By Andrew James, Astronomical Society of New South Wales, Inc.
(This is a special series appearing on   Doug Snyder's  Planetary Nebulae Observer's Home Page)

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Neat Southern Planetaries XX

Among Musca’s small PNe collection, are three main objects. I rate “The Spiral Planetary” NGC 5189, followed by IC 4191, with NGC 4071 lying a close third. (Making, in some ways, Musca’s “nebulae triumvirate”) IC 4191 makes quite a contrast to the much larger NGC 4071, as it appears stellar, except when using very high powers and large apertures. Visual magnitude is given as 10.6, while the photographic magnitude is 12.0.With care it can be found in 10.5cm telescopes, though it is claimed in AOST2 that it can be glimpsed even in 7.5cm. IC 4191 is classed as a “Type 2 - smooth disk in the Vorontsov-Velyaminov PNe classification, and for me, it appears a strikingly blue or aqua-blue “star-like” nebula. In 20cm, this colour was still obvious, and once found, it became instantly noticeable using any wide, low-magnification field. Using 30cm, I thought the colour was much bluer than the planet Neptune, and incidentally, larger as well! I think the presence of the red star so close to the planetary certainly influences the perception of the small disk’s colouration. This is probably similar to the problems faced with colour contrasting double stars - like the famous northern pair Beta Cygni (Albireo) (19307+2758) or the wider Gamma Crucis (Gacrux) (12312-5707). I attempted using an occultation bar to obscure the star, and thought that the colour did slightly fade. Using the highest power possible for the seeing conditions, I also once tried moving the star in and out of the field of the eyepiece. This was only partly successful. It would be worthwhile to do a “blind-test” on the object at a Star Party, and compare the colour with or without the star’s influence.

Positive identification of the PNe is particularly easy with an O-III filter, by just flickering the filter across the field of view. (See the Position in Figure 2.) Here the stars almost vanish while the planetary stays constant. (Comment: When doing this with the telescope, for some reason, I always think of Clyde Tombaugh, the discoverer of Pluto. Call me crazy, but the appearance of the PNe jumping in and out of visibility seems exactly like the observational methods of using old blink comparator.) In the case of IC 4191, I thought the usefulness of the O-III filter was not as effective compared with other planetaries of similar brightness and size. Using the O-III with 30cm and higher magnification, the disk appeared quite smooth and uniform, with perhaps slight increases in brightness towards the centre. I also suspected that the south edge of the disk was also slightly fainter and fades more rapidly.

Few modern amateur descriptions exist. One example was written by Queenslander Greg Thompson, who describes IC 4191 as having a starlike centre, surrounding by a small but bright and slightly bluish circular glow, which fades rapidly towards the edges. (“Southern Astronomy”; Sept./ Oct. p.39 and Field Sketch)

Astronomical Objects for Southern Telescopes (AOST2) (p.293) lists IC 4191 (#576), and says of its nature and colour;       “...this small planetary nebula can be picked out by its bright bluish disk from a profuse star field. The nebula has a bright elliptical prism image which 7.5cm shows as a tiny point, and is an easy stellar object...at the north-west apex of a narrow triangle with two fainter stars.”

Jenny Kay of the Canberra Astronomical Society Inc. (“Southern Cross”, June 1988) says of IC4191;

“12.5" [f-5] -- 50X: The planetary requires some care to pick out in the busy star field, being almost stellar in appearance. Once located, it can be noticed that the planetary is only slightly larger than a true star, and equal in brightness to a nearby 11 mag field star. It also appears to display a very deep, blue colour. There is a very strong response to the UHC filter. 83/151X: Some care still required not to mistake this very small planetary for a star. The planetary is 5" in size at the most, relatively bright, with a round disc of uniform brightness ...151X offers the best view where the planetary is a little more pronounced.”

In a Personal Communication, Californian amateur Kent Wallace, observed IC 4191 from Western Australia on the 21st January 1999, stating that;        

“[Using the C-8 SCT] at 62.5X and 100X [I] can see a very faint star, but needed the O-III and UHC filter to identify it as a PNe. Good response to the O-III and UHC filters. No response to the H$. At 200X, I could see a very small disk when using the UHC filter and averted vision.”

Detailed Analysis of IC 4191. 

The truth about the PNe nature of this object was finally tied down as recently as 1948 by the Russian astronomer Vorontsov-Velyaminov (VV), who listed IC 4191as VV67. Next observation of any significance, was one made by Karl K. Henize between 1961 and 1962 using the 74" Reynolds Telescope at Mt. Stromlo Observatory near Australia’s capital, Canberra. His observational results were a long time coming, and two significant papers by Henize were published. These were “Observation of Southern Planetary Nebulae”; A.J. Suppl, 14, P.125 (1967) April (1967), and a second paper authored by Westerlund, Beng. & Henize.“ Dimensions of Southern Planetary Nebulae”; A.J. Suppl., 14, p.154 April (1967)

The latter paper produced images and observations made between November 1961 and June 1963, during the time when Henize was visiting Mt. Stromlo (1961-1962) from Dearborn Observatory, of the American Northwestern University. In regards IC 4191, Henize classed the PNe’s structure, by placing it second in nine-categories of PNe types- “Ellipses gradually brighter in centre in order of core brightness”. Henize also found the core’s diameter as 6"arc seconds, about the size as seen in most amateur ’scopes. Observations from Mt. Stromlo found the outer halo dispersed its light to about 10"arc seconds. Henize thought he had seen some elongation, and using the twenty minute photographic exposures, made an estimate of the position angle as 50^O degrees. He also commented that he thought the nebula appeared “peculiar triangular shaped.” This is particularly odd PNe description, but his impression is seen clearly in the image produced in the original paper. In the professional telescopes appears a very faint outer envelope can be seen to extend by some 18"x11"arc seconds. However, this is invisible in all amateur telescopes. Here IC 4191 remains about 5"arc seconds across. Morano (1990) measured the Ha  diameter as 11"arc sec., finding strong emissions in O-III and Ha . This is also especially prevalent within the inner parts of the nebulosity. Like most “average” planetaries, the expansion velocity of the gaseous shell is 12.0 kms^-1 (1976), while the radial velocity, measured from the O-III lines, is -12.7±6.5kms^-1 (1983) in approach.

Images in Ha" are particularly interesting, as IC 4191appears the irregular second disk abut 21"arc second across, and possibly surrounded by even faint nebulosity. IRAS observed in 1982 a moderate flux at 25±6.0  µm, and also confirming Henize’s “triangular”description. The O-III image is also irregular, but slightly smaller than the corresponding Ha" image. No IUE satellite observations were ever made, but the radio observations confirm the moderate PNe radio source whose flux is 152m.Jy. (2cm./ 14.7GHz.) and a flux of 170m.Jy. (6cm./ 5 .0GHz.)

Line intensities are measured as Hß =100; He(II)=12; OIII=8; OIII=567; He (I)=19; Ha =380; N(II)=139; S(II)=5 S(II)=11against the standard Hß  flux of ~10^-11 mW.m^-2 (1985). These lines are not very intense, but because of the “stellar” nature of the PNe, the use amateur filters still prove quiet effective. The “Hß /[OIII]” ratio or “N”, is the measure of the brightness of the Hß  and OIII lines is 5.8 or 6, which is on the small side compared to many planetaries. (See Postscript NSP 19) Also the spectral lines and brightness between NGC 4071 (NSP19) and IC 4191 are about the same. As IC 4191 is much smaller in area, therefore it appears much brighter in the telescope.

Examination of the intensities of the emission lines also can be used to determine the so-called electron densities (using measures of the deep-red [SII] lines) and the nebula temperatures. First appreciated by Menzel, Aller and Hebbin in 1941, and further elaborated by M.Peimbert in 1967, these two quantities have become an important means of examining the nature of gaseous nebulosity. It allows some explanation of the mechanism of ionisation within the nebula’s confines, but also allows calculation of the relative abundances of the various photoionised elements. Typical values tend to produce electron temperatures (T_e) between 5 000K and 20 000K, and electron densities between 10⁴ and 10⁶ e^-.cm^-3 (number of electrons per cubic centimetre) are typical. As a method, this relates strongly to the atomic structure of matter and the general conditions within the nebulosity. Such “galactoastrochemistry” tells us much about the evolution of the PNe and PNN combination, including the element abundance of the original star and the PNN’s age. For IC 4191, the electron temperatures are taken from McKenna et.al. “[NII] and [OIII] Mean Electron Temperatures.”; PASP, 108, pg.610-614 (July 1996), finding values as [OIII] T_e=9 940K and [NII] T_e=10 350K, place this PNe in the mid-range of the 106-odd PNe examined in this paper.

Little is known of the central Planetary Nebula Nucleus (PNN), though the ESO-Catalogue (AG92) states a visual magnitude of 16.4v, while the other measured “B” magnitude is 16.8 [Hence, the B-V=0.4] Earlier texts normally state “16.6v:: mag”, however, it remains totally invisible in all amateur telescopes. The spectral signature of the PNN is uncertain, and the detail of the surrounding nebulosity is only inferred by its emissions. Zanstra temperatures for this object were first determined by Gleize et.al. (A&A 222, 237-246 (1989)) as 99 000K and 116 000K for HeII, giving a mean PNN Zanstra temperature of 108 000K. In 1978, the “uncertain” distance was determined to be about 2.1kpc. All values since this date have done little to improve this distance estimate.

Surrounding Field of IC 4191

L 5393/ NSV 6103/ HIP 64117/ SAO 252163/ PPM 359894/ BD -67^O1369/ HD113919/ GSC9241:1247 (13085-6748), which I saw closer to red than orange in colour. Some references give the spectra class of Ma (M-Class with absorption lines), thought others also give M1. (AOST2 says M2III) It is also listed in the NSV as a dubious suspected variable star at 8.9p magnitude. Presently, the variability still remains uncertain. It is listed in the Hipparchos catalogue, whose parallax is 3.6±0.67m.arc seconds, suggesting a distance of 200 parsecs - about a sixth the distance to IC 4191. The B-V magnitude is 1.710.

Eta (0) Muscae/ ?131/ HIP 64661/ SAO252224 (13152-6754) is a bright and wide pair that is visible in the smallest of telescopes. Discovered by Dunlop in 1826, and listed in his double star catalogue as ?131, this magnificent gem is some 39.6' NW (along PA 114^O) from IC4191's position. Annoyingly, it is not listed in nearly all of the common star atlases, (Ie. Sky Atlas 2000.0, but it is listed in Vol.2 of the Catalogue.)

Magnitudes are given as 4.8 and 7.3 in the WDS 96, though the PPM, Hipparchus and Tycho Catalogue give the secondary magnitude as 7.6.

This brilliant blue and white pair is separated by 60"arc seconds along position angle 332^O. Spectral class of the two stars is B8 and A0. As little has changed in the last 180-odd years. The last serious measures were made by John Herschel ten years later in 1836, finding no differences in the relative positions. Careful inspection of the field finds three stars, one 11th, and two 13th mag, The 11th mag star was easy, even in 7.5cm, but the other two proved far more difficult. I could only just see them with averted vision in 20cm. Apertures larger than this should have little problems. The primary is suspected to be an ellipsoidal binary, which varies between 4.76 and 4.81 in an unknown period. Such eclipsing systems vary due to the observed changes in the component’s areas as they orbit each other.

Eta Musca’s field contains a handful of interesting objects, and these are three faint red variables all are clearly visible in dark skies using 20cm. These are;

DY Mus/ GSC9242:218 (13170-6753) is a possible Semi-Regular variable (SR:). The tentative magnitude range is between 13.5 and somewhere below 15th. I did a complete literature search on this object, and found very little of interest. Observers may like to attempt several separate observations to see if you can find any variability.

FX Mus (13154-6750) lies some 3.2' N (PA 16.6^O) of Eta Muscae. Known as a Mira variable that changes between 12.2p-15.8p in a period of 203.3 days, and fixed to the maximum on the 26th April 1964. (JDE2438511) The rise in brightness is quite rapid, increasing by 3.6 magnitudes in only about ten days. At maximum, the star is obvious in the field in 15cm or above. Somewhere between 30cm and 40cm telescopes are required to see the minimum brightness.

FY Mus (13170-6749) is another Mira variable, some 13.3' NE (PA 48^O) of Eta Muscae. I have not seen this star, as FY Mus must have been below the magnitude limit of 20cm on the night I searched for it in February 1982. The magnitude varies between 13.4p and below 16th, with the lowest magnitude so far observed being 16.1p. Period is about 352.7 days, starting from the 3rd June 1963 (JDE 2438213), though variability is certainly dubious.

A field chart, with all three variables mentioned above appear in Figure 3. This will help you find them in the eyepiece.

I 363 / HD112938 (13016-6751) was discovered by R.T.A Innes from South Africa in 1902, and can be found 43'W of IC4191. Although the field is liberally sprinkled stars, I 363 is easily identified, as it is the only bright star in a medium magnification eyepiece. Confirmation of the pair is easily made as there appears the 12th mag Mira variable FU Muscae. (Mentioned below) The white A2V pair of I 363 is listed as 8.7 and 10.2 mag, separated by 2.7"arc seconds along position angle 193^O Little has change in the star’s relative positions, and if these stars are truly associated, the period must certainly be long. A 15cm is required to easily resolve the pair, but it is worthy while trying this in 10cm.

FU Muscae (13017-6749) is merely 3' N of I 363. Sky Catalogue and Atlas 2000.0 nor Uranometria 2000.0 both don’t mention this pair, nor the variable. Little is known about this star, except its period is 199.5 days, starting from the maxima on the 21st June 1963. Table 1 gives the expected maximums of FU Muscae. Although most Mira type variables  changing brightness like clockwork, variations can be out by as much as one month. Minimum magnitude for FU Mus is thought to be below 16.1, but longer-term observations might find the minimum by one or two magnitudes lower. No finder chart or comparison stars exist for FU Mus, so it is best too roughly make estimates using the brightness of the pair. See if you can visually find any variability for a month or so around the predicted maxima. Approximate dates of each maxima is calculated for; 

-----------------------------------------------

 Table 1.

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20th Jan 2000 6th Aug 2000

21st Feb 2001 8th Sep 2001

26th Mar 2002 11th Oct 2002

28th Apr 2003 13th Nov 2003

30th May 2004 15th Dec 2004

2nd July 2005 17th Jan 2006.

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“Y”-shaped Southern Telescopic Asterism (12580-6750) is equidistant between IC4191 and Beta Muscae, and 24' west of I 365. Shaped roughly like a “Y”, I thought it was quite pretty, and its 1^O size is visible, using low power, in either 15cm or 20cm ’scopes. The southern part of the asterism is marked with the white 6.7 mag star HIP 63236/ SAO 252105/ HD 112383/ PPM 359770 (12574-6758). (Note: Only three of these stars are positioned in Uranometria 2000.0.) Following on further north, is a 9' long gaggle of five-odd 9th to 10th in an arc, ending with the orange 8.5 mag star HIP 63279/ HD 112472/ “Pair 1” (12580-6749). This last star’s colour is quite obvious in modest apertures. Alongside HIP 63279, and some 50" due east, is a distinct yellow 10.6 magnitude star, that forming a pretty contrast with its nearby companion. It is not listed as any known double star in the WDS96, and looking at the Tycho Catalogue information, it is very likely just an optical system. (“Pair 1” appears also in this star atlas, north of HIP 63236, though the magnitudes are shown as the same.) The “Y” asterism centre is another 10'N from “Pair 1”, and here the other two lines of stars stretch both NE and NW, finishing on each end with two solitary 9th mag stars about 3' apart.

Also below and west of HIP 63236, are two faint Mira variable stars. These are;

FR Mus (12580-6804) is the brighter of the two, and appears some 8' SSE of the brightest star in the field, HIP 63236. When I glanced at this field, FR Mus was just on the edge of detection in 20cm. List as a Mira, the 4th Edition of the General Variable Star Catalogue (GCVS4) states the magnitude varies between 14.1 and possibly below 16.1. The period is listed as 253.3 days.

FO Mus (12555-6758) is likely a suspected Mira variable, which appears 12'W of HIP 63236. When I examined the field, I did not see this star in 20cm, likely through the want of aperture. FO Mus is slightly fainter than FR Mus, and the GVSC list the magnitudes as 14.5 and possible minimum of 16.1. The period is given 347.8 days.

Cr268/ Harvard 8 (12379-6823) is a faint open cluster in Musca, lying some 50.0' roughly NE (PA 71^O) from Eta Muscae (PA 70.6^O), or alternatively 20' SE of orangery 4.9 mag star HIP 64820/ SAO 252240/ HD 115211 (13172-6647). If you choose this latter field star, you will find midway between the cluster and Eta Muscae, a telescopic asterism that looks like the constellation of Crux on its side, whose corresponding “Acrux” is the orange-red 8.1 mag star HIP64853/ PPM 360001. The “Catalogue of Open Cluster Data (5th Ed.)” by G. Lynga (1987), gives this 4' cluster a total magnitude of 9.5 - the magnitude for all 30-odd stars. In the telescope, the brightest ten stars range between 11th and 13th mag, while all others are below 14th. To the west finds a solitary triangle of stars, containing the brightest member, 11.5 mag GSC9002:127. About 2'E is another eight or nine stars in a line, some 4' long, which has a solitary reddish star towards the south.

All of these stars are visible in 10.5cm, though the remaining stars of the cluster might require at least 30cm.

NGC 4815 / Cr265/ C1254-593 (12580-6458)(U451) is an open cluster found just inside the boundary of the southern Coal Sack, and 1.1^O SWW of the wonderful pair Theta Muscae. NGC 4815 lies in Musca, but is 17.2' S of the intersection of the constellations of Crux, Centaurus and Musca. It is like many of the open clusters in this region, suffering significantly from interstellar absorption, making all the stellar luminaries appear yellow and reddish, Surely this region has one of the greatest absorptions of starlight known. I have selected this cluster because of its history and the appearance in telescopes between 15cm to 30cm, as “globular mimic”.

NGC 4815 was discovered by John Herschel, who twice described it as, “Cluster, rich, pS, resolved; irregular; gbM, 3', stars 13..18th mag.”, and “Class VI, p rich, irregular figure, gbM, 3', stars vS, comparatively insulated; has two bright stars 8-9th mag and 9-10th mag.” Dreyer in the original NGC (1888) describes it as “Cl, pL, pRi, iF, st10..18.” Southern observer R.T.A. Innes at the Royal Observatory in South Africa, and using 7-inch refractor, was next to comment on NGC 4815 in 1898. Looking objects inside the Coal Sack, he doubted that this was a real cluster, claiming that it was an irregular nebula surrounding two stars. Innes again claimed this in 1903. It is interesting that the below 20cm aperture, NGC 4815 really looks like a globular, that I confirmed using 20cm (C-8) in 1983. Some observers might like to compare this object to Musca’s true globular NGC 4372, (NSP 22), which has similar problems with resolution using c.20cm apertures.

Total magnitude is 8.6 mag, and for its tiny 3.0' size, you would expect it to be grandiose than it really is. A 7.5cm telescope reveals two bright 9th stars in the field, followed by perhaps a half dozen 12th mag stars. In dark skies, apertures between 10cm and 20cm, reveals some twenty-five stars in an “S” shaped asterism. To me it looks like an old garden snail. (Dare I say “The Snail Cluster.”?) Larger apertures start to show the faintly dull haze, which under reasonable conditions reveals the sprinkling of faint 14th-15th magnitude stars. An STScI image of the field of the cluster appears in Figure 4.

Brightest and the  most easterly star, is the orange-yellow 9.3 mag PPM 779227/ T8997:563:1/ “NGC 4815 1” (12580-6457) in colour. Almost due east by 1.3' is the orange-red 9.8 magnitude PPM 779225/ CoD -64 2088/ T8997:72:1/ “NGC 4815 2” (12577-6457). All the remaining stars appear as colourless dots. Until 1994, both these stars were considered part of the cluster, but there is some evidence that they merely field stars. The brightest star is now given as “NGC 4815 3”, whose visual magnitude is 12.38, followed by “NGC 4815 4” at 13.16. Both these stars appear on the very top of the Main Sequence in the Colour-Magnitude Diagram in Figure 5. (Note: NGC 4815 “xx” star number sequence uses the same methodology as NGC 4755 (See NSP18a))

In the literature, NGC 4815 is labelled as a rich cluster with about 100 stars. Trumpler Class is stated in many references as “1 2 r -”, but this has recently been downgraded to “1 3 m”. No known variables are attached to the cluster, but some recent work has been made on this particular cluster since 1991. The first paper of some significance was “Stellar photometric stability. II. Ages and distances for 13 open clusters with time series observations.” by Kjeldsen and Frandsen (Astron. & Astrophys., Suppl. Ser., 87, 119 (1991)), which aimed, among other things, to study significant “gaps” in the main sequence stars as seen in the resultant H-R Diagrams. These gaps are deemed to be quite important, and they suggest something about the evolution of all open clusters. Of the thirteen clusters were selected in this paper, including NGC 4815. Figure 5 presents the C-M Diagram obtained by CCD photometry. (G. Carraro and S. Orolani “Deep CCD BV photometry of the poorly studied open cluster NGC 4815.” Astron. & Astrophys. Suppl. Ser. 106, 573 (1994)) This paper shows the measurements of 2498 stars between 12.9 down to ~20.5 mag, measuring both the “B’ and “V” magnitudes. Figure 5 gives the distribution of magnitude “V” verses colour “B-V”, that clearly shows the small gap in the distribution of stars around 15th magnitude. B-V colour values for NGC 4815 are quite different than nearly all the observed young open clusters. (Compare this with the colour-magnitude diagrams of, say, the Jewel Box in Universe February 2000) The colour here is deeply reddened by the Coal Sack, and if it didn’t have this dark nebulosity, it would be certainly be much brighter and bluer.

Results obtained from this paper shows some similarity to the Hyades, giving a “middle” age of about 500 million years. The distance is presently estimated as 2.5kpc from the Sun.

In 1998, the latest paper (Chen, B. et.al. “The spatial distribution and luminosity function of the open cluster NGC 4815.”; Astron. & Astrophys., 331, 916-924 (1998)) makes a further examination of NGC 4815. Using the earlier data, the collaboration eliminated some of the field stars, and investigated the proper motion and other properties in an attempt to explain the shape and rapid “drop-off” in stellar density of the cluster. More importantly, this paper confirms the “mass segregation” of stars, and future investigation of this and other clusters might still yield new clues on open cluster evolution.

All in all, this is an interesting and different cluster, away from many of the brighter ones often seen in the Milky Way. I thought that the cluster reminded me of some of the open clusters in the northern constellation Cassiopeia that I observed in England back in 1997.

R 207/ “Bemusca”/ Beta Musca / Syd 1-207/ HIP 62322/ SAO252019 (12463-6806) rates somewhere in the Top 10 of southern pairs, and is surely a “must see” for amateurs. Culminating on 12th May at 9pm and 27th March at midnight, Beta Musca is the most northerly star of the main trapezium-shaped body of the “Southern Fly”, and appears 1.3^ON along position angle 40^O, from Alpha Musca (12372-6908). A renown binary system, R 207 was first seen by H.C. Russell at Sydney Observatory mid-evening on the 15th April 1878. In 1882, Russell claimed in “New Double Stars found while measuring Herschel's Cape Stars in 1882.”, that Beta Musca was, “...this is one of the closest doubles I know.” Measures made during the 1880's on this star were prolific - twice by Russell himself, and eight times by J.A.Pollock. As mentioned in AOST2, the first measures, and subsequent ones at Sydney Observatory, were as follows;

Date                       PA^O Sep Obs Mag

1880.344 317.27 0.54 R 4/4.5

1886.501 322.0 0.74 R 4/4

1886.594 325.8 0.52 P

1886.597 325.7 0.52 R

1887.512 328.1 0.88 P

1887.515 327.0 ---- P

1887.531 326.7 0.74 P

1887.550 327.4 ---- p

1887.572 326.7 ---- P

1887.586 325.0 ---- P

1887.600 326.7 1.06 P

Russell was well aware of the significance of this pair, and had the fortune to find R207 having its fastest orbital motion in the orbit during its closest approach (periastron) sometime in the early 1870's - later determined from the observed orbital motion as 1872.290 - 10th April 1872. As Herschel had missed this obvious but difficult pair, it was natural to assume that the star had begun to significantly widened since Herschel's observations.

Magnitudes are 3.6 and 3.9, with the combined magnitude being 3.04. Position angle is presently (2000) about 45^O, and the separation is c.1.3"arc seconds. Presently both stars can be easily seen in 20cm, but remains difficult in anything less than 15cm.

AOST2 says about ;

“...the stars reached a separation of 1.4" in about 1950 and now seem to be gradually closing again; 12.5cm will separate the stars on a night of good seeing. This is a long-period binary, and a fine object in a starry field.”

The Washington Double Star Catalogue 1996 (WDS96) states in the notes that the period is 383.12yrs., a=1.74"sec.arc., while the motion is direct. “A” has also been observed to have a variable radial velocity, so it might be also  binary itself. The “4th Catalogue of Binary Stars” (Worley and Heintz (1996)) gives the orbital elements as;

P=383.120yrs  T=1872.29

a=1.735"e=0.526 i=61.30^O ?=98.32^O   O=161.81^O

The most reliable data to date is from F.Mourao (1963), first published in (Bul.Astr.Obs. R.Bel., 5,143 (1964)). All the orbital elements remain uncertain, as only a quarter of orbit has been observed, and this is all just after periastron - the most critical position for determining orbital elements. I have used the latest data to draw the apparent orbit (Figure 6.) Closest approach was sometime in the beginning of 1872. The pair then widened until its first maxima were reached in the mid 1960's. Presently the pair is slowly decreasing in separation, and this will continue until 2029, where the separation reaches 1.25"arc sec - the second minima in the orbit. Again the orbit increases in separation, reaching the greatest apparent separation of 1.715"arc.sec in 2165AD. About the same time, observations of Beta Musca will increase in precision, and the true period, etc. of the system will be properly known. Over the next hundred years or so, after about 2165AD, the separation and position angle changes more rapidly. By 2258AD, the separation again reaches the minimum separation of 0.38"arc.sec, making it difficult pair in most amateur telescope below 30cm, and even larger if the seeing is poor.

The orientation of the true orbit is placed as along the dotted lines in Figure 6. This is inclined 8.3^O to the east-west line, with “P” and “A” being the date of the respective periastron and apastron of the orbit.

Hipparchos found the distance as 95.41±5.56pc (311.2±18.1 ly.) from the Sun, using the measured parallax of 10.48±0.65mas. The results here seem questionable. For example, the parallax measured in the Tycho catalogue gives the distance as 39pc±4pc (25.60±6.10mas), more the two-and-a-half times larger than Hipparchos. Using the dynamical parallax, the theoretical distance of the system derived from the magnitudes and orbital data, finds the distance of 50.6pc (164.9lty). I again calculated this using the “full” Baize-Romani method (first determined in 1945) and obtained 51pc. Such divergence in the values makes the mass and size of the orbit as doubtful. From the available data, I think that the Hipparchos distance is likely too large, and it is certainly much closer than the quoted error. It is possible that the proximity of the two stars might be causing an incorrect measurement. The Hipparchos satellite had significant problems, due to the configuration of the optical system, in measuring pairs between about 1.5 and 0.2 arc seconds. Beta Musca, because of the stars brightness, lies on the boundary of this region. Overall, I think the 51pc distance is likely closest to the truth.

The true dimensions of the orbit, compared to the Solar System, is huge. If the Hipparchos distance of 95.41pc is true, the dimensions of the true orbit have a periastron passage some 42.4AU apart, this increases to the furthest apastron distance of nearly 200AU - five times the orbit of Pluto! If the distance is closer at 50.6pc, then the proportion orbital size is smaller, dimensioned as periastron at 22.3AU and apastron as 104AU.      

Proper motions suggests the motion of travel is moving further south each century by some -40.40" in RA and -10.32" in declination, with Beta Musca is moving way from us at about +42km.sec^-1. Spectral class of both main sequence stars is B2V and B2.5V, implying effective surface temperatures of 24 400K and 20 950K, respectfully. The measured B-V for both star’s combined light is -0.178, reflecting its visual blue colour. Absolute magnitudes (M_abs) are -1.3 and -1.0, respectfully, making the two stars 275LR and 208L R times the luminosity of the Sun. Using this data, and applying the results from the dynamical parallax, produces the minimum value for the total combined solar mass of S 4.6M R, and individual masses of 2.33MR and 2.28M R, respectively. Furthermore, the minimum radii of each star is about 3.0RR and 2.6RR, roughly four million kilometres across. Limitations on mass, radius and absolute magnitude using evolution theory place possible value about ~30 to 40% higher than the ones quoted above.

In all, this is an interesting system and challenging for those with moderate apertures. Due to the brightness of the two stars, cleanly separating the two is highly dependant on the seeing. Using a 20cm C-8 at 225X and 333X, I have observed the pair on more than three or four dozen times, seeing the two Airy disks clearly separated perhaps  only 40% of the time. (Incidentally, I have tried without success to observed this pair using the 30cm (11.75") Refractor at Sydney Observatory in 1977 and 1978, also limitation by the seeing!)

Next: Sagittarius “Little Gem” - the Magnifique PNe  NGC 6445.

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