Re: (IAAC) Published magnitudes and visibility

Thanks for summarizing for the list some information about the
the stages of life for Planetary Nebulae, Dave. I gleaned some
information from it that will help my own observing, I hope!
>I always thought that because of dust or the presence of dust,
>the object would reflect rather than emit...
Others with a stronger astrophysics background than either of
us will probably have more to say on this topic, but here are
my $0.02 anyway: I believe a significant quantity of material
*behind* an emitting star may cause a "reflection feature" to
be observed (like the "blue part" of M20, for example).
OTOH, a signficant quantity of interstellar material ("dust"
mainly) in the *line of sight* between an object and observer
will cause the light from that object to be both dimmed (due
to absorption and light scattering by the dust), and also to
be reddened (due to absorption and re-emission of radiation
by the same dust). The relative effects of each - scattering
and absorption, versus reddening - will depend on the wave-
length of light in question, and also on constituent elements,
size and "optical depth" of the intervening material.
And note that dust reddening (and dimming) actually occur in
*all* deep-sky objects that we observe! After all, the space
between stars in our galaxy is really a very "dirty" place. :)
But luckily for us amateurs, there are some very nice objects
which are not too greatly affected by this fact.
Thanks for a thought-provoking post, Dave. Clear skies all!
Lew Gramer
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