David, and all,
The Keyhole Nebula was one of the trip's mysteries for me. I hadn't
done enough preparation to know exactly what to look for, although
almost all of the descriptions of the Eta Carina region mention it, so
I was looking for it. But it wasn't at all obvious at the eyepiece.
Let me say that differently. There was a lot of dark stuff that was
obvious, but no keyhole shape jumped out at me. So I also did a
sketch.
During the days between observations, I kept searching for a good
description among the resources that I had with me (not much).
Surprisingly, Hartung was not helpful at all; not in the index, and no
mention whatsoever in the description of NGC 3372 (although the
Homunculus is pretty-well described). The other descriptions that I
had from various sources also didn't mention the keyhole (or equated it
with the whole of NGC 3372). Finally, I looked at The Astro League's
brief description, which was reasonably direct. This matched my sketch
pretty well.
"...a small, curved, dark dust lane crossing the brightest portion of
the Eta Carina nebula. It is located West of the star Eta Carina. The
keyhole shape has become less apparent over the years because of
changes in Eta Carin and the associated nebulosity...." (A.L. Southern
Sky Telescopic Club)
When I returned home, I hoped to find another clear description. S. J.
O'Meara's book "The Caldwell Objects" (sorry about using the C word,
folks) wasn't very helpful, which was disappointing, because I thought
its goal was to provide good descriptions and sketches; neither one
satisfied me, although he does try to describe it. His photo is next
to worthless -- it even has the keyhole labeled, although there is no
indication what the label is attached to or how big it is, and the
sketch doesn't call out or label the Keyhole (and it is on much too big
a scale).
Next I went to the NGC/IC website to get Steve Gottlieb's description.
That was helpful (more helpful than O'Meara's) and was consistent with
the A.L. description quoted above. However, I still couldn't pick out
the keyhole from the many photos I had seen.
The description and side-by-side sketch and photo that you (David)
provided was the clincher. I can now see the keyhole (or what remains
of it) in the left photo, and also in the photo in Hartung's book. My
little sketch, as poor as it is, at least labels the little arc of dark
stuff as "keyhole ?" with clarity. Now I can "erase" the question
mark.
Bob
(BTW: from Herschel's sketch, I would have called this the
"guitar-case" nebula, not "keyhole")
--- David Kingsley <kingsley@No-Spam> wrote:
> In addition to the bilobed, dark spattered Homunculus, with one
> textured lobe so much brighter you could tell it must be the lobe
> closer to us, Bob, Albert, and I also enjoyed the surrounding
> nebula and dark lanes in the Eta Carinae region. One question that
> came up a couple of times while we were observing was the exact
> identify of "the keyhole" dark nebula. There clearly is a dark lane
> near Eta Car, which was shaped somewhat like a keyhole, but not a
> very good one. I just found a very interesting site that helps
> explain that mystery.
>
> The keyhole shape was named based on observations by John Herschel,
> who drug his fathers mirror and telescope down to Cape Town in the
> 1830s, and extended his father's sky surveys to the Southern
> Hemisphere. As it turns out, the period he was observing and
> sketching the Eta Car region was the same time that Eta Car was
> rising steadily in brightness to become one of the brightest stars
> in the heavens. Herschel's drawing during this period clearly shows
> a beautiful keyhole shape next to Eta Car. See
> http://www.quasarastronomy.com.au/extra02.htm, which includes one of
> John's drawings at the bottom right of the page.
>
> Although the keyhole shape was nearly perfect when Eta Car was
> bright, part of the bright nebulosity that defined the lower border
>
> of the dark keyhole was actually a reflection nebula illuminated by
> the flaring star. As Eta faded back to its current magnitude
> around mag 7 or so, this bright nebulosity was lost, and so was the
> key hole shape whose border was partly defined by the reflection
> nebula.
>
> I sketched a large number of objects at the eyepiece in Australia,
> including a view of the Eta Car region through Albert's scope in
> Wirrealpa. It was great fun to find the on-line copy of Herschel's
>
> drawing, and to be able to compare it to my own quick sketch of this
>
> region 170 some years later. I can see exactly the parts of
> Herschel's original black keyhole that still remain, and the exact
> portion that fell away when when Eta sputtered back to its current
> dimmer state. The Hubble picture of the Homunculus, or the much
> older drawing by Herschel of the larger surrounding region, are
> exactly the kind of follow up reading and learning that is so fun to
>
> do after interesting observing sessions. What an amazing region t
> look at, and what a rare opportunity to see the belching and
> sputtering of a star nearing the end of its life.
>
> If my house was on fire, the Australian observing notebook would be
> one of the things I would grab to save!
>
>
> --David Kingsley
>
>
>
Received on Sun Apr 17 19:18:08 2005