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

At 14:33 03/11/98 -0500, you wrote:
>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?

I think this would be caused by the fact the emission nebulae is not at a
very high temperature so that  the OIII lines are not excited. Normally
they would be excited by collisions and if the temperature of the nebula
was not hot enough you would not get sufficent material excited.
Alternatively the nebulae  could have a high density and the OIII lines
could  be quenched. I think that is unlikely in these cases. Without
wishing to plug the Webb soc again there was an article in one of the DSO's
on other nebulae that could be seen with the H-beta filter out side the
tradtional four
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