Nightfall

1998 April

Newsletter of the Deepsky Observing Section of the Astronomical Society of Southern Africa

THE STRETCH OF MILKY WAY from Sirius to the Pointers contains some of the most brilliant open clusters in the night sky, easy pickings for even the most casual binocularist. Here are numerous brilliant stars, too, forming easily recognised constellations and asterisms. Starting at the Pointers, and moving up the Milky Way to Sirius, we encounter Crux, the Diamond Cross, False Cross, Southern (right-angled) Triangle, and finally Canis Major itself.

The Southern Right-Angle, although not an "official" asterism, is nevertheless easy to make out. It consists of Lambda and Gamma Velorum and Zeta Puppis. The star at the eastern tip of the Triangle is Lambda Velorum (Alsuhail), a 2nd magnitude star that is a convenient starting-point for a star-hop to some fascinating clusters. To my naked eye, Alsuhail has a distinct orange-yellow colour; what do you see? The region surrounding this star, specially to the south and east, shows as a large mostly resolved glow, to one side of the Milky Way. It appears to be bound on the east and west by two massive, dark Milky Way dust lanes. In binoculars, the colourful Lambda is accompanied by a 4+ degree field of a dozen bright, scattered stars making an attractive sight.

THE PATCH OF SKY east of Alsuhail, bound by RA 9h12m to 10h and declination -39 to -50 , corresponds to chart 398 of the Uranometria 2000.0 atlas. On this chart, three open clusters (NGC 2982, NGC 2849 & Pismis 12) and a planetary nebula (NGC 2792) are plotted. Of these, only NGC 2982 can be seen in binoculars.

NGC 2982 lies nine degrees east of Alsuhail. It was first noted by Sir John Herschel during his 19th century visit to Cape Town. Observing from the foot of Table Mountain, he recorded it in his observing log as "a cluster of about 20 stars 11th mag and two of about 10th mag, forming an oblong nearly in parallel; place of preceding star about 10th mag." By "oblong nearly in parallel" he means the grouping is elongated in an east-west direction.

When the New General Catalogue (NGC) was reworked in the 1970s as the Revised New General Catalogue (RNGC) authors Sulentic and Tifft declared this cluster to be "non-existent", recording that it was not a true stellar grouping. Be that as it may; it is clearly visible in binoculars if the sky is good. From typical light-polluted suburban skies I recorded it in 11x80's as "Only a 10th magnitude star, with possibly an extension due east." Under darker skies (limiting magnitude 5.5 to the naked eye), but with noticeable haze, I imagined seeing a nebulous puff here. Some attention, using averted vision, shows at least two faint stars surrounded by a glow. Under even darker skies, with greater transparency, binoculars show it more clearly. The cluster is easy to find midway between two 6th or 7th magnitude stars, one of which is y Velorum. It is seen as an indistinct smudge about 10' across. Under mediocre conditions, it appears irregularly round and with great attention looks elongated roughly southeast - northwest. However, under excellent conditions and when well dark-adapted, this very faint, hazy object is clearly elongated east-west, in the ratio 1:2. The surrounding 4 binocular field is rich and textured and the cluster doesn't have enough contrast to stand out well.

The key to this cluster is aperture. Even a 6-inch will show it well. A low-power (42x, 50' field) eyepiece shows it clearly as a coarse grouping of about 15 stars of the 11th magnitude; it fits snugly into a 15' compass.

The other two open clusters, NGC 2849 and Pismis 12, are far more obscure; I'd be keen to hear what other observers find here.

A short distance north-east of Alsuhail is the planetary nebula NGC 2792. Don't bother looking for this one in binoculars - it's catalogued at 13th magnitude. However, don't be discouraged if you have a telescope - it appears somewhat brighter visually and I expect it to be visible in a 4-inch.

At low power, a 6-inch shows it readily as an 11th magnitude star on a rich field of mixed magnitudes. Higher magnification brings out the pale round disk, which appears featureless. (Look for the neat tiny double star nearby). Greater aperture shows it as an intriguing object. John Herschel again: "observed with Mr Maclear and another gentleman . . . pretty faint, exactly round, equal to a star 9th mag, but of a dull light. At first I was inclined to think it double, but with 320 [magnification] it exhibited a uniform round disc; nor did a friend to whom I showed it see any division. Stars to- night perfectly well defined." On a different night, he called it "pretty bright, round, 6 arcseconds diameter, equals in light a star about 9th mag, a very careful and good observation." He recorded it a third time, and wrote in his log: "Viewed past meridian. It occurs in a field with about 40 stars. Diameter 4 or 5 arcseconds at the utmost; 10 arcseconds is too large certainly . . . the night is good and it bears magnifying. With 320 power the disc is dilated into a dim hazy round nebula; yet there is a peculiarity in its appearance which completely separates it from all nebulae of the same size. A very remarkable object."

Although these are the only objects plotted on the Uranometria chart, there are several others that can be picked up in this part of the sky. Several of these objects were first recorded in the 1970s by astronomers working at Uppsala Observatory, examining photographic plates taken with the European Southern Observatory's 1-metre Schmidt telescope at La Silla, Chile. Also known as the Quick Blue Survey, it was designed to be a counterpart to the more famous northern Palomar Observatory Sky Survey (POSS). One of these new objects is ESO 315 SC 14. This little cluster lies north-east of Psi Velorum, and is readily seen with a 6-inch telescope on a moderately busy field. Although small (3'), it is quite noticeable and clearly fuzzy even at low power (42x). Use high powers to show it as a single 10th magnitude star surrounded by five much fainter starlets. Larger telescopes should show a handful of even fainter members (image on previous page).

A far more obscure object is ESO 212 SC 2. In my 6-inch it is as invisible as the Galactic Equator near which it lies. This cluster is also listed as vdBH 63, indicating that the Uppsala team wasn't the first to spot it. Instead, it was discovered by Canadian astronomers on a photographic plate taken with the Curtis-Schmidt telescope of the Cerro Tololo Inter-American Observatory. In the van den Bergh-Hagen catalogue, which was drawn up after these plates were studied, it is described as being moderately rich and 1.5 arcminutes in diameter. If you have any luck spotting it, please let me know.

The cluster ESO 261 SC 7 is an altogether more obvious target. At 52x, the 40' field of my 6-inch shows about thirty 10th magnitude stars in the field, and then some 30 more fainter ones. Although it is widely scattered, it is a somewhat suspicious field, suggesting the presence of a real cluster. If all the stars were 3 or 4 magnitudes brighter, it would resemble NGC 2516 or IC 2602.

Look six degrees further east for Ruprecht 81. It is visible in the same low-power field as NGC 2982, and is about a third of its size. It is readily seen in the 6-inch as a small, irregularly round glow, 10 very small stars (perhaps more), resembling a very distant open cluster. It is elongated southeast-northwest and measures just under 3' x 1'. It reminds me of the Beehive (M44) seen with the naked eye.

Further south lies NGC 2932, another of John Herschel's discoveries. He wrote: "This is about the middle of an enormous cluster of a degree or degree and half in diameter, very rich in stars of all magnitudes, from 8th mag downwards, which merits registry as a sort of telescopic Praesape. It may perhaps be regarded as a detached portion of the milky way, which is here very much broken up." The verdict of the compilers of the RNGC is a thumbs-up: there is no real cluster here. On the one hand, I'm inclined to agree; I searched the area with the 6-inch and a 50' field of view, and found nothing resembling a large cluster. On the other hand, binoculars do seem to show a large stellar congregation. Tripod-mounted 11x80's show a vague, coarse gathering of about 10 stars, « across, only somewhat separate from the background. These few stars are fleshed out by "nebulosity" between the members. By comparison, within the field, just 1 west-southwest, lies a brighter grouping of stars, same size, but distinctly free of its neighbour's haziness. The whole area, for at least 3 around, is littered with fine, small stars, making a milky rich field. Let me know what you see.

Due east of NGC 2932 lies an interesting binocular field, surrounding the orange- yellow star m Velorum (mv = 4.6, near 09:52, -46 .5). Here, a noticeable if sparse stellar grouping of a half-dozen white brightish stars accompanies the one coloured member. Look close to m for a small companion, which has the appearance of colour. I suspect this is merely an illusion, because of its proximity to the coloured star, and also because it is probably too faint for colour detection anyway.

South of this field lies Ruprecht 160, which the 6-inch plainly shows as a coarse, 6 arcminute small-starred grouping. At 144x, there are two 9th magnitude stars (shown on the Uranometria) and then about half a dozen fainter ones arranged in an east-west elongated grouping (image on page 2).

Pismis 15 to the south-west is clearly out of reach of my 6-inch; as the image shows, it is a moderately rich but indistinct grouping of very faint stars, not well separated from the background. I haven't been able to check out this cluster with a larger telescope, and welcome any observations of it.

The final unplotted object is vdBH 60. This cluster is described by van den Bergh and Hagen as being 2.5' in diameter, and star-poor. Its position is somewhat similar to that of Pismis 11, differing by 13' in RA, and 18' in declination. The description of Pismis 11 is a closer match: 2' in diameter, circular, and containing 18 stars. I'd bet on it that these two objects are the same. The image below shows a 15' x 15' patch of sky around the co-ordinates reported by van den Bergh and Hagen; there is a clustering of stars south of the bright star. I'd be keen to hear how this grouping appears visually.

As always, correspondence is welcomed at PO Box 12838, Stellenbosch, 7613, or via email to auke@global.co.za

Do you have any comments on the objects discussed here? If so, please submit them!

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"Deepsky Observers Companion" (http://www.global.co.za/~auke) Copyright 1998 Auke Slotegraaf (auke@global.co.za). All rights reserved. Uranometria 2000.0 copyright (c) 1987-1996 Willmann-Bell, Inc. Page last updated 1998 March 26. Lawyers question of a witness: "How many times have you committed suicide?"