In a message dated 9/29/03, Craig Scull writes:
> Thanks for your reply Steve. I'm new to absolute visual
> magnitude, but I can appreciate that it's not to be used
> as a predictor of observed magnitude without first
> applying the conversion you described. Before I do this
> however, I have a few questions:
>
> If I apply a conversion from "absolute" to "apparent"
> visual magnitude,
Out of curiosity, here's the conversion to apparent visual magnitude --
m = M + 5 log D - 5, where M = absolute visual mag and D = distance in parsecs
The distance to NGC 6946 is roughly 6.2 Mpc (accurate to within ~0.5 Mpc)
according to a recent paper, so
m(v) = -13.2 + 5 log (6.2 x 10^6) - 5
= -13.2 + 34.0 - 5
= 15.8
So, visually this is a pretty faint object as my notes indicate. As far as
the absolute visual magnitude listed in the research paper, I believe they
model the galactic extinction and reddening due to dust in the milky way and
take that into consideration when computing the -13.2 absolute magnitude. My
derived apparent visual magnitude of 15.8 may be off by a couple of tenths,
though, due to possible errors in the distance to NGC 6946. But, it should
fall within the 15.6 - 16.0 range, unless I'm missing some additional
factors. Any other comments?
Steve
how often will the calculation lead to
> a value that reasonably matches what one would see
> through the eyepiece? Does it consistently under or
> overestimate?
>
> At first glance this calculation seems like it could be
> biased by certain factors:
> 1. The modeled absorption effects of the intergalactic
> medium -- are there debates in this area or is it pretty
> nailed down?
> 2. Error in the measured distance - are there debates
> about measurement techniques that significantly affect
> the values I would be using?
>
> In other words, for practical purposes I'm trying to
> determine the predictive validity and utility of this
> calculation for amateur astronomers.
>
> Craig
>
>
> --- Steve Gottlieb <sgottlieb@No-Spam> wrote:
> > In a message dated 9/29/03, Craig Scull writes:
> >
> > > Seek and yee shall find. Try the 4th entry on a
> > Google
> > > search for "6946 Globular"
> > > http://arxiv.org/abs/astro-ph/0104133
> > >
> > > Open the PDF and look at the photo of the globular
> > near
> > > the end of the article. That thing is HUGE! It's
> > also
> > > relatively bright at Mag 13.2.
> >
> >
> > The listed magnitude in the paper M(V) = -13.2 is the
> > "absolute" visual
> > magnitude -- its the apparent visual magnitude *if* the
> > cluster was located
> > at a nearby distance of exactly 10 parsecs or ~33 light
> > years. To convert to
> > an apparent visual magnitude, you use the distance to
> > the galaxy. I'm fairly
> > certain the object I observed in N6946 was fainter than
> > V = 13.2.
> >
> > Steve
> >
> >
>
>
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