Re: (IAAC) IC 434 - 17.5inch F/4.5

Again thanks for these fine observations, Yann. BTW, bravo on catching
the single useful hour of darkness between moonset and dawn yesterday!
Also, I was curious about IC 466 - you mention it contains both refl-
ection and emission features, and yet its appearance only changed at
the very edge when using emission-band filters. What can be concluded
about the physics inside this nebula from this visual effect?
And now for the Dumb Amateur Astrophysics Question of the Week...
I have always been curious as to the astrophysical explanation behind
the narrow H-beta emission spectra of a few certain nebulae in the sky:
these include the "H-beta four" nebulae which traditionally justify the
use (and so purchase) of an H-beta filter versus an OIII or UHC. Why do
these nebulae (e.g., IC434 around the Horsehead, or the California) emit
so exclusively and narrowly in the Hydrogen beta band?
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