And the keyhole

by David Kingsley

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!

Posted on sf-bay-tac Apr 17, 2005 00:27:17 PT
Converted by report.pm 1.2 Apr 23, 2005 21:13:16 PT