Re: (IAAC) How Bright are the Galaxies - Really?

Here is a very helpful reply from Brian Skiff on the topic of "what magnitudes
mean to the galaxy observer", and specifically on naked-eye visibility of M81.

Enjoy folks, and clear skies this weekend!
Lew Gramer

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Date: Thu, 6 Dec 2001 13:21:48 -0700 (MST)
From: Brian Skiff <bas@lowell.edu>
To: dedalus@latrade.com
Subject: Re: (SNES) How Bright are the Galaxies - Really?

>> SkyMap Pro, is now available. Version 8...
>> I find the numbers for M81 to be of particular interest (6.9 vs. 7.8)...

     It looks as though Marriott has (mistakenly?) adopted blue magnitudes
rather than V.

>> Overall I find the new higher magnitudes to be much more realistic.
>> M33, as an example, makes much more sense at magnitude 6.2 than at 5.7...

     [Here we] are confusing total magnitudes of extended objects with what
is perceived.  Simply lowering the total brightness to conform with the
perceived brightness-plus-surface-brightness impression won't work since
galaxies vary in mean surface brigntness.  You can of course adjust different
galaxies by different amounts to conform to what you think you see, but
that isn't very helpful.  You need to consider both total brigntness and
mean surface brightness and most importantly how the eye detects stuff at
low light levels, largely a contrast phenomenon.  In other words it's has
as much to do with brightness _differences_ as with light-levels per se.
     All this stuff is gone over in excruciating detail by Nils Olaf Carlin 
at Mel Bartels' Web site starting here:


When you read the numbers out of a catalogue---assuming they are really
total V magnitudes and mean surface brightnesses (in units of mag/square
arcminute usually), you will simply have to adjust your personal scale
of "this mag/sfc br means about this difficult to see".  Despite the
apparent conflict, I would adjust your scale to V-sub-T, i.e. total V
magnitudes, not to B or some other (particularly non-total) magnitudes.
Because most people fail to take this perception effect into account,   
recent reports about Comet LINEAR's brightness are also too faint.
Comparing the comet to open clusters (having bright backgrounds) to the
comet in another part of the sky, where the natural and local backgrounds
are different will almost guarantee the magnitude estimate will be wrong.

>> Doing a search for M81's "photometric data points"...

     That's the place too look.  Note that the NED header pages for an
individual galaxy usually list the total B magnitude, or some other blue-light
magnitude that's not necessarily an integrated value.  This isn't specified
and is a known problem, as it were.  One of these days we'll get to fixing

>> ...inclusion of the revised New General Catalog (NGC) database,
>> published earlier this year...

>> ...which Doug mentions from the revised NGC...

     There is no such revision that I know of.  What is the source?

>> I'd imagine that for those
>> folks trying to compile databases for amateur use, this must lead to
>> all kinds of nasty confusion...

     Right, especially if they don't know anything about photometry!

>> Is it possible that NED has simply
>> not been updated with more recent total V magnitude measures for M81...?

     It's been a couple of decades since anyone actually measured M81 for
total brightness.  The numbers are bascially unchanged since the RC2 from
1976.  Same goes for nearly all of the brighter few thousand galaxies,
whose magnitudes (in general) are well in hand at the couple-tenths of a
magnitude level.

>> ApJ costs a few thou/year!

     "Only" $1500/year, and papers more than two years old are available
for free on-line back to issue number 1 in 1895.

>> ...to magnitude estimates in OTHER BANDS such as 'I', 'V', etc...
>> In other words,
>> all the recent research I could find only relates to how bright
>> the galaxy M81
>> would appear in infrared, ultraviolet, x-ray or other non-amateur telescopes!

     Just ignore all that stuff.  Again, no one has measured M81 in the
manner of interest in a long time.

>> I suspect the V magnitude value of "about 6.9" may have to stand...

     The figure from RC3 is Vt = 6.94 +/- 0.03.  That uncertainty is a
formal one, and in fact total magnitudes of galaxies seem to have a minimum
error of around +/- 0.15 or thereabouts.  But, yes, V=6.9 will stand until
someone shows that the raw data that went into that integrated value is
somehow wrong---not likely, since the data were compiled from many mutually
consistent sources.

>> And what *is* the "experienced
>> amateurs'" consensus on M81....

     You mean about its total magnitude?  Doesn't matter, since there is
real data to go on.

>> ...and in particular its naked-eye visibility?

     Several observes have found it reliably, particularly indicating in
their reports the process of sorting out the neighboring mag. 7 and 8 stars
in order to isolate the galaxy.  Most who attempt this don't make this
effort, so their reports aren't reliable.  I've copied out an old s.a.a.
post about finding M81 to the Lowell ftp area:


     So basically folks need to calibrate their eyeballs a little more
carefully to account for surface brightness.  Read the Carlin/Bartels/Clark
stuff at Mel's Web site.  OBserve galaxies noting their apparent 
difficulty as compared to the combination of total brightness and the
mean surface brightness.


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