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Southern Cross - October 1998

• Constellation of the Month - Aquarius
• NGC 6589 et al: A nebulae complex in Sagittarius
• Book Reviews

Constellation of the Month - Aquarius

Albert Brakel

Aquarius the Water Bearer is a large zodiac constellation 980 square degrees in area. Apart from its Water Jar asterism just south of Pegasus, it does not have any conspicuous pattern of stars, and its brightest star is only 3rd magnitude. Normally it is best located by its position between the Great Square of Pegasus and the star Fomalhaut, but this year an additional help is the planet Jupiter near the Pisces - Aquarius border. The stars Alpha, Zeta and Eta Aquarii of the Water Jar lie almost right on the celestial equator, and can be used to determine the diameter of the field of your telescope at a given magnification - simply let one of the stars drift centrally across the field from one side to the other (with the motor drive off), and the time this takes in seconds divided by 4 gives you the field diameter in minutes of arc.

Aquarius is sufficiently far from the plane of the Milky Way that it does not contain any open clusters. It does, however, contain 3 globular clusters, 2 remarkable planetary nebulae, and lots of faint galaxies, as well as the usual sprinkling of double stars. Let's start with the Messier objects in the constellation, and see what they looked like through a C8 from a moderately light-polluted backyard..

Messier Objects and Globular Clusters

M2 (NGC 7089) (RA 21h33.5m, Dec. -0 49'). This is a fine globular cluster some 8' in diameter and magnitude 6.5, lying about an Alpha - Eta length west of Alpha Aqr, and visible in the finderscope as a small fuzzy blob. It appeared as an unresolved milky glow at 77x, started to get grainy at 117x, and with 190x stars started to be resolved across it.

NGC 7492 (23h08.4m, -15 37'), three and a half degrees following Delta Aqr, is a globular cluster some 4' across, but with a rather faint 11.5 magnitude, and not resolved. That's not surprising, because it's very distant at about 80,000 l.y. (half-way to the LMC) according to the most recent figure.

In the far west of the constellation, three and a half degrees SSE of Epsilon Aqr at 20h53.5m, -12 32', is globular cluster M72 (NGC 6981). At magnitude 9.4, being fainter as well as smaller than M2, it was not visible in the finderscope and difficult to spot using 77x power in a less than dark sky. Use of a deep sky filter or higher powers helped increase the contrast a bit, but it remained as a faint glow.

Just over a degree following M72 we find M73 (NGC6994) (20h59.0m, -12 38'), the least remarkable of all Messier objects and one of the curiosities of deep sky cataloguing. It is actually a small group of 4 faint stars (mag. 8.9) in the shape of a reverse Greek letter lambda, or reverse "y", depending on how you view it. The stars may lie by chance near the same line of sight, but more probably they are physically related, as noted by Eric Pozza five years ago. Messier claimed to have seen "a little nebulosity" in the group, but photographs have not detected any; perhaps Messier's telescope had started to dew up without him realizing it. Though the group is unspectacular to look at, at least you'll be able to say you've seen it.

Planetary Nebulae

The well-known Saturn Nebula NGC 7009 (21h04.2m, -11 22') is located about 2 degrees NE of M73 and a bit over a degree preceding Nu Aqr, and while not visible in the finderscope, it was readily distinguished from stars at low power (77x) as a small bright circular patch with a distinctly greenish tinge. The magnitude of 8.3 and a diameter of only 25" ensures that its surface brightness is high, and the colour is strong enough for even eyes not used to picking up subtle hues to do so without difficulty. With higher powers, an east-west elliptical elongation became more apparent. The nebula gets its name from two arms of nebulosity projecting like ansae from either side of the disk on photographs, but these are very difficult to observe by eye. Hubble images show that the "ansae" are two blobs of gas called "fliers" that are moving faster away from the central star than the rest of the gas in the envelope, for reasons that are still not clear. Those with 25 cm or larger apertures could try for them from a dark sky site on a night of exceptional seeing.

The Helix Nebula (NGC 7293) (22h29.6m, -20 49') would have to be the most interesting object in Aquarius, and it even appeared on an Australian stamp in 1992, as part of the National Space Week issue. On photographs it has a shape resembling a helical coil, due to multiple outbursts from the central star. As the nearest planetary nebula to us (about 520 l.y.), it has a diameter of 13' and an integrated magnitude of 6.5, making it in theory the brightest planetary in the sky. In practice, the light is spread over such a large area that the perceived surface brightness is low and has a low contrast against the sky background. To find the Helix, look in the southern reaches of Aquarius between the stars Upsilon and 57 Aqr, about a third of the way from Upsilon.. A nebular filter will definitely help. I detected it with 77x power as a very subtle circular glow almost half the diameter of the Moon. With 117x and a Lumicon deep sky filter, the dark central "hole" became visible. Using an O III filter made the nebula brighter, and the central "hole" could now be seen with 77x, but it was still a low surface brightness object. While the eastern and western sides were the brightest, the helix structure was not obvious. The central star is 13th magnitude, and several brighter foreground stars are scattered across the nebulosity.

Double Stars

12 Aquarii (21h04.1m, -05 49') is a yellow and white double, magnitudes 5.9 and 7.3, separated by 2.4". They were split with a power of 117x.

29 Aqr (22h02.4m, -16 58'), another yellow and white binary, was just separated at a magnification of 77x in good seeing conditions, but looked better at 154x. They are 3.7" apart, and the primary is an eclipsing variable that ranges from mag.6.4 to 6.8 over a 22h 40 m period, while the secondary is mag.7.2 .

41 Aqr (22h14.3m, -21 04'). Yellow and straw-coloured unequal mag. 5.6 and 7.1 stars, with 5.1" separation. Resolved at 77x, they improved at 117x and 156x.

51 Aqr (22h24.1m, - 04 50') , about 5 degrees south of the Y of the Water Jar. White, components mag. 6.5 and 6.7, at a very tight 0.7". I was unable to resolve them in 1995 using 380x on a good night, or this year using 235x on an average night. But the pair is widening, so it is worth checking in future years. Very good seeing and high power will be needed to split them, so if the air is not steady enough, leave it to a better night.

53 Aqr (22h26.6m , -16 45'), about 12 degrees south of 51 Aqr. The two yellow mag. 6.4 and 6.6 stars were once thought to be an unconnected optical pair, because the proper motions appeared to be different, but they are now known to be in orbit around each other. The separation was 10" in 1823, 4.1" in 1968, and is now down to 2.2". They were just resolved at 117x, in less than ideal conditions.

Zeta Aqr (22h28.8m, Dec. -00 01'), almost right on the celestial equator, is the star at the centre of the Y in the Water Jar asterism of Aquarius. It is an equal, bright yellow pair, with magnitudes 4.4 and 4.5 at a distance of 2.0". It was divided by 117x on a night of mediocre seeing, but was better with 154x.

94 Aqr (23h19.1m, -13 28'). Attractive yellow (mag. 5.2) and pale orangeish (mag. 7.6) components situated 12.6" from each other, which appeared wide even with 50x . Very suitable for small 'scopes.

107 Aqr (23h46.0m, -18 41') is a pretty pair, bright with magnitudes 5.7 and 6.7, at an easy 6.7", for which a magnification of 77x was quite sufficient.

Variable Star

R Aqr (23h43.8m, -15 17') in eastern Aquarius is a Mira-type red giant, with an invisible white dwarf companion that is pulling gas off the outer layers of the larger star. The brightness varies irregularly between mags. 6 and 12 (roughly), with an average period of 386 days. It is currently getting brighter, and should reach maximum by December, when it will be too close to the Sun in the western sky to be observed. The system is enveloped in the very faint nebulosity Cederblad 211, which Californian amateur Steve Gottlieb was reportedly unable to see even with a 17.5" aperture from the Sierra foothills. With the Bobroff 20", who knows?

Galaxies

The galaxies of Aquarius are not particularly bright, and are difficult to spot with a medium-sized telescope in a suburban sky, so I had to resort to a darker site . For example, NGC 7596 (23h16.9m, -06 56'), a lenticular galaxy, was for all practical purposes invisible in the suburban sky, but calling upon all my skills for observing faint diffuse objects, I was aware of a vague "something" intermittently there at the indicated position. From the darker site it was less difficult, appearing as a faint diffuse misty patch with some elongatation.

About 2 deg. to the south, NGC 7606 (23h19.1m, -08 29') has a more pronounced elongation, and was just a little brighter thanks to a nuclear region. It is an Sb spiral that is not quite edge-on.

NGC 7723 (23h38.9, -12 58') and NGC 7727 (23h39.9, -12 18') are a pair only about a degree apart in eastern Aquarius. The first is an Sb-type spiral in tilted orientation, the second is a slightly larger barred spiral.

Finally, in the far southwest of the constellation, about 6 deg. west of the Helix Nebula, is NGC 7184 (22h02.7, -20 49'). It's another large Sb spiral, and resembles a grey needle of light with a sharply-defined core. Although supposedly dimmer than the other galaxies (mag. 12 vs. mag. 11), its edge-on orientation concentrates the light into a smaller area, making it no more difficult to see.

NGC 6589 et al: A nebulae complex in Sagittarius

Jenni Kay, FRAS

Problem: NGC 6589 et al is a nebulae complex which has more catalogue designations than there are apparent objects. The historical "New General Catalogue of Nebulae and Clusters of Stars (1888), Index Catalogue (1895), Second Index Catalogue (1908)" (NGC/IC) by J.Dreyer, assign no less than three NGC designations and three IC designations to this complex. However, direct visual observations of the complex through a 12.5" f/5 Newtonian, as well as viewing a high resolution 3.9 Metre Anglo-Australian Telescope (AAT) image of the complex, reveal three distinct objects. What follows is a review of the historical data, followed by the modern catalogue data; and finally, my analysis of all the data and my conclusions as to which objects exist, and which are either nonexistent, or duplicates. Please refer to the annotated AAT image for a better understanding of the area.

The historical data as taken from the NGC/IC catalogues.

NGC 6589: 18 16 16 -19 47 48 (NGC) Swift "Double star in centre of extremely faint, pretty large nebula."
NGC 6590: 18 16 19 -19 52 48 (NGC) Swift "Double star in centre of pretty faint, pretty large, round nebula."
NGC 6595: 18 17 05 -19 52 26 (NGC) J.H #2002;HGC#4395. "Faint; pretty large; considerably extended; double star involved."

IC 1283: 18 17 16 -19 44 19 (IC) Barnard #3111 " star 9.3 nebulous."
IC 1284: 18 17 40 -19 39 56 (IC) Barnard #3101 " star 7.6 in nebula, 15' diameter."
IC 4700: 18 17 06 -19 52 20 (IC) Barnard #4239 " star 9.4 in dense nebula."

The modern catalogue data:

The ESO Uppsala Survey of the ESO (B) Atlas
NGC6595 = IC 4700: 18 17 04 -19 51 56
IC 1284 : 18 17 39 -19 40 18
NGC6589 = IC 1283: 18 16 51 -19 46 40

Lynga Open Cluster Catalogue
NGC 6595 : 18 16 58 -19 52 52

Lynds Catalogue of Bright Nebulae
LBN 43 = NGC6590: 18 16 57 -19 43 52
LBN 46 = NGC6589: 18 16 57 -19 38 51
LBN 47 = IC 1283 : 18 16 57 -19 28 52

Analysis

While the above positions and identifications show that the modern catalogues could not agree as to which designation belongs to which object, the problem originated with the listing of six designations in Dreyer's NGC/IC catalogues. It would be reasonable enough to suspect that Dreyer assumed that all six "new" discoveries corresponded to six separate objects in the sky. Since the historical co-ordinates differ from each other he had no reason to suspect there were duplicate observations. Furthermore, the six discoveries were found by three different astronomers, each using a different sized telescope, and with various position-making error margins. However, by comparing the historical descriptions and positions of each designation to the objects in the sky, it becomes apparent that there are multiple duplicate observations. Following is an examination of each NGC/IC designation:

NGC 6589 : At Swift's given position there is nothing in the sky. However, about 9' east lies a bright nebula, with a double star involved. Swift's description matches this object quite well, and it is probably this object he was looking at but he was out in his position measurement. But, before giving Swift credit for discovering this object, Herschel logged this object too, and measured a better position. Therefore, I would propose this object be given to Herschel, giving the name as NGC 6595, with NGC 6589 being a duplicate observation.

NGC 6590 : Again there is nothing at Swift's given position, but ~9.5' east lies the second, bright, nebula with a double star involved. This is probably what Swift intended for his discovery. NGC 6590 is a valid designation for the object here.

NGC 6595: Somewhere along the line this object was called a star cluster, which was carried forward into the Lynga catalogue. However, there is no star cluster here, and Herschel did not call it a star cluster in the Herschel General Catalogue. The position and description of Herschel's object, Dreyer naming it NGC 6595, is a perfect match for the bright nebula, with a double star involved which does lie here. As previously mentioned, Swift logged the same object but gave it a poor position. Since both astronomers were probably looking at the same object, both descriptions fit the object in the sky, but as Herschel logged the better position, I would suggest the primary designation be given to Herschel. Therefore, NGC 6595 is a valid designation for the object here.

IC 1283: At Barnard's precise position there is a bright 9.1 mag star lying near the edge of the primary nebula IC 1284. Barnard describes his object as a " * 9.3 neb" and he may have considered this to be a discrete object within the primary neb. Did the "9.3" (9.1 mag) star appear nebulous through his telescope, whereby he logged this as a new object? At this stage, in my opinion, it seems that Barnard's object may be only a star, therefore, I would class this as a nonexistent object.

IC 1284 : Barnard's position and description matches the object here. IC 1284 is a valid designation for the object here.

IC 4700: Barnard's position and description matches the already discovered NGC 6595 (=N6590), and appears to be another duplicate observation of this object.

Conclusions

A visual observation with a 12.5" f/5 Newtonian.
Location: Lobethal, SA.
Skies: Limiting naked-eye magnitude 6.1; ANT II.

The N6589 nebulae complex lies about 5.5 degrees northeast of M8 (Lagoon). Overall view: 83X (47' field of view) - the nebulae complex is not so bright that it catches the eye, but in a study of the field, each member of this complex can be picked out easily enough. The brightest member is NGC 6595, being a relatively bright, relatively large, irregularly round hazy glow, with a prominent double star at the centre. NGC 6589 is the next obvious nebula, appearing as a rather faint, relatively large, possibly round, hazy glow with a double star of uneven brightness involved. IC 1284 is noticed with averted vision as a faint, extensive nebulous patch, with some scattered stars involved. Within the IC 1284 lies a very bright 7.5 mag star which seems to have a nebulous halo. The UHC filter enhances the view, showing the nebulae more strongly.

151X (20' field of view), offered the best view for detail, and following is a closer look at each member of the complex at this power.

NGC 6589

As seen at the lower power, this nebula appears as a rather faint, relatively large, round hazy glow with a double star of uneven brightness. The stars may be around 10/12 mags. Not seen at the lower power however, is a second, faint round glow, east-southeast of the first. The two round glows may be bridged by a narrow strip of nebulosity, or they are barely detached. Both hazy glows are similar in size and brightness. The first may have been more easily detected having the prominent double star involved. This second glow has a faint 14 mag star at the centre, and could easily be overlooked if not given care and attention. Both round, hazy glows are 1.5' in diameter. N6589, being made up of both nebulous patches, has an overall size of 4.0' x 2.0'.

NGC 6595 =NGC 6590 = IC 4700

This is the most obvious nebula in the group appearing as a bright, round, hazy glow, with a double star involved. Both stars are about 10 mag. The nebula has a size of 3.0'.

IC 1283

There is only a star here, which lies in the southwest region of the IC1284 nebula. The nebulosity around or near this star does not appear to me to be any stronger or more obvious in any way to warrant an additional designation.

IC 1284

Requiring averted vision, the nebula is faint, very large, about 15' in diameter, with a very irregular overall shape. There are about twenty stars ranging from 7.5 - 12 mag which are scattered across the nebula. The brightest 7.5 mag star, which lies near the centre of the nebula, seems to be wrapped in a soft, nebulous halo.

Book Reviews

Ross Gould

Following is a slightly revised version of some book reviews, given on the theory that these volumes may still be obtainable from Brian Warner at the Cape Observatory.

John Herschel, 1792-1992: A Bicentennial Symposium. Royal Society of South Africa, 1992, 144 pages, illustrated.

John Herschel was a late example of the "Renaissance Man", the polymath who excels in a variety of fields. His work in astronomy is very important, but he made significant contributions to chemistry and to photography, was a capable artist, translated Homer in his late years, and influenced the thought of such notables as Charles Darwin and John Stuart Mill. 1992 was the bicentennial year of the birth of John Herschel, providing reason for a symposium reviewing his life and work. This book has interesting and very readable articles which cover a range of matters, particularly those connecting with John Herschel's South African years of surveying the southern sky, but not confined to that place and period. There is also much here on various matters scientific and photographic in the period, and some glimpses of South African Colonial society, contrasting with the English establishment. Although some of the material is familiar, a good deal of it goes beyond what is commonly known and provides new insights and information.

Included are stories of other notables of the time, for example the disputatious and ferocious Sir James South, who nevertheless had an amicable collaboration with John Herschel in his early astronomical work in England. South provided the precision equatorially-mounted refractors used in accurately measuring doubles, first in the north, later in South Africa as well - the rough alt-azimuth mounted Herschel reflectors were far from ideal for such work, useful though they were for finding new pairs. It is worth repeating that the 18-inch reflector JH used for his survey work was the most effective and powerful instrument in the world at that time. William Herschel's 48-inch had been largely unsuccessful.

The large reflectors of Lord Rosse's (his first was a 3-foot reflector), and William Lassell (a 4-foot), were built after John Herschel had returned to England. The Herschel 18-inch telescope had been used by both father and son for their surveys of the northern sky, and now examined the southern regions which had been sampled very inadequately from Parramatta by James Dunlop a decade earlier, with much inferior instrumentation, and (as John was to discover) inadequate care in measuring the position of objects, so that most of Dunlop's "discoveries" could not be identified.

The noted astronomical historian Michael Hoskin provides an assessment of John Herschel's overall contributions to astronomy, seeing him as building on the pioneering foundations his father had laid, and also through his writings having "a major influence on the direction of astronomical progress". Brian Warner overviews JH at the Cape, including his non-astronomical work and activities. He provides detailed and fascinating material on Herschel's methodology in his "sweeps" of the sky, and reproduces an astonishingly long list of what had to be done in telescope preparation to start a sweep. A modern amateur who thinks his telescope complicated would find this list instructive. L.J. Schaaf, historian of photography, reviews JH's work in photography, and his drawing with the camera lucida, providing as well a contrast with Fox Talbot, one of the inventors of photography. John Herschel's huge contributions to photographic processes are well described, including his experiments with colour in the 1840s.

An article by MJ Crowe describes the methodology of JH's work as a scientist, and his influence on others (Whewell, JS Mill, Darwin, James Clerk Maxwell, etc). There is much on the condition and methodology of science in pre-Victorian and early-Victorian Britain. Crowe concludes that "among leading British scientists of the first half of the nineteenth century, it was only John Herschel who mastered and productively employed the full range of ... mathematical, observational, and experimental techniques that have made modern physical science possible". The essays on Herschel and Cape Flora, and Herschel's influence on Education, provide side views of this very multi-faceted man.

Appendices include an address by JH to mathematical students; and a bibliography of his writings, which is much more comprehensive and informative than previous listings. The only obvious deficiencies of this volume are its lack of a contents page, and the absence of an index. Although not a straightforward biography (Gunther Buttman supplied that in 1970), this work through its varied parts provides a first-rate overview of John Herschel, who is too often seen only as a shadow of his illustrious father. This volume helps to give a more balanced view of one of the great scientists of the 19th century, who had achievements of the first order, and an unparalleled range of abilities.

This book might still be ordered from the Royal Society of South Africa - its price was given as 12 pounds stg inc. p&p for UK. The address was The Assistant Secretary, Royal Society of South Africa, P.D. Hahn Building, University of Cape Town, 7700 Rondebosch, South Africa.

Maclear & Herschel: letters and diaries at the Cape of Good Hope 1834-1838, edited by Brian and Nancy Warner. Cape Town: 1984.

Charles Piazzi Smyth, astronomer-artist: his Cape years 1835-1845, by Brian Warner. Cape Town: 1983.

These two books centre on the period when Sir John Herschel did his great southern sky survey from the Cape of Good Hope, in the years 1834-38. In 1833 the British Admiralty offered the position of Cape Astronomer to Thomas Maclear; a medical doctor by training, with an interest in astronomy, he decided to accept the position partly because he knew John Herschel, and his family were planning to spend some years there. John Herschel was the most notable astronomer of the time, and he provides a centre around which other people and matters can be examined. Herschel's own diary of the period can be found in Herschel at the Cape, ed. David Evans et al. (Univ. of Texas Press, 1968).

In an alternative universe, I reviewed Brian Warner's Astronomers at the Royal Observatory, Cape of Good Hope (see Southern Sky, March/April '94). Included in that work is an outline of two figures drawn in much greater detail in the books reviewed here: Charles Piazzi Smyth and Thomas Maclear. Thomas Maclear was a conservative personality. The great period of Maclear's life was his early years at the Cape, when Herschel was also there (they both arrived in 1834). They were not close enough in physical distance to see each other daily: a correspondence resulted, and there is a nice contrast of styles between Herschel's precise but energetic letters (usually first drafts), and Maclear's more plodding notes (revised before sending). That in itself is worth the admission price. Maclear's diaries help to place the letters, and fill out the picture; as well there are Herschel's letters to him, being most of JH's letters in the Cape period. All this provides, in Warner's words, "a rare view of the daily lives and interactions of two 19th century scientists who were foremost in their fields." The Maclear & Herschel volume would benefit from being read in conjunction with Herschel at the Cape, but is sufficiently complete to be enjoyed by itself if this is unobtainable. It has also a useful section of biographical notes at the end, outlining most of the people mentioned in the main texts.

Warner's book on Piazzi Smyth is a fine portion of a life. Young Charles Piazzi Smyth, son of Admiral Smyth of Cycle of Celestial Objects fame, was only 16 when he arrived in 1835 to take up his position as First Assistant at the Royal Observatory under Thomas Maclear. Smyth, as well as becoming a notable astronomer, was a gifted artist who made many sketches during his Cape years, and 70 of these are reproduced in Warner's book. And he was a pioneer of photography who took some of the earliest photographs in South Africa. It should be remembered that photography "went public" in 1839, when Daguerre announced his process. John Herschel had already returned to England from South Africa: hearing of Daguerre's invention (though knowing no details) he invented his own photographic process within a few days. Continuing to work on this, he wrote to Maclear about his techniques. The letter arrived at the Cape in July 1839. Other material also came to hand, and by November 1839 Piazzi Smyth was experimenting with photography. However it was not until early 1843 that he appears to have succeeded in producing permanent photographs (some earlier examples have faded). These appear to be the earliest South African photographs.

Much of the book is devoted to work by Maclear and Piazzi on checking the results of Lacaille in measuring the arc of the meridian in South Africa in the 1750s. Newtonian theory had predicted that the Earth, because of its rotation, should be flattened at the poles - if Newton was correct, the length of a degree at high latitude should be greater than a degree measured nearer the equator. Lacaille's work suggested an Earth less curved in the southern hemisphere than the northern, which was odd. The Astronomer Royal, Nevil Maskelyne, in 1775 had suggested a resolution: that the mass of a large mountain can affect measures by attracting the plumb-bob which establishes the prime vertical. Sir George Everest, visiting the Cape in 1820, pointed to this effect and recommended a much longer arc be measured. Maclear was given this task when he became Cape Astronomer. Piazzi was much involved in the work, especially the extension of the survey through difficult country well north of Cape Town. The difficulties of survey work in primitive conditions in the early 1840's are shown, with the contrast from the sections on scientific society in Cape Town.

Back at the Observatory, Piazzi developed an irrigation system which allowed extensive gardens around the Observatory; he also made observations of the Great Comet of 1843, and was involved in the erection of the Herschel Obelisk (where Sir John's telescope had stood), and other matters. There is a short section on Piazzi's later life after the Cape Years - he was appointed to Edinburgh, succeeding Henderson (who had been Maclear's predecessor at the Cape). Piazzi Smyth lived until 1900.

Both books will appeal to those who wish to extend their knowledge of 19th century astronomers and their work, particularly in colonial circumstances with all the difficulties that implies. Smyth and Maclear are contrasting personalities. That John Herschel casts a large shadow in each book (as in each life) adds to the interest of the works. They are recommended for the historically minded, and those who like to fill in the details of lives and events, to see what lies behind the science. For the general reader, and those wanting an overview, Warner's Astronomers at the Cape (mentioned above) is recommended.