Relationship between magnitude and distance



Last night I was enjoying the view of two galaxies, NGC5740 and NGC5746.
Both fit into the same field of view in a wide angle EP. NGC5746 is a
splendid edge-on while NGC5740 assumes a more typical orientation. I was
struck by the differences in brightness. Located in an approximately 4.1
NELM of my night sky, NGC5746 was fairly easy to pick out with a 9mm
eyepiece while NGC5740 was very tough to discern. Now, it would seem to me
that even if both had the same absolute magnitude if it were possible to
"flip" 5746 into the same orientation as 5740, wouldn't 5746 be brighter to
my eye simply because it is observed edge-on? More importantly, knowing
that galaxies come in all sizes, different shapes, and distances from us,
is there a general rule of thumb which roughly matches visual magnitude
with distance? For example, would a person be horribly way off in assuming
that the "average" 12th magniutde galaxy is maybe 85 to 100 million light
years away while the "average" 13th magnitude galaxy is around 100 to 125
million light years distant? The distances I just cited are ones which
were made up for illustration of my point and question. Again, I am
talking about some average galaxy size and taking into account a common
orientation to our line of site for that galaxy.


--
Martin R. Howell
"Photographs From the Universe of Amateur Astronomy"
http://members.isp.com/universeofamateurastronomy%40isp.com
.

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