SELECTED SOUTHERN DOUBLES and VARIABLES
R.A. 14 Hours

β Cen / Beta Centauri / Hadar / Agena / VOU 31 / HIP 68702 / SAO 252582 / PPM 360515 / HD 122451 (14038-6022) is the 11th brightest naked-eye star in the sky. Its position marks one of the so-called “Southern Pointers”. It remains the bright star on the Australian National Flag, and lies between Alpha Centauri and bright star of the Southern Cross. In earlier times,these stars were also associated with Argo Navis — likely because of the general proximity to the horizon. The southern declination meant that two thousand years ago, Hadar appeared some 5° above the horizon from the Egyptian city of Cairo. Chinese observers once called this star the “horse’s belly”. This star today has two common names in use are either Hadar or Agena. Equal usage of these names in the literature have existed for the last hundred years or so. Lately, Hadar is becoming more common. Hadar also in Arabic means Ground, as used with a stone bowl with a mortar and pestle used to reduce grain husks into flour. (Arabic observers also give Alpha Centauri name as Wazn, the tool also known as the Weight.) This name common proper name was created by the American popularist Elijah H. Burritt who named this star Agena. It appears to have come into usage around 1880, but according to Richard Allen the meaning of the name is “unknown”.

Beta Centauri is a noted double star, whose duplicity was first discovered by J.G. Voute in 1935. Listed as VOU 31, the current separation is 1.3″ showing the respective magnitudes of 0.9v and 4.1v. (Later, some sources like the Washington Double Star Catalogue (WDS96) says the magnitudes are the brighter 0.7v and 3.7v. I could not find this later source.) Separation has remained fix since discovery, though the position angle has decreased slightly from 259° to 247° in the last sixty-five odd years, suggesting physical connection. If this is true, the period is likely lengthy. This pair will remain interesting to watch in the 21st Century.

From the observed data, the brighter component contributes most of the starlight. The temperature for β Centauri A is 25 800K and whose diameter is 14R⊚ or 19.5 million kilometres across. Absolute magnitude is estimated to be about -5.1 being about 9 500 times brighter than the Sun. β Centauri is a blue B1III giant which is placed at the very top of the stellar Main Sequence. Recent photometric observations show β Centauri is the brightest example of the short-period variables types known as Beta Cephei’s or BCEP. For this star the mean magnitude is 0.61V and this varies by the smallish 0.045 mag over the regular period of 0.157 days (3h 46m). Little knowledge is known of the nature of the companion.

VOU 31 remains challenging for moderate apertures. Hartung claims to have seen this with 20cm aperture using a neutral density filters — but I personally find this hard to believe. I would have assumed that Hartung could better use aperture stops — a most useful device for amateurs know. This device allows the extinction of pair separations or limiting magnitudes by reducing the telescopic aperture. By using several cardboard circular diameter holes — simply made to correspond to various common telescope apertures, an observer may estimate the apparent resolution of the pair in question. Although this can be made theoretically, it is extremely useful to have an observation to back-up this up. It is also a device whose basis is for the variable star extinction aperture, which functions like a gigantic camera iris for telescopes. Aperture stops are quite simple devices that are very useful to have on hand, especially if you are writing observational text like in these southern pairs’ web pages! I can also be applied to determine the minimum aperture to see a particular observed object, such as, galaxies and other nebulous deep-sky objects.

I have tried separating the pair, even under perfect seeing conditions, more than a dozen times with 30cm, and still could not resolve it. I did glimpse this pair at the ASNSWI observing site at Bowen Mountain way back in April 1981 using 30cm. It was really tough though!

The only time I can say I really saw it cleanly separated was by using the 40cm Cassegrain at the now closed Black Birch Observatory in the South Island of New Zealand, and that was during the well deserved coffee break being taken by the other observers! Another 10.5 magnitude star appears 2.5′E, which is just visible in 15cm.

In distance, some references still quote values as low as 123pc (400ly), but the Hipparcos satellite data firmly places its distance as 161±13pc or 525±43ly, which is based on the parallax of 6.21±0.56mas.

R Cen / HIP 69754 / SAO 241580/ HD 124601 / PPM 3424701 (14166-5954) is often one of the first variables that southern observers start for their variable star observing programme. Its position is easy to find, as R Cen is roughly just slightly north of the imaginary line between α and β Centauri, but more implicitly is closer to β Centauri — some 1.6° to the NEE (PA 74°). R Cen is contained in a quite starry field. The deep blood red colour of this star is remarkable, reminding me of X TrA or even EsB 365 next to β Crucis. Throughout the near predictable 550-odd day period, the star changes colour with magnitude being reflected in the M4e to M8IIIe emission spectra. AOST2 says that;

“…near maximum is a fine red star… while near minimum looks crimson.”

I playfully experimented with an O-III filter, and found the star had simply disappear from view. The prism image, under steady conditions, shows many dark lines with one or two brighter ones - at either side of the observed spectrum.

According to the 4th edition of the General Variable Star Catalogue (GVSC4), R Cen is a Mira variable with the visual magnitude varies between 5.3v and 11.8v magnitude in the period between successive minima of 546.2 days. Earlier references often quote the earlier period of 531.0 days, and this was found almost fifty years ago. The observed light curve is quite different from many other Miras, as it also shows another less deep light curve some 160 days from the deep minima. After this 0.4 drop in brightness, the light then again increases by another 1.4 mag. in about 100 days. Speculation by the theorists thinks this behaviour is similar another variable star class — the RV Tauri’s. These appear as “semi-regular Cepheids”, having the characteristics of a consistent period with irregularly bright maxims and minims. The cause of these two periods is uncertain. Russian theorist Tsesevitch in 1955, proposed that these dual pulsations are caused by shock waves emanating from the star which appears to speed up and slow down as the wave moves through the extended outer thin atmosphere of the outer layers. In turn, this pressure varies the emission spectral lines, that are seen to change over time.

For distance, some references still quote values as low as 123pc. (400ly.), but the Hipparcos satellite data places it at 161±13pc. or 525±43ly. (π is 6.21±0.56 mas.) away from us. During the collection of the Hipparcos observations, the magnitude found was 7.18 with the inaccurate measured parallax of 1.56 mas, giving the distance of 640pc. or 2100 ly.

NOTE: An example of R Cen (or 1409-59) light curve on this star either at * Ref 1 or * Ref 2*. Although the latter reference is in Spanish, the text can be translated easily, and the general gist can be worked out with a little patience. Similar information can be found other southern variables like R Car, L₂ Pup and S Car. When I looked up the observations of R Cen the page told me that some 209 observers had contributed the total of 5983 observations! If you wish to see the power of visual observations — just looking at some of these light curves, show this as 5.2v to 11.5v between JD 2440000 and 2453000. In this time, the amplitude of the light curve shows a gradual decrease over time, where the range is more like 6.0v to 9.5v. In the beginning of the light-curve there are deep minima followed by a shallow one and next one again deep. The mechanism seems to be regularly switching on and off that is no doubt something to do with the internal workings inside the star.

Δ159 (14226-5827) is a bright easy double star 3°NE of Beta Centauri (PA 54°), and forming the apex of a naked-eye equilateral triangle with Alpha and Beta Centauri. To me this pair looks like a fainter version of Alpha Centauri, with similar colours and spectral classes. AOST2 says the colours are yellow and white, though I see them as yellow and deep yellow - similar to the G8 III and F5 spectral class. For me, this was the third double star I ever observed, behind Alpha Centauri and Alpha Crucis, with more than twenty different observations in the 1980s alone! The Washington Double Star Catalogue gives the respective magnitudes as 4.9 and 7.0. Since discovered by James Dunlop in 1826, the separation has changed from 9.5 to 9.2 arcsec while the position angle has similarly reduced from 161° to 159°. To me the separation seems less than 9.2 arcsec. The field is marked by another 7.6 mag star (HIP 70228/ HD 125545/ SAO 241669) in the field, seen using medium power 11′ nf.

Some complications exist in the catalogues, and I spent sometimes trying to sort out the mess in the magnitudes and positions. The Guide Star Catalogue (GSC) lists 159 as 5.0 mag, while the Tycho catalogue (T8690:3220:1) inexplicably states the magnitude is 6.91. The Hipparcos Catalogue gives Δ159 as a separate star - HIP 70264, which correspond well with the star SAO 241673/ HD 125628/ PPM 342793 (14h 22m 37.120s -58° 27′ 33.00″) HIP 70264 is stated to have has the visual magnitude of 4.76, B-V of 0.795±0.003, while the distance is 86 pc. or 280 ly. (π=11.56±3.09 mas.) and an usual combined spectra of “G8/K1 +F/G”. A second ‘stellar’ measure is given with the parallax of 0.70 mas and the B-V being 0.455. I can only assume this refers to the companion star. Megastar 4.0 also shows a whole collection of 15th to 16th magnitude stars surrounding Δ59 to about 2.5′. These are obviously artefacts from the measuring machine during the production of the GSC Catalogue.

TDS 9199 (14258-5449). While writing this text, I found that the latest WDS03 gives a new TDS (Tycho Double Star) pair in Lupus. Magnitudes are given as 11.39V and 12.28V and this roughly matches the previous combined magnitude of 11.4v. In 1991, the stellar positions were 2.2 arcsec along PA 224°. I have not seen this star but have identified it using the accurate position of 14h 25m 46.0s -54° 48′ 39″. It is the next star some 58 arcsec SW (PA 233°) from HJ 4675. The stars here might be difficult to see in 20cm because of the brightness of the stars — especially as fainter pairs are sometimes notoriously difficult to split. I suspect 20cm should see this duo but 30cm might be better.

HJ 4675 (14259-5448). What lured me to NGC 5593 in Lupus was that the brightest component is the double HJ 4675 near the centre of the cluster, whose components measure 10.0 and 11.0 magnitude. Although the pair was discovered by John Herschel, his measures were omitted for several years because his positions of 5.0 arcsec and PA being did not seem to reconcile with later data from Russell and Innes data. In the WDS01 the measures of Russell and Innes are given as the first and last positions within the period of (23) twenty-three years. Last measures were produced in 1915 and were given as 8.2 arcsec and 338°. To me these positions look similar today (2003). The Hipparcos/ Tycho results somehow did not resolve the stars. HJ 4675 was easy to resolve in 20cm, and suspect even 7.5cm could see them with care.

NGC 5593 (14259-5447) (U430) is an open cluster in Lupus 1.8°;W from Δ163. This cluster is a bit odd because it looks just like a small collection of eleven (11) 10th-11th magnitude stars in a line some 7 arcmin in length. Throughout the cluster there seems about six, maybe seven, 14th magnitude stars. There seems no grading of magnitudes in the cluster itself. In 20cm I had much trouble identifying the field.

Catalogues class NGC 5593 as “ III 2 p” and lists about twenty stars. Except for the nearby presence of the pair, this is a merger example of an open cluster.

V745 Cen (14272-6204) is an eclipsing binary lying 45'N and 10′W of the nearest star to the Sun Proxima Centauri. This 8.1 magnitude star is some 4′S of the variable’s position identify it easily, with the field containing a few much fainter stars. V745 Cen varies in the period of 3.0251 days and the light-curve changes between 9.3 and 10.3v magnitude. Readers should also note that Burnham’s Celestial Handbook Vol. 1 pg.544 gives the older visual magnitude range as 9.8v to 10.8v. Both stars are separated by 12.2 million kilometres, while the mass primary star is 3M⊚ - the companion being 2.6M⊚. In luminosity, both are 424L⊚ and 54L⊚, while the temperatures of the two stars are 15 210K and 9 720K. Total mass of the system is 7.89 ΣM⊚, with the individual masses calculated as 4.75 M⊚ and 3.14 M⊚. Spectral classes are given between B3 and B8 and both have been determined to be giant stars.

NOTE: V735 Cen is plotted on in the top ‘Proxima Inset’ in Sky Atlas 2000.0 Map 25. V745 is located right in the middle of the figure!

Δ161 (14286-5439) No pair is located in this position in Centaurus and no likely matches with “8,8”. Dunlop’s pair is “nf” in Quadrant 1 whose only clue is the positions differences are ΔRA 2.86 sec and ΔDec 40.36 arcsec, producing the PA of 85° and the separation of 41.5 arcsec. The general positions are likely wrong and Δ161 remains presently unidentified.

Δ162 (14339-4628) Dunlop identifies as 295 Centauri but the star is in Lupus. At 7.5 and 10.5 magnitude, the present alignment is PA 241° and is separated by 72.2 arcsec. Spectrum is G6/8 III.

η Cen / Δ164 Eta Centauri (14355-4210) is an optical pair that is very suitable for small binoculars or telescopes. Eta Cen lies 21′N of the more southern Lupus border. Colours are blue and white for these 2.3 and 9.0 magnitude stars, and both are separated by 2.1′ and aligned towards the southeast. The blue primary is Eta Centauri / HIP 71352 / PPM 319680 / SAO 225044 / HD 127972 with the precise position of 14h 35m 30.5s -42 09′ 27.9″. This star has the B-V of -0.157 and the spectrum of combined spectrum of B1Vn + A. The companion is only listed as T 7814:3668:1 with the V magnitude of 9.04. The general field is stacked with stars including a number of other bright field stars. Understandably not now listed in the WDS.

R 249 (14371-6230)is a faint pair 1.7°S (PA189°) of α Centauri and is contained in the most profusely populate fields that I have seen. The field stars are all oriented along 1.6×0.4° wide ‘line’ approximately the NW to SE, averaging magnitudes between 9.5 and 13.5. (This cannot be seen clearly in Sky Atlas 2000 nor Uranometria 2000.0, but is visible in the Millennium Star Atlas.)

Some problem exists on the discovery date by H.C. Russell. This pair could have been seen by Russell as early as the 11th June 1871, as it is close to the 11th magnitude pairs; R 245 (2 arcsec) and R 246 (15 arcsec) As no evidence is written on this earlier observation, then the stated discovery date must remain ten years later on the 9th August 1881.

R 249 is an orange pair (K0 III) with magnitudes of 8.2 and 9.8, who Russell positions at (1428-6203 (1880)) with a separation of 2.77 arcsec at PA 33.633° (8 and 10 mag.)

Other than Russell’s positions, little has changed in the 3.0 arcsec separation along PA 24°, which was last measured in 1960. I have used this double in the past for finding 11.3 magnitude Proxima Centauri / V645 Cen (14302-6242), which is 49′WSW (PA 260°) from R 249. Listed as HIP 1475 / PPM 360885 / SAO 252821, the combined magnitude is measured at 7.5±0.4, but was unresolvable using the Hipparcos instruments. Parallax is measured as 4.77±1.58 mas, giving 209±69pc. (680±225ly.) Using this distance to estimate the separation, the two stars are some 94 billion kilometres or 630 AU. apart. If this is binary, then period is around 15 000 years. Little wonder we have seen little motion over in the last 120-odd years!

Δ163 (14380-5431) is in located in Lupus near the southern boundary of the constellation. This pair can be found 6.3° due north of Alpha Centauri (Δ165) and the field is only 57′NW from to one of the very best southern double — the much neglected Δ168. It also mimics another bright Dunlop pair Δ169 in north-western Circinus, which is some 1.5°SE (PA 137°) from our Δ163. *(See both pairs below). Δ163 is also approximately halfway between the open clusters NGC 5749 / Cr 287 (14489-5430) and NGC 5593 (14259-5447), which are 1.6°E and 1.8°W, respectively.

Δ163 of 8.0 and 8.4 magnitude, which Dunlop called “8,8”, is an attractive object in binoculars or small telescopes. Present separation is about 65 arcsec along the near easterly PA of 102°. What is most attractive is the very neat equal bright colour contrast of the yellow and blue stars. What I find interesting is that the wide spacing between the components means that any colour contrast effects are nullified. If this were a close pair, this might be more spectacular. Telescopically it might be best to look the pair at very low magnification to make the proximity more spectacular. I experimented with 7×50; binoculars and my small 50cm solar eclipse refractor, and thought that the stars looked far more attractive in these than the 20cm. !

Spectral classes of the two are; ‘A’ F0 III and ‘B’ B8 III, whose colour as roughly similar to Beta Cygni ‘A’ is a K1 star. Looking at the proper motions, both are travelling in the same way, but the velocity of the primary is about four times bigger. Furthermore, the primary has the Hipparcos parallax.

This is certainly an optical pair that is wonderful for binoculars or small telescopes.

ALPHA CENTAURI is one of the most impressive stellar ornaments in the sky and remains the major observational highlight for any amateur astronomer. Every amateur in the southern hemisphere must have seen it — and likely most of our fellow northerners and world population have at least heard about it. Alpha Centauri lies some two hours in Right Ascension east of the Cross and merely 13.5′ESE (PA 127°) from the interesting bipolar planetary nebulae, He2-111 (NSP16).

Alpha Centauri is sometimes referred by its proper name as Rigel Kent or less frequently by the long name, Rigel Kentaurus, but is is more commonly referred just as Alpha Centauri or α Cen. Rigel Kent, surprisingly, is not a very new name — unlike all the other bright 1st magnitude stars in the sky, as it was named this in the 20th Century by aviators who used it as the guiding beacon for visual navigation. The star is also catalogued as HIP 71683-HIP 71681 or SAO 252838. As a double star it is sometimes referred after Father Ricard, the discoverer, as RHD 1 - or by James Dunlop as Δ165 To me it remains Imperatorius Astrum - the Imperial Star.

Very distinctly rich yellow in colour α Centauri is ranked as the third brightest star in the sky at -0.29 magnitude. This bright beacon ranks falls in brightness by only behind Sirius and southern Canopus, but holds a particular reverence over all other stars as our closest stellar neighbour. (1). I have read many accolades about this particular star over the years but the one that really sticks in my mind is by E.J. Hartung who describes Alpha Centauri in his the definitive southern observer”s handbook “Astronomical Objects for Southern Telescopes” (AOST2) using just one word — “brilliant”. (2) Often, for some reason or another, several northerners have placed Rigel Kent behind the first magnitude star Arcturus — and this is seemingly often slyly done by listing the two stars individually. Either way, α Centauri (AB) and α Centauri “A” is brighter. Navigators amazingly did not name it Rigel Kent until the early days of aviation, who often used it as a stellar beacon for global positioning. α Centauri lies within a bright part of the Milky Way — so the telescopic field contains many many background stars.

In 2004, this binary is easily separated — even in small telescopes. A 5cm telescope can probably resolve it 50% of the time, though combination of the brightness of the pair and poor seeing sometimes make this difficult. I have seen it with a pair of 7×50 binoculars, but it had to be firmly fixed to a tripod. The minimum aperture for resolution is presently (1999) between 4cm. and 4.5cm. On good nights, my small 5cm. refractor has little trouble with powers greater than about 25× magnification. In the city some six stars occupy a 0.25° field and this increasing to about ten in darker country skies. I find that both stars can easily be found during daylight and separated cleanly in apertures above about 7.5cm or more. In daylight the colours lose their yellowness, appearing like brilliant white “diamonds” against the blue sky. Due to the overwhelming sky, I had some trouble separating the pair with the 5cm refractor. This is reverse to my understanding of optical and double star theory as this should become easier as the images cannot “blur” together so easily. Three attempts by me have been made — none successful. (Maybe I was just unlucky!) To find Alpha Centauri in daylight, an equatorial telescope is a distinct advantage, as the position can be simply dialled up using the setting circles.

In the decades to come the decreasing separation will make resolution far more difficult. Observers may tend to make calculations on telescopic resolution using Dawes Limit where visual separation is stated as 11.58 ″/ Aperture (cm) or 4.54″/ A (inch) Frankly applying this is useless, as the brightness of the two stars overwhelms true separation. Problems with this will apply to α Centauri just after 2010, and between the years 2023 and 2031. The minimum aperture to separate the pair when 10 arcsec. apart will be 11.5cm. telescope.

As separations reduce below 5 arcsec, this will become even more difficult, due to the problems with the seeing and so-called proximity. A good comparative test of this particular problem is the other “pointer” — Hadar / Agena (β Centauri). (See below) The value in the Dawes”s equation this instance should be set to about 13.5 ″/ A(cm). Around 2013 to 2017, and the years 2035 and 2039, the minimum size telescope required will be a 20cm but a 25cm will have no difficulties. A neutral density filter may have to be used to cleanly separate the two stars during periods of poor seeing. November 2037 is the closest approach in the orbit where separation decreases to a meagre 1.71 arcsec on the eastern or preceding side. (PA of 12°) Changes in PA for five or six months reaches 5° each month. For a few years, depending on the seeing conditions, the pair will be very hard to resolve. It is best to use a hexagonal diaphragm - a hexagonal shaped cover placed over the mirror or lens, or by using a neutral density filter. Estimating the constant used in the Dawes equation will likely be between 14″/A (cm). and 15″/A (cm).

At minimum separation, H.C.Russell did measured the pair on the 18th February 1878; 1.66 arcsec, but he was using a sizeable refractor! A 25cm may be able to glimpse the duplicity, and 30cm and above will have no real problems! It is fortunate for the observer that within two years this difficult period of observation will soon pass.

Alpha Centauri”s also has a rapid common proper motion producing a close approach or stellar conjunction with Centauri in 6200AD. The minimum distance reaches 23 arcmin or 1380 arcsec This is the best stellar conjunction for 1st magnitude stars for the next 400 000 years, and becoming truly the best double star in the sky!

Brightness of Alpha Centauri

Past observations by the astronomers such as Lacaillé, John Herschel and even H.C. Russell have suggested that the difference in magnitude was about three to four magnitudes, with the average of all WDS(96) observations being 2.49. The “average” magnitudes of the two components in the last 300 years has alleged risen in brightness, with relatively small changes in the difference in magnitude. Today, it is clearly only about 1.5. My own thirty-one observations of estimating the difference in magnitude, between the years 1981 and 1990, range anywhere between 1.1 and 1.8, producing an average of 1.4. Hipparcos (1996) measured this difference as 1.36, so this is about right. Why this difference is so large by these early and notable observers remain still a mystery. This problem with magnitudes is also briefly stated in Burnham”s Celestial Handbook Vol.1. Another issue, not mentioned by Burnham, is the way magnitudes were reported through most of the 19th Century. Unlike the modern magnitude system first set of by the variable, early observers tended to use “personal” magnitude scale, which could vary a much as six or seven magnitudes among visual astronomers. These variations were analysed in 1856 by Norman Pogson - the founder of the logarithmic magnitude scale that we use today. Here he found that the John Herschel (and to some extent William Smyth) underestimated the brightness of the first magnitude stars. (Hearnshaw., John, B.; “Origins of the Stellar Magnitude Stars”; S&T, 84, 5, p.494-499 Nov.(1992)) It is quite possible that the companion star has brightened in the last 100 to 200 years, though such changes would be, in many aspects, contrary to our current understanding of stellar evolution. The “A” component own variability could also have an small influence on the estimates, but it should be average out instead of being consistently out by such a large amount. The actual colour of this star may have also changed from deep yellow to light-yellow / yellow, as reported by both Herschel and Russell.(3)

Minor variations in brightness for α Cen was reported in S&T, 76, 6, p.509 Dec. 1984 in the “News Notes”, first reported in the French professional Journal “Comptes Rendu”s in 7th June 1984 by the collaboration of several French astronomers. Lead by Eric Fossat, working in the European Southern Observatory in Chile discovered these small periodic variations over the six day observing window. Although the primary has the same spectral class as the Sun, being a G2 star, however, the star oscillates slightly in brightness every twenty minutes by about 0.65 magnitudes. Such oscillations are not uncommon in G-type stars, and such brightness fluctuations have been detected in the Sun, which varies in periods around five minutes. Since discovered in the Sun in 1965, the variations have been found in several stars using high-speed photometry centred on the narrow double-yellow Sodium lines in the combined spectrum. In the Sun, this is achieved using a bolometer — a very small telescope attached to an electronic photometer, and is very useful in measuring small changes in temperatures and energy fluxes. It seems that the “A” component is the culprit, whose data was subtracted from the combined light. The origins of these brightness fluctuations is unknown, but several projects in the field known as helioseismology, are slowly revealing the inner workings of the Sun and the how energy and material is transferred from the solar furnace in the core to the surface.

Distance of Alpha Centauri

Prior to the early 1830”s, direct knowledge of stellar distances was not known. The only way to achieve this was by measuring tiny shifts in position against the background stars, using Earth”s three hundred million kilometre orbit as a base line. If the size of this tiny angle is known, simple trigonometry would reveal the elusive stellar distance. As it happened, one of the first successful parallax measures was of α Centauri by Thomas Henderson (1798-1842). Born in Dundee, Scotland, his early career began as a humble solicitor”s clerk, but his interest and vocation soon changed to astronomy. Around 1830, an astronomical triumvirate of sorts was formed between the German astronomer Friedrich Wilhelm Bessel (1784-1864) of Königsberg Observatory, Thomas Henderson at the Cape of Good Hope, and the Russian double star aficionado - Friedrich Georg Wilhelm Struve (1793-1864) of St. Petersburg”s Dorpat Observatory.

Each selected one or two particular bright stars to investigate. Logically, the closest stars were likely the brightest ones, but also assuming that these stars had similar luminosities but each lying at different distances. Sirius and Arcturus, and even Aldebaran (then named Palilicium), were immediately discounted - primarily because of their brilliance overshadowing the field stars for their suitable measurements. The second reason was their high proper motions - as first determined by Edmond Halley. when he compared Claudius Ptolemy’s Almagest data against John Flamsteed”s measured astrometric positions. On similar lines, Struve chose Wega (today”s Vega), Henderson chose α Centauri, and seemingly against logic, Bessel eventually selected 61 Cygni. Bessel’s pick proved to be a valuable insight, because unlike the single stars, 61 Cygni was a known binary star - and this gave Bessel two stars to provide parallax measures. Another important issue was that stars having very large proper motions would likely be close to the Solar System. Information on the high proper motion of 61 Cygni was discovered by Sicilian astronomer Giuseppe Piazzi (1746-1826) in 1792. who christening it “volatilis asterum” - “The Flying Star”.

Henderson”s southern star selection was based on geography, as in 1831, he was royally appointed as the Director of the observatory at the Cape of Good Hope. Making several observations using a mural circle (a 17th-18th century positional telescopic device) between April 1831 and mid-1834 he obtained some useful results but did not immediately act on them. On returning to Scotland he was promoted to the higher position of Astronomer Royal. After settling into this job, Henderson then began to arduous take of analyse his α Centauri data, and to his surprise, found an initial parallax of 0.76 arcsec — suggesting an astonishingly distance of 4.2 light-years. Like the Ancients of old, such intimidating distances made some astronomers fear humiliation from their peers and the wider scientific and religious communities. In his hands was the first known stellar distance, but doubting the veracity of the results made him very reluctant to publish. This continued for another four years.

Belatedly in 1835, Struve started measuring his own positions using the high-quality 23cm refractor and his homemade filar micrometer. Struve’s methods proved fairly time intensive. so it was not until mid-1837 that Struve found Wega”s parallax to be π = 0.125±0.055 arcsec. (26.0±7.9 ly.) This he published in “Mensurae Micrometricae” in late 1837. Unluckily, his chosen star was far more luminous than our Sun, making it notably further away than the precision possible by his observational methods. Struve also doubted his own results, but unlike Henderson, he had legitimately analysed the available data stating:

“We can therefore conclude that the parallax is very small, and it probably lies between 0.07 and 0.08 arcsec. But, indeed, we cannot give it yet absolutely.”

In early 1838, Bessel began his 61 Cygni observations using a 11.6cm. heliometer (An 18th Century equatorial telescopic device which has a split objective). By November of the same year, he found an accurate parallax of π = 0.314±0.014 arcsec. and the distance of 10.4±0.95 ly. This quite precise result was published in December 1838. Having no personal doubt on his methodology, history records Bessel as the discoverer of the first stellar distance. Soon after seeing Bessel’s result. Henderson’s doubt was quashed, and he published his results in February 1839. (Henderson”s remains a warning in the “cut-throat” world of both science and professional astronomy “publish or perish”.) However, worrying about “divvying the spoils” in finding these three results cannot reduce the importance of these discoveries - as 1838 marks the first steps in finding distances beyond the realm of the Solar System.

Comparison with the very recent Hipparcos data reveals that results from all three observers were very close to current values. These were as follows;

After Sir John Herschel was presented with the Gold Medal of the Royal Astronomical Society in 1841. his speech summarised and prophetically said of these discoveries;

“I congratulate you, and myself, that we have lived to see the great and hitherto impassable barrier to our excursions into the sidereal Universe that barrier against which we chafed so long and so vainly - almost simultaneously overleaped at three different points. It is the greatest and most glorious triumph which practical astronomy has ever witnessed… Let us rather accept the joyful omens of the time and trust that, as the barrier has begun to yield, it will speedily be prostrated. Such results are among the fairest flowers of human civilization”

In 1848, after the examination of most of the bright stars. α Centauri was deemed the closest. By the early 1850”s. new parallax measurements and the ascertaining of the seven orbital elements changed Henderson”s original result to 0.74 arcsec. giving the often familiar 4.2 and 4.3 light-years. For almost one hundred years, this value remained until more precision was obtained. The literature now gives the distance as 4.396 light years, rounded to 4.4 1y.

Presently the Hipparcos data give the adopted distance as 1.347 8±0.002 6pc. or 4.395 5±0.008 2 ly. from the most accurate parallax known to date of 0.742 12±0.00140 arcsec. This distance is universal adopted, but in reality decreases measurably from year to year.

Statistical Data on Alpha Centauri

COMPONENT	Alpha Cen A HIP71683 SAO252838A	Alpha Cen B HIP71681 SAO252838B	Alpha Cen C Proxima HIP70890
R.A. (2000)	14h 39′ 40.90″	14h 39′ 39.39″	14h 29′ 47.750″
Dec. (2000)	-60° 50′ 00.65″	-60° 50′ 22.10″	-60° 42′ 52.90″
Solar Mass {M)	1.07	0.87	0.4
Spectra	G2 V	K1 Vd	M5 VII E
Total Mag.	-0.04	-0.04	12.1 – 13.12 B
App.Mag. (v.)	-0.29	1.35	11.01(var)
Abs.Mag. (MV)	4.38	5.74	15.4
B-V Mag.	0.71	0.9	1.807
Luminosity (L*/L⊚)	1.5	0.4	0.00001
Radii (R⊚)	1.22	0.92	0.35
Separation (AU)	35 (Max)	55 (Min)	13 000
Radial Velocity (RV) kms.-1	-26	-18	-16
Proper Motion (pmRA.) (mas.)	-3 678.19±1.510	600.35±26.10	-3 775.64
Proper Motion (pmDec.) (mas.)	+481.84±1.24	+952.11±19.75	+768.16±1.82
Distance (pc.)	1.3478±0.002 5	1.3478±0.002 7	1.295
Distance (ly.)	4.3964	4.3964	4.223
Period (Years)	79.92	79.92	100 000
Parallax (mas)	742.12±1.40	742.12±1.41	772.33±2.42
Note 1: Distances here are quoted from calculations made by Jahreiss and Morrison (1993) using the Gliese Catalogue. (The Gliese Catalogue specialises with the closest stars to the Sun.)
Note 2: Proper Motion can be either expressed in terms of mas (milli.arcsec.) per year, arcsec. per Century or arcsec in decayears (10 years) (′d.yr-1)
Note 3 :Proxima is a UV Cet Type Variable namedV645 Cen.

ENDNOTES

1. My fellow Australians, however, seem to have forgotten this reverence, as it is the only star of the “Cross and the Pointers” left off the Australian Flag!
2. He must have been a modest man in my opinion judging by his particular elegant literary style.

POSTSCRIPT : Some Personal Experiences

When I was five or six years of age, I was fortunate enough to look through a telescope and gaze upon Alpha Centauri. This immediately revealed two very bright stars, appearing like two bright car or truck headlights at some distance, daintily painted against the velvet-blue backdrop of the city skies. My imagination was directly sparked. These two stars were genuinely surprising, and to make sure it was not an illusion, I looked along the telescope tube to see one solitary star, and from that time on I was hooked to “up there”. This interest became even more important in my later astronomical adventures when I had my own telescope and began observing double stars.

I recall many wonderful inspirations, but the best story involves travelling to New Zealand for the 1984 Royal Astronomical Society of New Zealand (RASNZ) conference. On my arrival in Wellington, which is located on the southern tip of the North Island, I was picked up by my friend at the airport. He told me he was scheduled to be the night guide at Carter Observatory for the usual public viewing night, and persuaded me to come along. If you do not know Wellington, Carter Observatory is located within the Botanical Gardens, on the steep hills directly above and behind the commercial business district of the city of Wellington. You can still catch the cable car up from the middle of the city to the base of the observatory. Quite a unique experience! As New Zealand’s first National Observatory, the main telescope is an elegant 19th Century 23cm equatorial refractor, mounted on a good solid equatorial pier. The tracking was excellent, based on a gravity weight drive that had to be manually cranked-up, but this has now been converted to using, and far less romantic, to electrical power.

I recall my astronomical friend making some excuses about a minor problem with the telescope drive. and kindly asked if I would act as the night-guide for a moment or two. Today, I still don”t know if it was an excuse for a break, but in a short time, a little boy, perhaps seven or eight came along with his father. Carefully he began to climb the twenty-odd steps into the observatory dome and looked amazed at the size of the telescope. The father of the boy explained that his son wanted to look through the “big telescope”. I recall they were not too sure of me because of my Australian accent, but astronomy is astronomy, so this insecurity quickly passed.

The northern part of the sky had inconveniently become covered by some very heavy cloud. I disabled the right ascension lock and directed the telescope towards to the southern part of the sky. Selecting Alpha Centauri, I noticed that the sky towards this direction was very bright, as the telescope was pointing directly above the city. In moments, I had the finder centred on the star and glanced quickly through the telescope to see that the finder alignment was true. The field was vividly sky blue from the lights of downtown Wellington below, and this field reminded me of sketches that we sometimes see in old astronomy books. The images of the two stars were suffering dreadfully from poor seeing conditions, but they did indeed look like incandescent searchlights. Not forgetting the “customers”, I invited them to look. For any adult, the telescope was at a comfortable height, without the need for bending or even “tippy-toeing”, but the boy was far too small. His father picked him up gently by placing his hands under the boy′s arms and then guided him carefully towards the eyepiece.

“What do you see?” I quietly asked.

“I see two stars, and they are really large and really bright”, he enthusiastically replied.

His dad put him gently down to have a look himself, and I began speaking to the boy’s father explaining what he was looking at.

I had not noticed what the small boy was doing, but when I did, I can still visually picture the very rare sight of this little boy being literally “star-struck”. He was simply sitting on the observing stairs, at one corner of the observatory, with his elbows on the knees and his hands angelically open under his chin. His one comment was simply perfect “Wow!”.

Instantly my mind flooded back to my own first experiences. I imagined, and hoped that the boy”s personal path along the “starry path” would soon follow mine. Moments like these you cannot forgetten.

My own discoveries occurred in the year that I first started reading astronomical books. I found that this star was the closest to the Sun, as stated in nearly all introductory astronomy texts. What did happened 4.3 years ago? The mind was simply lost. More revealing, at least to me, was that the two stars were in an actual orbit, travelling full-circle in just under eighty years. This was just like the planets in the family of the Sun, and all other astronomical bodies, each under the eternal dance of the universal force of gravitation.

By the age of thirteen I had my own 7.5cm reflector. Its “first light”, as in all my subsequent telescopes, has been Alpha Centauri, followed by other pairs like Alpha and Gamma Crucis and the Jewel Box. I cannot recall the apparent positions or distances of the stars during my first few observations, but by 1971, I was old enough to understand — the pair was approaching maximum separation of just over 22 arcsec. From 1971 until about 1995. the position of the apparent orbit has changed little in the pair’s telescopic appearance. A serious observer, recalling the background field stars, could possibly pick up these small changes, however, I admit I still have trouble discerning serious differences in the orbit. At the dawn of the 21st Century sees notable changes and now I do see the changes.

Over the next few decades Alpha Centauri will undergo dramatic and obvious changes, producing significant problems for those with both small and large telescopes. Twice, within fifty years, the familiar pair will almost merge into an evidently single star.

I have even dreamt about peeking at Alpha Centauri when it was much closer together, but looking at this pair every now I have proven (with my own eyes) that I have seen a binary star in action - something that is a illusion because sometime must pass before this can be seen for yourself.

RST 3894 (14427-6457) lies inside the larger dark nebula in Circinus B 145E and there are few interesting stars except for the pair. This star is easily found by drifting 0.8°E of α Cir. The magnitude of the two stars is 10.5 and 11.0, and separated by 2.2 arcsec at PA 160°. This pair is easily visible in 7.5cm and little has changed in the positions since Rossiter first measured the pair in 1936.*

α Cir / Alpha Circini / Δ166 (14424-6458) is a bright 3.4 mag star which is a wide binary with a beautiful colour contrast. The primary appears distinctly yellow while the companion is reddish. Discovered by Dunlop in 1828 and first measured by John Herschel in 1837, little change has occurred in the last 160 years except for the decreasing position angle by some 18°, showing retrograde motion. As of 1994, the current separation is 15.7 arcsec and the PA is 246°. Once in the Third Catalogue of Visual Binaries this system was quoted as having a thousand-year long orbit, only to be rejected in the latest 4th Catalogue. If truly binary, the period must be several millennia. Recently, the ‘A’ component was thought by double star observers to be variable. (I.e. WDS96) As yet it is not listed in the NSV. The pair is easily visible in 7.5cm. In larger apertures, it is absolutely beautiful in a star-studded field. Between the “Golden Horseshoe” and α Circini is a dark nebula. Typically dark nebulae are boring, however, this one is interesting because of a major drop in the number of stars both east and south of α Circini, and this is visually obvious some 1°E of α Circini.

NGC 5749 (14489-5430) (U431) is a cluster 1.6°E of Δ168. This cluster looks like a small gaggle of stars shaped in like a ‘A’. In 15cm there are about eleven (11) stars from 9th to 12th magnitude, and in the 20cm I saw about twenty-five (25) stars. Larger apertures may well begin to see some of the fainter stars that total about thirty-odd. To the east of the 8′arc minute cluster there are four pairs each about 1′ apart that are arranged in a straight line. In the very centre is a 10th magnitude orange-red or red star. Stars within the cluster are an odd random mixture of spectral types, which might appear better in dark skies. Identifying the cluster is the 9.6 orange-yellow star, HIP 72519 - appearing 6′ or 7′ east from the open star cluster’s heart.

Data on the cluster says the total magnitude is about 8.8, which to me seem to suggest this is an eye-catching open cluster, but when I looked at it I was a bit disappointed. It is classed in the Trumpler System (1930) as III 2 p with the number of stars being about thirty (30).

In all, NGC 5749 is an unspectacular cluster.

ASTERISM: The Golden Horseshoe (1451-6612) is another brighter asterism that lies between α Circini and γ Triangulum Australis, and some two-thirds the bisected distance in a straight line between these stars. To the naked eye the brightest star in the asterism is the 6.2 mag bluish ζ Circini (SAO 252951). The Golden Horseshoe has a diameter c.0.8° and contains some twenty-five stars - three of 6th magnitude, six are 7th, eleven of 8th and five of 9th. It also contains a number of double stars.

In the finder, this asterism is obvious, though the larger telescopes may have serious trouble in getting the object within the telescopic field. A small telescope, say 7.5cm or 10cm using low power will have no trouble placing it into one field.

Another ‘mini-asterism’ lies in the northern part of the ‘horseshoe’, which I call the ‘Golden Snake’. This is an ‘S’ shaped line of stars that extends NNE from the 7.6 mag star SAO 252951 (GSC9019:467) by some 12′ in length towards α Circini. Twisted like a snake, the line of twelve 11th and 12th mag stars end in a ‘forked tongue’. A 7.5cm should see these easily in dark skies, a 10cm if you are in a sky-lit suburb.

I 369 (14487-6635) is the bottom star of the straight line of five stars in the southeastern part of the field of the horseshoe. This is the Innes triple, whose magnitudes of the wide AB pair are 5.90 and 9.0, separated by 30.0 arcsec at PA 80° (WDS01). Checking the observational data, Brian Mason of the US Navel Observatory confirmed this is correct. The spectral types of the AB system are B2.5Ve and B7/8V. All proper motions are also similar, suggesting that the three stars are really associated. However, little has changed in the positions since Innes observations in 1902.

HJ 4707 (14544-6625) is the John Herschel pair on the opposite end of the horseshoe. This nearly equal yellow and white pair, has quoted visual magnitudes of 7.5 and 7.9. Since the first observation in 1837 the separation had slowly decreased, until sometime is the early 1960’s. I.e. 1.5 arcsec to a minimum of 0.54 arcsec. Since then the separation has began to increase that according to the calculated ephemerides in the 4th Catalogue of Visual Binaries, the current separation is 0.84 arcsec at PA 294°. Then if this is correct, an aperture of 20cm could just resolve the pair under good seeing conditions and using medium-high magnification. Visually, I can just see it in 30cm, suggesting the separation is about 0.65 to -0.7 arcsec and that the PA may also be out by +10° or so. It is possible that the binary star ephemeris is incorrect.

To my eyes, the pair appears strongly yellow in colour. It is no doubt, from the forty-one measures made to date, it is likely a long period binary. In 1948 Woolley and Mason calculated the retrograde orbit has a period of 288 years. Another known problem is the observed magnitude difference. Herschel’s observations state a Δm of 0.4 — and this figure is given in the IDS. The most recent observations suggest no difference at all I.e. Both 7.00V magnitude, respectively. My own observation suggests that the difference is nearer Herschel’s. Could this be a suspected variable?