Coonabarabran, NSW, 8/23-24/2006: New Longest Entry - Eta Carinae

by Marek Cichanski

After writing that long description of the Tarantula, I found myself having another long stare at Eta Carinae the next night. Just for something to do, I decided to try my hand at writing an even longer description. I guess that I was sort of play-acting at being John Herschel, and imagining that I had to 'paint a word picture' of the object. Or perhaps I was trying to write something 'Burnham-esque' or 'NSOG-esque'. So, for what it's worth, here's my attempt to describe Eta as completely as possible. (There's a bit of general travelogue afterwards.)

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Eta Carinae Region (a.k.a. 'Eta Argus' in the parlance of J.H.), 12" f/4.7(?) dobsonian, 31mm type 5 Nagler, OIII filter for observations of nebulosity:

This remarkable complex of nebulosity and star clusters almost perfectly fills the field of view of the eyepiece. The apparent surface brightness of the nebulosity varies from considerably faint to very bright, but most of the complex is considerably bright, or brighter.

Although the outer 'border' of the complex defines a roughly equant shape, the nebula is not round by any means. The complex contains much dark nebulosity, and perhaps the most distinctive feature of the complex is a 'v' of dark nebulosity that divides the bright nebulosity into two principal regions. The arms of this 'v' extend towards the west and the NNE. The angle between the arms of this 'v' is approximately 105 degrees. The W-pointing arm is 55 arcminutes long, and the NNE-pointing arm is 27 arcminutes long. Both arms are about 9 arcminutes wide. Both arms are remarkably dark, but the W-pointing arm is slightly more so.

The bright nebulosity contained within the arms of the dark 'v' is the brightest nebulosity of the entire complex. Of this region, the brightest part is a V-shaped region that mimics the dark 'v', is of similar dimensions to it, and lies immediately along the insides of the dark 'v's arms. The coincidence of these bright and dark 'v's is remarkable, but any possible significance of this coincidence is unknown.

The orangish-yellow star Eta Carinae, subject of intence scrutiny from the time of John Herschel to the present, lies about 2 arcminutes north of the apex of the bright 'v'. Immediately west of Eta Carinae lies the Keyhole Nebula, a bi-lobate, dumbell-shaped area of dark nebulosity about 12 arcminutes long. The southern lobe is pretty uniformly dark, but the northern lobe contains patchy bright nebulosity, some of which postdates the 19th-century outburst observed by Herschel.

The bright nebulosity lying to the S and E of the dark 'v' is of relatively uniform surface brightness, although it certainly contains many patchy bright and dark areas. Perhaps the most distinctive feature of this region is a narrow, straight, SE-pointing strip of dark nebulosity that very nearly joins the apex of the dark 'v', nearly causing the whole to form a 'Y' shape. It is about 25 arcminutes long and about one-third as wide as either arm of the dark 'v'.

Two other noticeable patches of dark nebulosity lie within this bright region, they are both wider and shorter than the one just described, they parallel it, and they lie to the S and E of it, respectively. A large, wine-gourd-shaped area of dark nebulosity lies immediately to the south of the west-pointing arm of the dark 'v'. Its wider, more bulbous end is on the south, and its narrower, west-curving 'neck' is on the north.

The region of bright nebulosity south and east of the dark 'v' is, as noted above, of variable surface brightness, but it is generally dimmer than the bright nebulosty that lies within the arms of the dark 'v'. It varies from pretty faint to considerably bright. Immediately east of the dark 'gourd' is a notable patch of bright nebulosity. Perhaps the patchiest bright nebulosity in the entire complex is that which lies east of the 'almost-a-Y' junction.

The complex lies in a rich area of the Milky Way, and has several open clusters associated with it. The most prominent such stellar groupings are probably the group of stars that lies adjacent to, and immediately east of Eta Carinae, and Collinder 228, a loose, bright group that stretches eastward (this might be southward - I suspect a blunder here) about 20 arcminutes from the apex of the dark 'v'. To the northwest of the west-pointing arm of the dark 'v' there are three(?) groups of stars that suggest stellar associations.

The Eta Carinae complex has fascinated observers since the 19th century, and even a visual examination through an amateur instrument raises numerous questions. These principally revolve around the three-dimensional structure of the complex and its evolution through time. Is the complex entirely a star-forming region, or are some of its features due to mass-loss and/or the death of stars? What is the cause of the curious bright and dark 'v's, and what is the reason for the remarkably sharp dark-light contacts along the inner edge of the dark 'v'?

The Eta Carinae complex is one of the most spectacular and satisfying targets for observation in the southern sky, and will reward the observer many times over for his or her time spent in careful study and wondered appreciation. ****

At the risk of sounding a bit self-important, I'll be curious to see how long it takes this to be copied around the Net with little attribution. Something rather like that has already happened with my ORs from last year's Oz trip, but that's another story for another time.

Yesterday Dan and I had a bit of 'release time' from our observing 'duties'. We actually had clouds and rain! So, we went up to the AAO and had a look at the Anglo-Australian Telescope, and then went over to the Warrumbungles and hiked up to a viewpoint called Fan's Horizon. Then it was back to the cottage for, wonder of wonders, a night of sleep! It felt so weird to get in bed at 10:30, and wake up at 7:00. Weird, but good.

The weather has fined up again, and it looks like there are a few more good nights on tap. So, it's back to the usual observing 'grind' (tongue firmly in cheek), and then back to Sydney and home. I'm really psyched to launch a series of final all-out assaults on the LMC.

I hope that there's another good weekend of Bay Area observing on tap. Wherever y'all are - MB, Coe, Coyote, Willow Springs, Fremont Peak, wherever, you're all down here in spirit. Where one TACos goes, all TACos go!

Cheers,
Marek