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Southern Cross - July 1998

• Constellation of the Month - Hercules
• My Best View Yet of NGC 6188

Constellation of the Month - Hercules

Michael Nelmes

6.30pm on a Saturday night in July or August is a good time to look at Hercules - the constellation, not the one that comes before Xena on Capital TV.

Hercules is often thought of as a northern hemisphere constellation, and I confess I'd never looked at it before being asked if I could do this month's COTM. I have a good north sky, being on the northern tip of Canberra, and a low horizon. So I had a look and there it was, in its entirety, an easily recognisable trapezium with legs and arms outstretched. But you do need a low horizon: one cluster I will mention below, M92, reaches only 10 degrees (half a handspan) above the north horizon from Canberra. In early August it culminates at around 7.30pm., usually before the freezer-suit weather sets in.

Among the most ancient constellations, Hercules (originally Heracles, but the Roman name for the Greek hero has stuck) is a large group bordered by a menagerie of other constellations: Draco, Bootes, Corona Borealis, Serpens Caput, Ophiuchus, Aquila, Saggita, Vulpecula and Lyra. It is double star heaven, except for the problem of its low elevation, but also includes some globular clusters and a planetary nebula or two. The following observations were made with a 4.5-inch Newtonian reflector, around first quarter Moon and in average to good seeing conditions.

M13 (NGC 6205).

The Great Hercules Cluster is the best globular visible to northern observers. I saw it as mottled rather than properly resolved into stars, and with little central condensation, like a small Omega Centauri but round.

Halley found it 50 years before Messier. At 6th magnitude and about 10 arc minutes across it's well seen in binoculars, but certainly not spectacular through the 4.5-inch; in fact the term "great cluster" is often spoken with a hint of sarcasm by those of us who are pampered with overhead views of Omega Centauri and 47 Tucanae.

M13 contains about 30,000 stars and lies 20 to 25,000 light years away. Despite the apparent crowding of the central stars, Burnham makes the point that in relative terms there is as much space between them as between grains of sand spaced 2km apart.

M92 (NGC 6341).

As with M13, this globular cluster was not first found by Messier, despite bearing his initial. Another binocular object, at magnitude 6.5 with a diameter of around 5 arc minutes, I saw it as mottled in the telescope. The core is much more condensed than that of M13.

Look for the "spiral structure" reported by Lord Rosse last century through his Leviathan reflector (which incidentally was recently restored); I suspect the spiral galaxies were getting to him. Distance is 35,000 light years. Alpha Herculis (Ras Algethi).

This magnitude 3 to 4 irregular variable red giant has a 5th mag. companion 4.6 arc seconds away, providing a rare colour contrast. The primary has a diameter of some 400 times that of the Sun. As a result it's one of the coolest of stars - hence the deep orange colour. Burnham calls it "one of the finest double stars in the heavens".

Delta Herculis.

Magnitudes 3 and 8.5, this double is not a true binary but two "line of sight" stars at different distances and moving in different directions. Minimum separation in 1960 was 9 arc seconds; it is greater than 10 now. The secondary is quite faint through the 4.5-inch.

Rho Herculis.

Magnitudes 4.5 and 5.5, at 4 arc seconds separation. Hartung calls them pale yellow but I could see no colour. The pair is best split at higher powers, say over 120X.

Mu Herculis.

Magnitudes 3.4 and 9.8 separated by 34 arc seconds. When using a small scope, the secondary is best seen at low power. The secondary is actually a pair of 10th mag red dwarfs an arc second or so apart, orbiting every 43 years, which I couldn't split. The distance of the system is 30 light years.

95 Herculis.

A binary system of giants of magnitudes 5.1 and 5.2, I saw them as yellow and white, although the pair is famous for being seen as different colours by different observers, and even apparently changing colour. As Burnham says, star colours are "delicate and elusive". A nice contrast and easily split at 6.3 arc seconds, which corresponds to a true separation of 775 AU at the estimated distance of 400 light years.

Hercules also contains planetary nebulae, but despite some searching I couldn't find the brightest one (NGC 6210). I intend trying again through a bigger scope, when the Moon is out of the way. But I think this selection will provide a worthwhile observing session, although those in the "deep south" might want to wait for the next deep sky night.

My Best View Yet of NGC 6188

Jenni Kay, FRAS

NGC 6188 is a large and irregular mass of emission and reflection nebulosity positioned in a very dense part of the bright southern Milky Way along the Ara/Norma border. In photographs, such as those taken by David Malin of the Anglo-Australian Telescope (AAT), this region appears as bright as the Orion nebula (M42), the Eta Carina nebula, or the Lagoon nebula (M8). But, such deep images of NGC 6188 are misleading, this nebula is much, much fainter than those other brighter showpieces.

Searching for faint detail is a challenge for me, and it's because of this challenge that I return to NGC 6188 night after night, year in and year out. It has become a favourite target of mine, an annual quest, drawing me back to study it and find the faint detail I know is really there. The main hindrance which has veiled the views I've been after has been dew. That and my 8" SCT just hasn't been able to pull in the faint detail. Last year I acquired a new 12.5" f/5 Newtonian, and on one, rare, dry and very transparent night, I finally saw faint detail beyond my expectations.

As aides at the telescope, I had print-outs of two David Malin photographs of NGC 6188 as downloaded from the AAT web site (see fig. 1 and 2). The AAT web site URL is http://newt.phys.unsw.edu.au/~mgb/astroimages.html

Most Malin photographs from the AAT site have fields of view of one degree, but I noticed that these two had been cropped to a much smaller field size. I had also printed out a sky map from a popular computer sky map making program only to discover that the map didn't match the Malin photographs. Checking yet another popular sky map making computer program revealed the same error.

Comparing the field stars in the maps to those in the Malin photos clearly showed the nebulosity in the Malin photographs extends much further to the west than the NGC 6188 symbols on the two maps. In fact, the full size of the NGC 6188 complex is almost twice the east-west width as shown on the computer generated maps. Having noted the marked differences between the photos and the generated sky maps, I prepared to make my observation of NGC 6188.

Following is my visual observation of NGC 6188, and the star cluster NGC 6193 which is associated with it. The observation was made from my semi-rural home site in Lobethal, South Australia. The sky conditions were above average; dry and very transparent, where typically the limiting naked eye mag is 6.1, ANT II-III.

NGC 6188 (BN) 16 40.5 -48 47 At low power, 50X; 60' FOV, the neb is noticeable as an extensive hazy background glow, with two very bright nebulous halos shrouding bright stars. The first nebulous halo has a close double star centred in it, the double is 5/8 mag, and is the brightest member of the star cluster. 6' west of the double lies a bright 8 mag star centred in the second nebulous halo. There is a strong response to the UHC filter from both nebulous halos. In the general 60' wider view, the field is rich in stars against the smooth, hazy background sky.

Bumping up the magnification, and using an UHC filter, reveals the best detail. At 105X; 37' FOV, the nebula appears to be broken into two, extensive parts; the eastern and the western part. Beginning with the eastern part, this area is the brightest. Engulfing the two nebulous halos with bright stars, the eastern part extends a little further east to encompass the whole of the star cluster, and ends in the SW in the shape of a peninsula. The tip of the peninsula is about 19' SW of the neb halo plus double star.

All along the western side of the peninsula, and moving northwards about 12', is a well defined, crisp dark edge to this nebula. This dark edge runs north to south in a very jagged line. Further north, passed this point, the neb continues but becomes much fainter, and eventually is lost in the dense milky way background. In general, the eastern neb is at least one degree in size north to south, and at it's widest it is about 22' east to west.

Returning to the peninsula in the NW, there is a long curving chain of bright 9/11 mag stars in a semi-elliptical shape, open to the SW. The arch of this chain of stars crosses a dark band, a break in nebulosity, between the eastern and western neb parts. In fact, this chain of stars bridges the two neb parts, with the stars in the east marking the SE edge of the western neb part. This extensive neb is much fainter to the eastern part, but with care, it can be seen to be equally large in size. From the chain of stars, the neb widens out in a fan shape, extending north and northwest. The eastern edge of this neb lies north to south and runs in parallel to the well defined dark edge of the eastern neb part. Northwards this neb fades and becomes lost in the dense milky way background, but it is probably at least one degree in size.

I checked a small, unnamed, nebulous star, as shown on Malin's photograph, which corresponds to the GSC star 8333:702 at 16 40 0.14 -48 46 58.3. On the photograph it appears as an obvious, bright, neb star, but through the eyepiece I can only see the 10 mag star, with no hint of nebulosity at all magnifications from 105X - 151X. Perhaps this is a patch of reflection nebulosity which is too weak for me to detect.

NGC 6193 (OC) 16 41.3 -48 46

At low power, the cluster appears more of a loose grouping of stars rather than a cluster per se. There is a very bright double star, 5/8 mag, at the southeastern edge, which is wrapped in a halo of nebulosity. The cluster is large, about 20' N-S by 12' E-W. There are generally bright stars involved, all similar in brightness at about 9/10 mag, with a membership count of thirty-five. The cluster is not well detached, blending into the general star field in the north. At 151X; 20' FOV, some of the very faint stars can be seen, although there are only a few of these. In particular, immediately south of the double star there is a very faint 14 mag star showing a purple colour. There are four other very faint 15 mag stars around the double star. It can be easily seen that the nebulosity of NGC 6188 is strong here, and extends to the north and west of the cluster.

* * * * *

Ed Note: Last year I wrote a note about the bright double star in NGC 6193. This is Dunlop 206 (16 41.3, -48 46). In looking at this multiple star, from a suburban site, the nebulosity adjoining the cluster was no more than suspected with 7-inch aperture and no nebular filter. To quote part of my description from last year -

The pair in question is Dunlop 206, magnitudes 5.7 and 6.9 at 9.6" separation - easy in any telescope, and one of Dunlop's better pairs. Closer examination shows more components. Hartung mentions one - mag 9.0 at only 1.6" from the mag 5.7 star - and with his usual optimism this "was clearly resolved with 10.5cm". Curiously he doesn't mention the Dunlop companion. As well, there are three fainter companions - mag 10.5 at 13.4", mag 11.4 at 13.9", and mag 12.5 at 20.8". I have seen all six stars with the 18cm refractor at Downer, at 180x. The star cluster, being rather loose and wide, benefits from a lower power. As Jenni's descriptions indicate, more aperture, a UHC filter and a dark sky make a huge difference to the nebulosity visible here... Ross Gould