Rings and shells in Comet Holmes

David Kingsley

> A picture should be able to resolve the ring issue conclusively...

I mentioned this image by Teri Smoot of TAC-SAC early this morning. It was taken last night, when the ring was visually detectable but harder to see than tonight. But the brighter edge to the inner disc was evident even then in the image.

http://home.earthlink.net/~montanacg/index.html

In Kwok Sun's book on planetary nebula, he mentions Curtis's early morphological studies of M57 suggesting that the classic planetary must be a true ring, not a hollow sphere. For a sphere you expect the middle portion of the object to look about half as bright as the outer ring (summed material from the front and back faces of the shell contributing to the appearance of the middle portion, compared to longer line of sight along edges of the shell contributing to brighter edge appearance). In contrast, the dark center of M57 actually appears more like 20 times fainter than the outer ring, suggesting it was is a true ring lying flat against the apparent plane of the sky, not a hollow shell.

Comet Holmes may be a great chance to see what an expanding shell looks like when it really IS a hollow expanding sphere. The contrast between the middle portion and the outer ring is much less than M57, but that's what you'd expect for a more spherical object.

My eyepiece reticle eyepiece measurements agree with the timing measurements showing that this sucker is growing fast. It nearly doubled in visual size from last night to tonight, so we now have morphological evolution playing out before our very eyes in a naked eye object !

--David Kingsley