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Southern Cross - November 1998

• Constellation of the Month - Tucana
• An Impact Structure on the north coast of NSW?
• NGC/IC Star Clusters and Nebulae of the SMC - Part 1

Constellation of the Month - Tucana

Steve Crouch

Tucana can be hard to observe for equatorially mounted telescopes given its proximity to the south celestial pole but it contains three of the most spectacular deep sky objects in the sky - the Small Magellanic Cloud, and the globular clusters 47 Tucanae and NGC 362. There is not much else that is particularly inspiring but the preceding three objects more than make up for this.

I've only mentioned a few of the large collection of star clusters and nebulae in the SMC but for anyone that wants to take the time, there are hours that can be spent with a medium to large telescope. I would recommend a very large-scale map if you can get one because it is easy to get confused in the many crowded fields. Some of the currently available software packages such as GUIDE and MEGASTAR are ideal in this regard. The Herald-Bobroff Astroatlas is probably the best (and certainly the most economical) paper atlas to use in navigating around the SMC.

In the list that follows, I've added a new category, "SMC Object", because it is more convenient to treat them separately from the rest of the constellation. The observations were done using a C14 and 35mm and 27mm Panoptic eyepieces, giving magnifications of about 110x and 144x.

1. Double Stars
   
   Delta Tucanae (22h 27.3m, -64d 58m) is a wide unequal double (4.5, 8.9, 7".2) that should be easy in all apertures. The colours are yellow and white.
   
   Beta Tucanae (00 31.5, -62 58) is a very bright wide double (4.4, 4.5, 27") with a third star (5.2) 10' to the south-east.
   
   All three are themselves double but very close. Only the second star is worth trying to split as it is now near its maximum separation of 0".6. Kappa Tucanae (01 15.8, -68 53) is also worth observing. The components are yellow and orange and the separation is 5".
   
   
2. Galaxies
   
   I only managed to observe NGC 7408 (22 55.9, -32 32). This galaxy is definitely not bright and all I saw in the C14 was an ill-defined haze. It is apparently a barred spiral.
   
   
3. Globular Clusters
   
   The outstanding example is, of course, 47 Tucanae, also catalogued as NGC 104 (00 24.1, -72 05). This strongly condensed globular is a spectacular sight in almost any telescope. I actually prefer it to Omega Centauri because of the more apparent central condensation. Slightly to the northeast and in the same low power field is the much fainter NGC 121 (00 26.8, -71 32). The most remarkable aspect about this globular is its very marked elliptical shape. It is an easy object but not resolvable. There is an incredibly faint galaxy (PGC 1536) which I thought I suspected in the same field. As this galaxy has a magnitude of 16.3, I think that I must have been mistaken.
   
   Tucana's other spectacular globular is NGC 362 (01 03.2, -70 51). It has a similar degree of central condensation to 47 Tucanae and is well resolved. NGC 362 would, I believe, rank in the top 5 best globular clusters in the sky but, as its near neighbour 47 Tucanae overshadows it, it is not as well known as it deserves to be.
4. SMC Objects
   
   The following selection only covers the northeast central portion of the SMC, which is, in my opinion, the best part. The previously mentioned NGC 362 is a good starting point for navigating around this area. If you centre your telescope on this object and then go about half a degree to the south, you will come to one of the SMC's own globular clusters NGC 361 (01 02.2, -71 33). This object is faint but quite easy and shows some central condensation. Continuing another half a degree to the south brings us to NGC 371 (01 03.3, -72 05), a nice SMC star cloud. There is also emission nebulosity involved, as the use of an O III filter will show. Just to the northeast are two similar but smaller objects NGC 395 (01 05.3, -72 00) and IC 1624 (01 05.3, -72 02).
   
   If we go a short distance to the west of NGC 371, we come to NGC 346 (00 59.1, -72 11). If the SMC has a showpiece object, this is probably it. It is a bright emission nebula with stars involved and is easy to see in a 50mm finder. Some atlases do indeed plot it as a star cluster.
   
   Just to the west of NGC 346 are two smaller patches of nebulosity, NGC 299 (00 54.0, -72 10) and NGC 306 (00 54.9, -72 13). NGC 299 is the brighter of these two. My final object in this selection is NGC 330 (00 56.2, -72 29) just to the south west of NGC 346. This cluster is resolvable with sufficient power and can best be described as a small knot of stars.

An Impact Structure on the north coast of NSW?

Albert Brakel

A possible impact structure has been identified near Laurieton, between Taree and Port Macquarie, on the north coast of New South Wales. The finding was reported by P.C. Tonkin in last month's issue of the Australian Journal of Earth Sciences.

The structure is 30 x 35 km across, and dates from somewhere between 240 to 250 million years ago, the early part of the Triassic Period. (The Triassic was the geological period before the Jurassic, and the first third of the age of the dinosaurs). Within a few million years of its formation, the structure was filled by sedimentary rocks, similar in age to the sandstones of the Blue Mountains west of Sydney, and which like the latter have been eroded to form prominent cliffs in places. It has long been known as the Lorne Basin, but this is the first suggestion that it has an impact origin.

Evidence for impact includes the roughly circular outline, a central uplift of older rocks that may represent rebound of impacted strata, and fractures in the older rock filled by glass that contains small amounts of elemental iron-nickel, an iron-rich mineral, spherules and melted fragments. The glass is similar to that found at several impact sites elsewhere in the world, where it was formed by the rapid cooling of impact melts. The spherules are light-colored, glassy spheres or ovoids 1-2 mm across, embedded in the darker green-brown glass, and the larger ones have cores of crystalline quartz or feldspar. The melted fragments have glassy rims. The presence of metallic iron indicates formation at a temperature as high as 1515 deg.C., but does not by itself prove an impact origin. There are also a number of granite bodies clustered within the basin but not outside it, and dated at 210 million years old, younger than the sediments filling the basin. The suggestion is that they formed when molten rock from a depth of several kilometres took advantage of impact-formed fractures extending under the basin to rise up and solidify closer to the surface.

Tonkin is careful to say that further investigation is needed and that more indicators of impact should be looked for, such as melt sheets, planar deformation features, shatter cones and impact breccias. If an impact origin can be confirmed, the impactor would have been an asteroid 2-4 km in diameter, and the structure would be the first impact structure recognized in New South Wales.

Reference: Tonkin, P.C., 1998 -- Lorne Basin, New South Wales: evidence for a possible impact origin?
Australian Journal of Earth Sciences, v.45, p. 669-671.

NGC/IC Star Clusters and Nebulae of the SMC - Part 1

Jenni Kay, FRAS

Making visual observations through the telescope of Star Clusters and Emission Nebulae in both the Small Magellanic Cloud and Large Magellanic Cloud is an ongoing project of mine. This all began in 1996 while I was using an 8" f/10 SCT, an instrument that is generally considered to be a rather common and moderate sized telescope. Late one night in September, after a full night's observing in Sagittarius, I turned the 8" SCT towards the Small Magellanic Cloud (SMC), more out of curiosity rather than expecting to see much detail in it. To my surprise I found the SMC to be littered with numerous small hazy knots, all differing in brightness, but easy enough to pick out against the bright background glow of the Cloud itself.

The following observations are the results of two years work sifting through the SMC making visual observations through the 8" SCT. I would add here that there are additional clusters and nebulae easily visible through the 8" SCT which are not NGC/IC objects, these other objects bear such designations as Kron, Henize, etc. I have not included these other objects in this article as this article concerns itself only with NGC and IC objects seen in the 8" SCT.

All observations were made from my semi-rural home site at Lobethal, South Australia. The skies here typically have a limiting naked eye magnitude of 6.1; with seeing at Antoniadi II-III. The coordinates given for each of the NGC/IC objects, and the object class type, were taken from "The ESO/Uppsala Survey of the ESO (B) Atlas" (ESOB). I have found that the ESOB offered accurate positions more consistently than many of the other popular modern catalogues.

NGC 104 (47 TUC) : 00 24.1 -72 05 Globular Cluster.

Known as one of the finest objects in the southern sky, this globular cluster is an easy target for the smallest of optical instruments. In at least 7x50mm binoculars, the globular appears as a brilliant haze surrounded by a much softer and extended glow. At 66X, through the 8" SCT, the stars sprawl across the field in a magnificent display. There is a brilliant, grainy core, 2.0' in size; enveloped in a large mass of stars and haze, 7.0' in size; and numerous outliers stretching to 28.0' in diameter. At 140X: there are at least 150 stars resolving well and these are scattered throughout. The whole view reminds me of a splendid firecracker bursting into trails and countless points. All magnifications offer appealing views but the higher the magnification the stronger the visual impact.

NGC 121 : 00 26.7 -71 32 Globular Cluster

The ghostly glow from this globular cluster can be found some 46' north of NGC 104 (47 Tuc). Where the latter is bold and brilliant, the former is soft and timid. At 66X, the globular appears as a small, round, somewhat bright haze, about 30" in diameter. This object is bright enough to be noticed in a general sweep of the area. At 110X, the globular seems a little brighter towards the centre.

NGC 152 : 00 32.9 -73 07 Open Star Cluster

66X: The very faint cluster is a challenge to detect but in knowing the exact position to look for it, the eye is pulled to a hazy "something". 110X: confirms this haze is real. The cluster is still a very faint, large, round, hazy glow, about 2.0' in size. I cannot resolve any individual star members, only the soft, collective glow is seen. Since the glow from this cluster is only slightly above the background sky it is best to use a slow rocking motion of the field, averted vision, and in moments of better seeing the cluster stands out well enough for a confident sighting.

NGC 176 : 00 35.9 -73 10 Open Star Cluster

66X: The very soft glow from this cluster is somewhat difficult to locate but in a careful study of the field the eye is drawn to a very faint, hazy smudge. 110X: with averted vision, and a slow moving field, a small, faint, hazy glow stands out well enough for a confident observation. The round haze is about 30" in size. At the northeastern edge lies a faint star; a 6.7 mag field star lies 11.8' to the east; and there is a 7.8 mag star 3.4' southwest of the cluster. These field stars act as good signposts to guide the observer to the position of the cluster.

NGC 220 : 00 40.5 -73 24 Open Star Cluster

With some care, this cluster can be seen as a small, round, relatively faint haze that lies immediately southwest of an 11.5 magnitude field star. Care is required as a quick look at this object may only notice a single elongated haze with the 11.5 mag star lying in the centre. However, there are in fact two separate clusters here, two round hazy glows, with the second haze northeast of the star being NGC 222. 110X: NGC 220 appears as a smooth glow that is nearly 45" in size. In the rest of the field there is a light sprinkle of 12 magnitude and fainter field stars. To the southeast of the field begins a noticeable, extensive, milky background sky, which is the much denser parts of the SMC itself.

NGC 222 : 00 40.7 -73 23 Open Star Cluster

110X: 1' northeast of NGC 220, and immediately northeast of an 11.5 mag field star, lies the small, round, faint glow of this cluster. The size is almost 45" and the glow is noticeably fainter to that of its neighbour NGC 220.

NGC 231 : 00 41.1 -73 21 Open Star Cluster

It has taken me many attempts to see this cluster, and I have only ever seen it once on a particularly transparent night. 110X: northeast of the two hazy glows of NGC 220/ 222 lies a single chain of a few very faint stars forming a curving line open to the west. Slightly west of this chain is the softest milky haze I have ever seen. The glow is hardly above the background sky itself. The extremely faint, round glow, about 1.0' in diameter, requires averted vision and a still eye, waiting for the image to build up enough to be certain it is real. Since all three of these NGC objects are grouped closely to each other it could easily be taken that they are all belonging to one irregular mass. Surrounding this group there are only a few field stars which further makes the group stand out as a single object. At 66X, all sign of NGC 231 is lost, too faint.

NGC 241 (= NGC 242) : 00 43.5 -73 26

This object is nonexistent being a duplicate of NGC 242. The position for NGC 241 is only 1' northwest of NGC 242, but there is no cluster, or the faintest haze, to be seen for NGC 241. Furthermore, NGC 242 is the only object to be found in the near vicinity of the NGC 241 position. On an historical perspective, it was Sir John Herschel who discovered both objects. However, while he noted NGC 241 on one night, and NGC 242 on three other nights, he did not log both objects on the same night. With the two clusters supposedly only separated by 1 arcminute, this suggests Herschel was looking at the one, same object. Furthermore, Herschel gives both objects very similar visual descriptions, which adds strength to the case that Herschel was looking at the same object. The conclusion that NGC 241 = NGC 242 has been noted by some of the modern catalogues.

NGC 242 : 00 43.5 -73 26 Open Star Cluster

66X: The cluster stands out well in the field appearing as a small, somewhat bright, hazy knot, about 30" in size, and a little elongated southeast to northwest. 110X: there are two stars resolving in the haze, or there are two very small brighter knots, but these come and go in moments of good seeing. Apart from a very close pair of 11-12 magnitude stars 1.5' to the southwest, there are few other features in the field to point to this object. Even so, the hazy knot is bright enough to be easily picked out in a slow sweep of this area.

NGC 248 : 00 45.4 -73 23 Emission Nebula

The nebula lies along the eastern side of a prominent triangle of three 9-11 th magnitude field stars. Placed a little southeast of the northern most and faintest star in this triangle, the cluster appears as a small, round, somewhat bright haze. 110X: with averted vision, the haze appears binuclear; as two brighter knots wrapped in a faint glow. The overall shape is a little elongated southeast to northwest, with a size of 30". With an OIII and UHC filter in place the haze is enhanced. In the wider view there are other soft hazy patches nearby belonging to NGC 256, NGC 265, and NGC 269. The whole field is a charming mix of stars and small hazy glows.

NGC 249 : 00 45.5 -73 05 Star Cluster + nebulosity.

In a field with only a few stars lies a lonely pair of soft, round glows. The glow in the west is the cluster plus nebulosity NGC 249, the second is NGC 261. 110X: NGC 249 appears as the larger haze of the two, being about 1' in size. This haze shows a smooth, even glow throughout. With an OIII, or UHC filter in place both glows appear well pronounced, standing out boldly in the field like two bright cat's eyes.

NGC 256 : 00 45.9 -73 30 Open Star Cluster + nebulosity.

About 8' southeast of NGC 248, and 1.5' southwest of a bright 9.4 magnitude field star, lies this star cluster. 66X: The cluster appears little more than a faint, fuzzy, fat star. 110X: Better view - the cluster is a small, round haze, which is 20" in size, and there is a slight brightening towards the centre. There is no response to an OIII or UHC filter, therefore any nebulosity present is too weak for me to detect. NGC 265 is nearby to the east.

NGC 261 : 00 46.5 -73 06 Emission Nebula

5' east of NGC 249 lies the emission nebula NGC 261. The nebula is round, 45" in size, and seems a little brighter towards the centre. There is a strong response to the OIII and UHC filters. Immediately east of the nebula is a small group of a few 11-12 magnitude field stars.

NGC 265 : 00 47.2 -73 29 Open Star Cluster

6' east of NGC 256 lies the star cluster NGC 265. At 66X: the cluster appears as a very faint, hazy glow, and could easily be overlooked unless purposely searching for it. At 110X, and with averted vision, the haze is more obvious. It is round, large, about 30" in size, and seems a little brighter towards the middle. NGC 269 is nearby to the southeast.

NGC 267 : 00 48.0 -73 16 Open Star Cluster + nebulosity

This cluster plus nebulosity appears as a faint, hazy glow and could easily be overlooked. To locate this cluster follow the prominent chain of bright stars forming a wide arc to the northern end, and the soft hazy glow lies a little northwest of the last star. The soft haze is only somewhat noticeable at 66X, but changing up to a higher power improves the view. 110X: The cluster is a round haze, 1' in size and shows as a little grainy throughout from star members on the verge of resolving. There is a weak response to OIII and UHC filters.

NGC 269 : 00 48.3 -73 32 Open Star Cluster

Nearly 6' southeast of NGC 265, and immediately west of an 11 th magnitude field star, lies the star cluster NGC 269. At 66X there is only the slightest hint of a round, hazy glow. This object could easily be missed except in a careful search for it. At 110X, and with averted vision, the haze is faint, about 30" in size, and there is an even glow throughout. Overall, it is the whole field that is worthy of mention. There is a trail of bright stars in the east; three, small, milky patches; and a light sprinkle of faint field stars. NGC 256 and NGC 265 are nearby.

NGC 290 : 00 51.2 -73 10 Open Star Cluster

At 66X this cluster requires some care to locate in the field as it appears little more than a soft, fuzzy star surrounded by some true stars of similar brightness. At 110X the cluster stands out more easily as a small, round, relatively faint haze, about 20" in size, with a slightly brighter centre. 8' to the east-northeast lies a prominent pair of 10/11 magnitude stars. These point to the cluster and can be used as a good signpost. In the wider view the whole field is quite splendid. Surrounding the small, timid glow there is a generous number of 12 th magnitude and fainter field stars mixed with a few brighter stars.

NGC 292 : The whole SMC. Centred at : 00 52.6 -72 48

Here I will describe the SMC as seen by the naked eye, 7x50mm binoculars, and an overall impression through the 8" SCT.

Naked eye : From my semi-rural observing site the SMC stands out very well as a large, milky haze mass, very much like a bright star cloud from the Milky Way. The overall shape is a little elongated northeast to southwest and is 5 degrees in diameter. The haze appears smooth and even throughout. 47 Tuc can be noticed as a faint, fuzzy star at the northwestern edge detached from the SMC.

7x50mm Binoculars: 47 Tuc stands out brilliantly, and there are four, small, brighter knots in the SMC in the northeastern region. These brighter knots are NGC 330, NGC 346, NGC 371, and NGC 395. Apart from these four objects the SMC appears as a smooth, bright haze. All around the SMC there are numerous bright field stars which adds to a most superb view.

8" f/10 SCT : While sweeping through the SMC I have found the western region offers a different general impression, or feeling, to the eastern side. In the west the fields are full of small, faint, hazy glows creating a soft and gentle impression. Whereas, in the eastern region there are some very bright, bold and dramatic sights which create a more stirring impression. These two sides of the SMC can be divided from the cluster NGC 330 - the first of the bold, bright clusters in the east. Both sides of the SMC offer impressive sights, but along with views for the eye there is this change from the calm western region to the more excitable eastern region.

NGC 294 : 00 53.0 -73 23 Open Star Cluster

With some care, a slow moving field, and averted vision, this faint cluster can be picked out in the field. At 66X it appears as a small, relatively faint, round, hazy glow, about 30" in size with a slightly brighter centre. At 110X the cluster is more easily seen in the field. Overall, the soft hazy glow among the faint field stars creates a gentle and noteworthy sight.

NGC 299 : 00 53.4 -72 12 Open Star Cluster

In the western region of a prominent star group lie two very small hazy patches. The first hazy patch in the west is the star cluster NGC 299, the second to the southeast is NGC 306. At 66X, NGC 299 appears little more than a soft, fuzzy star and requires some care to pick out among the true stars nearby. At 140X the cluster is better recognised and can be seen as a small, relatively bright, round haze, about 25" in size, and it is a little brighter towards the centre. In a general sweep through this area the cluster might be easily noticed in using at least 140X magnification. The star group already mentioned as a signpost to locate NGC 299 is about 15' in size and made up of some twelve stars at 10-12 th magnitude.

NGC 306 : 00 54.2 -72 15 Open Star Cluster

About 5' southeast of NGC 299 lies this small and faint star cluster. At 66X the cluster requires care to pick out in the field, appearing as a faint, fuzzy star. At 140X the cluster is more obvious as a very small, round, hazy glow, about 15" in size. Not only is this cluster noticeably a little smaller to its close neighbour NGC 299, but it is also only half as bright. Meek and timid as this hazy glow is, it is the wider view of stars and two unequal hazy glows which makes for an attractive and worthy view to seek out.