells us that this is a physical pair, and it is located 69 light years away. In 1976 McAlister and colleagues discovered that B was a close pair (MCA 74) and it has subsequently turned out to be a binary of short period. It rotates every 6.3 years and the separation is never more than 0".2.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - September 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.

  kappa Pegasi (21 44 36.4 +35 47 39.5) is a famous visual binary, having been discovered by Burnham in 1880 using the 18.5-inch refractor at Dearborn. Apart from observations by the Greenwich double star observers around the turn of the century, and later at Herstmonceux and Christopher Taylor with his 12.5-inch it has eluded attempts at resolution from the UK. The separation barely exceeds 0".2 at any point in the 11 year orbit. Of more interest to the smaller aperture user is the Struve companion (STF 2824) which sits about 14" distant at PA 288. The writer has measured it with a 20-cm OG but, it has to be said, with some difficulty, owing to the faintness of B. The WDS gives 10.8 with the primary at V=4.1.

  pi Gruis (22 22 44.2 -45 56 53) would be a naked-eye pair if it were a little brighter. There are 2 stars of mags 6.5 (pi 1) and 5.6 (pi 2). Both are double stars in each case with companions about 4 magnitudes fainter and 3 to 5 arc seconds apart and both were discovered by Robert Innes. The easier of the two is pi1 (I 135) which also has the distinction of being a member of the rare S class of stars and varies between visual mag. 5.8 and 6.4. Hartung notes that the primary is bright orange. Whilst the binary nature of pi1 is not yet clear, pi 2 (I 382) seems to be physically connected. For the galaxy observer the edge-on spiral IC 5201 is about 15' SW of pi 2.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - October 2010

  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 pairs featured this month are both close but bright visual binaries which afford excellent tests of optics and seeing for the possessors of 20 to 25-cm aperture.

  72 Peg (23 33 57.19 +31 19 31.0) is equidistant from the top two stars of the square of Pegasus, alpha and beta, and about 3 degrees above the line joining them. Discovered in 1878 with the 18.5-inch refractor at Dearborn by Burnham, this yellow pair has slowly widened with increasing position angle. At the present the companion can be found at PA 105 degrees and separation 0".53. This is the maximum separation which this system will attain but the period of 260 years means that it will be some years before it appears to be significantly closer. The magnitude of the two stars, 5.7 and 6.1, and the declination of +31 means that in the autumn this pair is high in the mid-northern sky and thus provides an excellent resolution test.

  Lambda 1 Scl (00 42 42.89 -38 27 48.6) is a similar system to 72 Peg although somewhat fainter (6.7 and 7.0) and rather wider at the present time (21°, 0".7) having widened somewhat since discovery by Harvard College Observers in Peru in 1901. The primary is A0V and therefore appears white to the observer. The brighter star lambda 2 sits about 18 arc minutes following lambda 1 and provides the binocular observer with a fine sight. The parallaxes of both stars as determined by Hipparcos seem to rule out a physical connection.

  Bob Argyle - Double Star Section Director