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Southern Cross - April 2000

• Constellation of the Month - Robur Carolinum
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Constellation of the Month - Robur Carolinum

Albert Brakel

Just for a change we have an obsolete constellation of the month, Robur Carolinum (Charles' Oak), which lay in what is now eastern Carina. It was invented by Edmund Halley in 1679, in an effort to ingratiate himself with King Charles II. The young Halley had written a paper about planetary orbits, and had spent over a year on the island of St Helena compiling his Catalogue of the Southern Stars and observing a transit of Mercury. Although his work was clearly good enough to justify him being awarded a degree from Oxford, the rules were strict -- he had not stayed on campus for the minimum number of terms required. The new constellation did the trick, with the king expressly commanding the Vice-Chancellor of Oxford to award Halley a Master's degree (an M.A.). Alas, you can't get degrees that way any more, with all constellation boundaries now permanently fixed by the International Astronomical Union.

The Royal Oak tree became famous during the English Civil War, when Charles II was defeated by Cromwell at the Battle of Worcester, and the king climbed into it and hid there for 24 hours, something Charles was quite proud of.

The creation of Robur Carolinum did not please some others, notably Lacaille, who complained that it was constructed from some of the finest stars in the ship Argo, ruining that constellation. Argo was already missing its prow (where the Southern Cross is), and Halley's efforts had left only the stern end. Over the centuries it dropped out of the sky maps, but was still being shown on British Admiralty charts in the 19th Century. Halley's Alpha Roburis is now Beta Carinae.

The region includes one of the richest areas of the Milky Way. Star clusters and nebulae are obvious with the unaided eye, even from suburban sites, and wandering around it with binoculars is very rewarding. The large open cluster IC 2602 (RA 10h 43m, Dec -64 24'), for example, centred on the star Theta Carinae, is too big for the telescope and needs the wider field of view that binoculars provide. The cluster is often called the Southern Pleiades, but it looks nothing like the northern Pleiades, and provides a poor spectacle to the unaided eye; its bright stars are fainter on average and cover a smaller area of sky. It has a dozen or so bright stars and many fainter ones, with five on the eastern side forming an X shape.

Where a telescope will be needed is for examining the smaller cluster Melotte 101 (10h 42m, -65 09'), which lies on the southern edge of IC 2602. Though more compact, it still sprawls over a diameter of some 18'. It contains over 30 faint stars in the magnitude range 8-11, with hints of fainter ones at the limit of averted vision with 77x power.

But now let's stay with the telescope and head east. (I used, by the way, a 20 cm Celestron C8, from suburban Downer where there is a moderate amount of light pollution. Under different conditions and with instruments of different size you may be able to resolve, or not resolve, particular features of objects that can change their appearance.)

About 4 degrees east of IC 2602 is Mel 105 (11h 19.5m, -63 30'). This is a tiny cluster only 1.3' (or 80") across. With 77x and averted vision it appeared as a luminous unresolved haze peppered by tiny pinpricks of faint stars. 118x power showed it at its best.

IC 2714 (11h 18m, -62 42') is located less than a degree to the north. It is an irregular elongate 15' x 25' cluster of numerous faint stars, again set in a background haze of stars below the resolution threshold.

The Dunlop 323 region

If you now head 5 deg. NNW, you can't miss the naked-eye Dunlop 323 (or NGC 3532) (11h 06.4m, -58 40'). This breath-taking cluster covers nearly 1 degree of sky, and for its size would have to be the richest cluster in either hemisphere. Seemingly hundreds of stars of fairly comparable magnitudes are scattered more or less evenly and densely like sugar on velvet in the central portion. Most are whitish, but yellows are also present, and the two brightest, one on the southeastern edge (mag. 3.9) and the other in the central region, are orange. No wonder John Hershel called it "the most brilliant object of its kind I have ever seen".

Scan about a degree directly south, and the loose, sprawling cluster of Feinstein 1 (11h 06m, -59 45') swims into view. About 25' across, it comprises mag. 7 and fainter stars arrayed around a very rough E-W oval shape with a depleted centre.

Less than a degree SE you can find Collinder 240 (11h 11m, -60 15'), a cluster about 20' across of m.7 and fainter stars, a bit more closely spaced than those of Feinstein 1. The most striking feature is the mag. 4.6 sun on the eastern rim. To the west side, the stars are more concentrated into the subcluster NGC 3572 (11h 10m, -60 14'), which is dominated by a narrow parallelogram of stars aligned NE-SW, up to mag. 8.

Half-a-degree south is the smaller Trumpler 18 (11h 11.5m, -60 40'), forming a bar of faint stars emphasized by a line of three m.9 stars oriented NW-SE. In the same low-power field, just 10' to the SE, NGC 3590 (11h 13m, -60 45') appeared as a small number of stars fainter than mag. 10 immersed in an unresolved luminosity.

Some half-a-degree further south again is a complex of emission nebulosity bearing the numbers NGC 3576, 3579, 3584, 3581, 3582, and 3586. The 7' x 4' wedge-shaped group has a m.9 star at its southern apex, and the brightest patch is NGC 3582 (11h 12m, -61 12'). Hartung suggested it could be regarded as one nebula with dark lanes. Lumicon OIII and UHC filters were a definite help here. Use low power first to see the complex in the context of the surrounding star field, then zoom in with higher power. There is more diffuse nebulosity throughout the low-power field and beyond, with a bright knot some 25' to the east illuminated by a mag.9 star (NGC 3603).

The Eta Carinae region

It's time to move on to another region, and the obvious target is Eta Carinae. Plainly visible without optical aid, the Eta Carinae Nebula, NGC 3372 (10h 45, -59 40'), is by far the largest bright emission nebula in the sky. At 77x, it extends over at least three fields of view of billowing magnificence, especially with UHC and OIII filters. The UHC filter is the best compromise between seeing the nebulosity well, in addition to the embedded star clusters which are such a feature of the object. The complex consists of several large islands of nebulosity isolated by dark rifts of dust cutting across. The brightest area is the central wedge-shaped region containing Eta Carinae itself, as well as the smaller dark Keyhole Nebula that is not the perfect keyhole it once was in the 19th Century, but is still generally recognizable as such. Eta appears as a yellow star, but it is actually blue, being "reddened" by intervening dust. Around it is the small, orange, bipolar nebula called the Homunculus, that was easily seen with 154x, and with care, even with 77x. For an incomparable view of the Homunculus, with more intense color, you can't beat the Oddie 9" refractor. The famous Hubble image of Eta Car shows the Homunculus to be two expanding lobes of a dumbell.

The area immediately surrounding Eta is the quite attractive open cluster of Trumpler 16, which features a short arc of 5 stars just south of Eta. Ten arcminutes in the other direction is Collinder 232, dominated by 4 moderately-bright stars lined up in a NW-SE direction. However, the cluster is rather sparse, and a better one can be found a similar distance to the west. This is Trumpler 14, which is more attractive because it is richer and has a greater range of star magnitudes. Near its middle is the binary star h4356, mags. 7.2 and 8.9, separation 2.8", that could be split with 154x. The brighter star of this pair is HD93129A, one of the most massive stars known, that rivals Eta Car in mass and luminosity, but not of course in apparent brightness. Close by at 2' to the SE is the triple system h4360, with the two main components of m.7.8 each lying 12.6" apart; the NW component is itself a mag. 8.5 and 8.6 pair, 2" apart, for which I needed 235x to resolve them in mediocre conditions. Trumpler 15 lies immediately north of Col 232, and consists of pinprick stars roughly meridionally aligned.

A further degree to the west, van den Bergh 99 (10h 38m, -59 10') is a spread-out cluster with dozens of stars, and occupying most of a low-power field. It is noteworthy for its brightest member, HR4177 (10h 38.8, -59 11'), an orange-red mag. 4.9 star that has a greenish-blue m.6.5 companion about 5" to the NNE. The companion's spectral type is B4/B5, suggesting it should be whitish, so the perceived color must be an optical color contrast effect, though it's very pretty. There are other open clusters in close proximity to Eta Car, but I'll let you explore these for yourselves.

Proceeding half-a-degree to the north, NGC 3324 (10h 37m, -58 40') is an area of dim nebulosity visible even without a filter, with some embedded stars including a m.5.5 one.

The Diamonds and Ruby Cluster (NGC 3293) (10h 35m, -58 14'), another half-a -degree NW, is a stunner. This is one of the 'crown jewels' of the constellation, and it would make it easily into anyone's list of the top 10 clusters in the sky. A red star is set in among a dense collection of white and bluish-white ones, the shear beauty being guaranteed to satisfy even the most jaded observers.

IC 2581 (10h 27.4m, -57 38') comprises a gathering of moderately faint stars around a bright yellow one (mag. 4.7). To the west are two other reasonably bright stars that form an equilateral triangle with it, but in this rich area of the Milky Way it is hard to tell if these belong to the cluster or not.

NGC 3199 (10h 17.1m, -57 55') is a broad crescent-shaped emission nebula about 7' long, concave to the east, in a field of faint stars. It responds well to UHC and OIII filters. The southern end is noticeably brighter than the rest.

Planetaries, Galaxies, a Globular, and Binaries

NGC 2867 (9h 21.4m, -58 19) is a pale bluish planetary nebula, about 8" in diameter, mag. 9.7, and easily seen with averted vision. The stars on either side are comparable in brightness, but it can be distinguished from them by the way it dims greatly with direct vision, unlike the stars. The 14th magnitude central star is only for the larger telescopes.

IC 2501 (9h 39m, -60 05'), a stellar planetary nebula, needed the OIII filter to identify it by blocking out the nearby stars.

Another planetary I tried for, NGC 3211 (10h 18m, -62 40') was too faint for me to detect.

There are several galaxies in the constellation, but being this close to the disk of the Milky Way, they suffer greatly from extinction of their light by galactic dust. NGC 3136 (10h 05.8m, -67 23') was barely perceptible, and nearby IC 2554 (10h 09m, -69 00') not visible. In the far south, I saw a dim glow near the position of NGC 3059 (9h 50.2m, -73 55'), but with higher magnification this turned out to be some very faint stars. On a previous occasion I did manage to detect it from a darker sky site. There are a couple of galaxies close to Halley's Alpha Roburis (Beta Car), but these are overwhelmed by the light of the star.

More rewarding is the mag. 6.2 globular cluster NGC 2808 (9h 12m, -64 52'), 5 degrees directly north of Beta Car. It is only 5' in diameter, but it's bright. The rather sharply-bounded centre is strongly condensed, and surrounded by a broad milky halo. Stars did not begin to be resolved in it until I bumped up the power to 235x, when I could just start to pick some out around the edges.

With such an Aladdin's cave of treasures in this well-bestowed former constellation, I have not dwelt much on the double stars, except to mention some along the way. There are of course a great many of them in any rich part of the Milky Way like this one, so I'll close by mentioning at least three more.

Upsilon Car (9h 47.1m, -65 04') is a brilliant pale yellow pair of suns of mags. 3.0 and 6.3. They are 5" apart, looked elongate with 77x, resolved with 117x, and easily split with 154x. In the same field, about 5' to the southeast, is the fainter m.9.3 and m.9.5 equal pair h4252, readily resolved even by 50x, with their wide separation of 12".

Finally, in the deep south at 10h 53.7m, -70 43', h4383 consists of whitish stars (mags. 7.2 and 6.6) at a closer distance of 1.7".

Even the large number objects in this report cover only a fraction of those available for perusal in the old, now-forgotten Charles' Oak. I have included some old friends, but also many that may be new to you, outshone as they are by their more illustrious neighbours. Don't overlook these hidden gems, and have fun fossicking in the region for more of them.