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Southern Cross - December 1998

• Constellation of the Month - Eridanus, Southern Half
• NGC/IC Star Clusters and Nebulae of the SMC - Part 2

Constellation of the Month - Eridanus, Southern Half

Ross Gould

The considerable length of Eridanus led to it being split in two for our COTM series — in the next Southern Cross (February), Albert Brakel will lead the tour of Northern Eridanus.

The Celestial River is one of the oldest constellations, and is one of the largest in its modern form. It begins at the equator near Orion, then rambles southward past the rich galaxy meadows of Fornax to its distant end in the region of 1^st magnitude Achernar.

Most of the objects suitable for amateur ‘scopes in Eridanus are either galaxies or double stars; this is especially the rule in the Southern half, remote from the Milky Way. Eridanus is rich in galaxies, and a goodly number of these are visible with medium apertures, some of them showing structure. The double stars are fairly bountiful in number, and include one major showpiece and several other attractive pairs. A sampling of these is below.

Double Stars

If we start at the readily found 1^st magnitude Achernar, there are several doubles nearby. About 4 degrees north from Achernar is Dunlop 4 (0138.8, -5326), an easy pair of mags 7.1 and 8.4 at 10.4" separation. The primary is yellowish, and there are two fainter stars offset from the pair, helping the effect. Just over 1 degree West of Dun 4 is the wide uneven pair h 3444, mags 7.8 and 10.8 at 39". Higher power will show the primary as a very close slightly uneven pair of mags 8.4 and 8.8, at 0.9" in PA 044 in 1986. I found it just double in 1996 with 18cm aperture at 180x; increasing power to 330x showed an obvious neat close pair. Between Dun 4 and h 3444 is h 3449, which has an orange primary (m7.8), and a very faint (m12.8) companion at 26" due south.

Returning to Achernar, if the telescope is moved one degree to the north (and just east) the quite nice pair p Eri (Dun 5) will be found. These mag 6.0 stars are deep yellow in colour, separated in 1982 by 11.2". A 7^th mag star is southwest in the field.

Theta Eridani (02h 58.3m, -40 18) is a showpiece pair, a fine slightly unequal double that is a fine object even in a 6cm refractor. Both stars are white, a brilliant couple in a medium telescope. There has been little change over time, with only a slow increase in angle. The magnitudes are 3.4 and 4.5, at 8.3" and PA 89 in 1975. The field has only a few faint stars.

Three degrees West and just south from Theta, is the fine matched 7^th magnitude white pair h 3527 (02 43.4, -40 31). Separated 2.1", it has two faint pairs in the field, a wide one 20’ northeast; and a 10" uneven pair 30’ east. About a degree south and a little east from h3527 is CPO 30 (0245.6,-4113), another 2" pair, but fainter and uneven (m8.2 and 9.4) — this proved easy and rather nice at 140x with 16-inch aperture. Low power shows it as one of two stars, the other orange, a little south of a loose gathering of fairly bright stars.

Located southwest of 4^th magnitude e Eridani, the triple Jacob 8 (h3556) (03 12.4, -4425) is a good object for medium apertures. John Herschel discovered the wider pair, not as easy as the 3.5" separation implies because of the magnitude difference (mags 6.0 and 9.0). The bright yellow primary shows a small companion close south at 64x, easy at 140x with a 16-inch Newtonian. Higher magnification is needed to split the primary, a 44 year binary: it is nearly equal in brightness (mags 6.5 and 6.9), and the ephemeris gives only 0.7" at present (in pa 170 for 1997). With 18cm, 330x was needed to show it just separated.

Galaxies

I had hoped to check a number of galaxies in this area, but the continuing cloudy weather prevented this. The following notes are therefore a mix of past observations and published descriptions.

Bordering Eridanus in its middle part is Fornax, and the Fornax galaxy cluster spills over the border into the river. Several good fields are here, but even better ones are in Fornax, including the wonderful barred spiral NGC 1365, a very fine object in larger amateur ‘scopes under an out-of-town sky. Once we’re back in Eridanus, perhaps the best of the Fornax overflow is the arc of galaxies NGC 1389, NGC 1369 and NGC 1386. (1386: 0336.9, -3600). NGC 1386 is about 15’ north of 1369. A similar distance north of 1386 is NGC 1389. All three are spirals, of varying form, and accessible to 25cm; 1386 and 1389 should not be hard with 20cm. The largest is 1386, elongated and 2’ long.

NGC 1291 (0317.3, -4108) is a barred spiral galaxy with a visual magnitude of 8.5, making it visible even to small ‘scopes. It won’t show much structure in moderate apertures, and is only 2-3’ across. Photographs show a fairly large object, about 11’x9’. NGC 1291 is easily found by going 3 degrees east of Theta Eridani, and slightly south.

A pair of galaxies that might appeal to the astrophotographers (with long focal lengths), and visible with 10cm aperture, are NGC 1531 and 1532 (0412.1, -3252). These are 1 degrees northwest of 4^th magnitude Upsilon-4 Eridani. With medium apertures some structure — or at least shape - begins to show. 1531 is oval and about 1’ long; nearly at right angles is the long thin 1532. A photograph of the pair (from a UK Schmidt plate) was reproduced (though not well) in Southern Sky magazine for Nov/Dec 1993, p.34. NGC 1532 is an Sb spiral, nearly edge-on, and some amateur observers with larger telescopes have claimed to see the dust lane in it.

Other galaxies are scattered through the southern parts of Eridanus — the keen observer might like to pursue those shown on the Herald-Bobroff or Uranometria atlases. Surprisingly often they can be picked up, if you have at least 20cm aperture and a very dark sky. Galaxy hunting is much easier with that combination, and the CAS 14-inch is a useful ‘scope for the endeavour.

NGC/IC Star Clusters and Nebulae of the SMC - Part 2

Jenni Kay, FRAS

The following is the conclusion to a two-part series on visual descriptions of NGC/IC objects in the SMC. As mentioned in last month's article, all observations were made through an 8" f/10 SCT, from my semi-rural home site at Lobethal, SA. The typical skies here have a limiting naked eye magnitude of 6.1; with seeing conditions rated ANT II-III.

I would like to thank Steve Crouch for his very fine CCD image of the NGC 456 field, which accompanies this article. Steve obtained this image with a C14 and an ST6 CCD camera, taking 10 X 25 second exposures. Personally, I have never seen a finer image of this field.

NGC 330: 00 56.3 -72 28 Star Cluster

This star cluster is one of a few principal NGC objects in the SMC that is very obvious and eyecatching. In a 6x40mm finderscope, the cluster stands out well as a brilliant, nebulous knot. At low power, 66X, the cluster appears as a small, round, luminous haze. Never before have I seen such a small, tight cluster as bright as this, and certainly there is nothing like it in our own Milky Way. At 140X, the cluster shows a very bright, grainy core about 1.0' in size and a little elongated southeast to northwest. There are also a good many far-reaching outliers resolving well. These outliers range from 12-14th magnitude and are scattered around the cluster, with a higher concentration to the south, west, and north, creating a peppering effect. With the outliers included the cluster is nearly 6.0' in size. Overall, the view is remarkable. This cluster is placed in a rich star field among numerous 12th magnitude and fainter field stars. To the northeast lies NGC 346, which is another SMC showpiece.

NGC 339: 00 57.7 -74 28 Open Star Cluster

In a field quite barren of stars lies this extremely faint star cluster. A patient and still eye is recommended to wait for an image to build up of this elusive cluster for a confident sighting. In time, and in moments of better seeing, the cluster appears as a relatively large, round haze about 2.5' in size, and there may be a slight brightening towards the centre. After many nights revisiting this field, I would consider this cluster to be the most difficult object I have searched for in the SMC.

NGC 346: 00 59.1 -72 11 Star Cluster + nebulosity

This cluster plus nebulosity can be easily picked out in a 6x40mm finderscope as a very bright, small, hazy spot. At all magnifications the view is attractive and intriguing, but higher power shows the greatest detail. At 140x, this object appears as a bright, small, hazy knot, with much softer, nebulous extensions. The brighter region is a little elongated southeast to northwest, and there is a sharp, almost straight edge on the eastern side. In the northwestern part of this brighter region lie three stars, tightly grouped, forming a triangle. East of these stars is the marked sharp edge, whereas west of these stars the haze gradually weakens. About 1.5' northeast of the sharp edge, lie two 11th magnitude stars. This pair of stars lies in the much softer nebulous extensions.

With an UHC filter in place the entire object is bright, large, round, engulfing the brighter and softer regions into one hazy mass. With the filter the haze is equally bright throughout, but without the filter there are marked variances in the brightness intensity of the haze. The brighter region is about 2.0' in size, with the entire nebula about 3.0'. In the rest of the field the view is rich in field stars, and thebackground sky seems hazy, especially towards the southeast to east region. IC 1611 and IC 1612 lie nearby to the south.

IC 1611: 00 59.8 -72 20 Open Star Cluster

The cluster lies about 10' southeast of NGC 346. At low power, 66X, the cluster requires some care and attention to locate, as it is a faint and elusive object and is quite lost being set in a very rich star field. Furthermore, finding the cluster is hindered even more due to a background sky which appears washed out and milky from the nebulosity which is very extensive here. At 140X, the cluster is a little more obvious, appearing as a small, round, faint haze, which is 30" in size. Nearby are two 12th magnitude field stars, which along with the small glow shape a noticeable triangle. The first star is 1.0' to the east-southeast; the second is 1.0' to the southeast. In general, the whole view is magnificent, this being a dense region of the SMC.

IC 1612: 01 00.0 -72 22 Open Star Cluster

About 2.0' southeast of IC 1611, lies this very faint star cluster. At 140X, while using averted vision, and in moments of better seeing, the cluster appears as a very faint, round, relatively large haze, about 40" in size. At lower powers the cluster is lost, undetectable among the numerous stars and milky background sky. Another star cluster, Kron 42, lies 2.5' east of the IC cluster, and being a little more obvious as a small, round haze, it may be mistaken for the IC cluster. This mention of it is to help prevent any mis-identification of the IC cluster.

NGC 361: 01 02.2 -71 36 Star Cluster + nebulosity

About 3.0' southeast of a pair of 8/10th magnitude field stars, lies the very faint star cluster. At 110X, and in using averted vision, the cluster can be noticed as a very faint, relatively large, round haze, about 45" in size, and perhaps a little brighter in the middle. In moments of better seeing a pair of stars fleetingly resolve in the haze. Since this cluster is so faint, I needed a reliable star chart to guide me to the precise position to look for it and then patience and care to see it.

NGC 362: 01 03.2 -70 51 Globular Cluster

In the 6x40mm finderscope, this foreground globular can be easily picked out appearing as a very bright, fuzzy star. At 66X, the cluster has a brilliant, round, smooth core that is wrapped in a large and softer haze which is richly peppered in well resolved stars. At 140X, the brighter core appears grainy being on the verge of resolving into tightly packed stars. The core is 30" in size, while edge-to-edge the cluster is 6.0' in size. In the rest of the field there is a light sprinkle of bright field stars throughout. The view is outstanding at all magnifications.

NGC 371:01 03.5 -72 03 Star Cluster + nebulosity

This cluster plus nebulosity is another of the SMC showpieces. In a 6x40mm finderscope, it stands out well as a bright, small, round, hazy knot. At 66X, the cluster is a large, round haze with stars. With direct vision, the haze seems grainy; but, with averted vision, the haze is a glitter of countless stars. At 140X, there is a prominent chain of at least four stars which cross the hazy glow from the southeast to the northwest. These stars lie a little east of centre. There is a very good response to the UHC filter where this haze becomes very pronounced. The cluster is 5.0' in diameter. Crowning this cluster in the south are numerous bright field stars, which lie in many chains and rows. Overall, the view is superb at all magnifications.

NGC 376: 01 03.9 -72 49 Open Star Cluster

The cluster is obvious at low power, 66X, appearing as a very small, relatively bright, round haze, about 20" in size. There is a very good response to the UHC filter. Higher magnifications do not reveal any further details. In general, this is not a rich area with few field stars nearby, but despite the cluster's small size it probably would not be overlooked in a casual sweep of the area. NGC 419 lies nearby to the east-southeast.

NGC 395: 01 05.1 -71 59 Star Cluster + nebulosity

7' northeast of NGC 371 lies this cluster plus nebulosity. At 66X, the cluster is obvious as a relatively large, round, somewhat bright haze, with a size of 2.0'. Higher magnifications do not show me any further details or resolve any of the star members. The nebulosity responds well to the UHC filter appearing as a pronounced, smooth haze. IC 1624 lies close by to the south.

IC 1624: 01 05.4 -72 02 Emission nebula

2' south of NGC 395 lies this small patch of nebulosity. At 66X, the nebula is barely perceptible, with much uncertainty that anything is really there. At 140X, the view is much improved with the nebula seen as a faint, very small hazy smudge, about 25" in size, and could be slightly elongated east to west. Now and then there seems to be two or three stars almost resolving in the haze. In general, the whole view is superb. The region is rich in stars, NGC 395 is nearby, and the showpiece NGC 371 is only 10' to the west.

IC 1626: 01 06.2 -73 18 Open Star Cluster

This star cluster is too faint for me to detect.

NGC 411: 01 07.9 -71 46 Open Star Cluster

This rather faint cluster can be spotted easily enough in a search for it, but in a casual sweep it could be overlooked. At 66X, the cluster appears as a faint, round, relatively large haze, with a size of about 1.0' in diameter. At 110X, the cluster is a more obvious, and there seems to be a little brightening towards the centre. In the wider view there is an 8th magnitude field star 5.0' to the southeast, NGC 422 lies 5.0' to the east, and along with NGC 411 this trio of star plus two hazy glows shapes a triangle. In the rest of the field there is only a light sprinkle of stars.

NGC 416: 01 08.0 -72 21 Globular Cluster

The cluster stands out well, set in an attractive and rich star field. At 140X, the cluster appears as a relatively bright, round haze, about 30" in size, with a brighter centre. 30" north of the cluster lies a 13th magnitude field star. Higher magnifications such as 140X reveal good detail in the cluster, while lower magnifications showing a wider view reveal the full splendour of the rich field. In the wider view the field is rich in 11th magnitude and fainter stars with a few very bright 8th magnitude stars in the west. Overall, it is the rich mix of bright and faint stars, plus the round, hazy glow, which creates a magnificent sight.

NGC 419: 01 08.3 -72 53 Globular Cluster

Some 20' east-southeast of NGC 376 lies this bright and obvious star cluster. At 66X, the cluster appears as a relatively large, round haze, with a brighter centre, and a size of 1.0'. Higher powers do not show me any further details or resolve any star members. 8.0' to the south lies a 9th magnitude field star, 8.0' southeast of the cluster lies a bright 7th magnitude star, and along with the cluster the trio forms a dainty triangle. The rest of the field is quite empty of stars, but the trio of stars and bright glow stand out well, and it would be hard to miss this sight in a general sweep of the area.

NGC 422: 01 09.4 -71 46 Open Star Cluster

5' east of NGC 411 lies this very faint cluster. At 110X, with averted vision, much care, and in moments of better seeing, the cluster can be detected with full confidence. It appears as a round, relatively large, faint hazy glow, about 45" in size. At 140X, the glow is too faint. Some catalogues equate this cluster with IC 1641, but this is probably incorrect. There is an extremely faint cluster, as seen on a Digitised Sky Survey image, 1.7' east of NGC 422, which is probably IC 1641.

IC 1641: 01 09.6 -71 46 Open Star Cluster

This cluster is far too faint for me to detect.

IC 1644: 01 09.2 -73 11 Emission nebula

At 66X, in the field there are three 10th magnitude stars widely spaced in a row lying east to west. On closer inspection it can be realised that the central star is slightly different, a little fuzzy. At 140X, this almost stellar like object is obviously the emission nebula, appearing as a very small, round, hazy speck, about 15" in size. There is a strong response to the UHC filter. On the two field stars: the first lies 7.5' to the west, the second is 9.0' to the east.

IC 1655: 01 11.9 -71 20 Open Star Cluster

This cluster is too faint at 66X and 110X. At 140X, while using averted vision, the cluster appears as a very small, extremely faint, round haze, about 20" in size. Since the rest of the field is star poor with nothing to guide the eye to it, a deep and reliable star chart is recommended to find the precise position of this very elusive cluster.

IC 1660: 01 12.6 -71 46 Open Star Cluster

After a number of nights trying to see this extremely faint cluster it was only on a particularly clear and steady night that I was successful in this pursuit. At 140X, while using averted vision, the cluster sometimes appears as a small haze, round haze, about 25" in size; while other times it appears only as a single, faint star. As a guidepost, nearby to the southwest lie three 13th magnitude field stars, which along with the cluster form a trapezium. The cluster lies at the northeastern point of this shape.

IC 1662: 01 12.6 -73 27 Open Star Cluster

This cluster is too faint for me to detect.

NGC 456: 01 13.7 -73 17 Star Cluster + nebulosity

In a field generously sprinkled with stars over milky nebulosity, lies this cluster and nebula, which is the first of three in a group. NGC 460 and NGC 465 lie in the same 20' field. NGC 456 is the first hazy patch in the west. At 110X, the hazy patch is relatively large, bright, and round, with a size of 2.5'. Across the haze lie three stars aligned southeast to northwest. At 140X, the first star in the southeast in this chain of three can be seen to be actually a very small, brighter, nebulous knot. Now and then, in moments of better seeing, a single star resolves in this brighter knot. There is a very good response to the UHC filter.

NGC 458: 01 14.9 -71 33 Open Star Cluster

The cluster stands out well at 66X as a round, relatively bright, somewhat large haze, about 50" in size. Higher magnifications do not show any further details or resolve any individual star members. 10' to the southwest lie three prominent 10/11th magnitude field stars shaping an arc. Apart from these three stars the rest of the field is quite barren. In a casual sweep through this region the cluster should be easily noticed.

NGC 460: 01 14.6 -73 16 Star Cluster + nebulosity

3.5' east of NGC 458 lies this star cluster plus nebulosity, which is the second object in a group of three. At 66X, the cluster appears as a bright, irregularly shaped, relatively small, hazy patch, about 50" in size. At 140X the haze seems brighter in the south which may be a single star almost resolving, or a small, tight bunching of stars. There is a very good response to the UHC filter. Some popular catalogues give an incorrect position for this object placing it one degree too far north.

NGC 465: 01 15.7 -73 20 Open Star Cluster

4.0' east of NGC 460 lies this star cluster, which is the third object in this group of three. At all magnifications I can only find about four 12th magnitude stars in a group, of a sort, and these are almost lost blending with the numerous nearby field stars. There is a soft milkiness among these four stars, however, since there is no response to the UHC filter, this milkiness may not be nebulosity but very faint unresolved stars. It is the wider view with all three NGC objects seen together which warrants praise. The mix of stars and hazy knots is superb and easily stands out against adjoining fields.

NGC 602: 01 29.4 -73 34 Star Cluster plus nebulosity

A little over one degree east-southeast of the magnificent NGC 456 group, lies this very faint star cluster. At 66X, the cluster can be spotted, but requires care and averted vision. At 140X, the cluster is more easily detected, appearing as a faint, large, round haze, about 2.0' in size. The haze seems a little brighter in the southeastern region and this may be a star on the verge of resolving, or a tight bunching of stars. There are few field stars in the rest of the view and so some care is needed in searching for this object.