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(IAAC) Obj: M56 - Inst: 17.5" f/4.5 dob



Observer: Lew Gramer; Steve Clougherty
Your skills: Intermediate (some years); Advanced (many years)
Date/time of observation: 11/12 May 2001, 23:00 Local
Location of site: Myles Standish Forest, S Carver MA US (Lat 42N, Elev 5m)
Site classification: Exurban
Sky darkness: 6.5 <Limiting magnitude>
Seeing: 4 <1-10 Seeing Scale (10 best)>
Moon presence: None - moon not in sky, then Minor - far from object
Instrument: 17" f/4.5 dob
Magnification: 57x, 125x, 222x
Filter(s): None.
Object(s): M56
Category: Globular cluster.
Class: X
Constellation: Lyr
Data: mag 8.4 13.0pm*; size 7.1 irregular
Position: 191636 +3011.1
Description:
The stately M56 was a study in contrasts (literally AND figuratively) on this
fortuitous night. "Fortuitous" because the New England weather had one of its
rare POSITIVE surprises in store for us, as a cirrus overcast that had dogged
sunset essentially disappeared during waning twilight. Our study in contrasts
came as we had to observe in and out of drifting contrails all night, finally
watching through the eyepiece with bemused interest as this and several other
objects gradually faded in the rising moonlight after midnight...
--
In the best of conditions this night, in the rare times during the moonless
hours when transparency seemed perfect, our impression of M56 was actually a
bit underwhelming... We commented to ourselves that it was less interesting
than other objects we had the chance to observe that evening, in particular
M92 and M62 (logs soon to be submitted for these objects). In fact the most
striking thing we noted about this view under these conditions was not the
globular itself, but the lovely Milky Way star field in which it sits! The
medium power field around M56 is sprinkled with stars mags 10-12, forming
several complex arrays of asterisms all around the diffuse little globular.
--
However, strangely enough as the contrast worsened (first due to occasional
high clouds, then later due to the rising moon), our interest in M56 itself
actually increased! Suddenly, a "bite" (or "dark lane") stretching from the
NE corner of M56's diffuse core almost all the way through toward the SW,
became prominent. (I've noted similar "slices" in the cores of other bright
globs before. See my prior logs for M2, M3, M5, M10, M12, M15, M22 and most
prominently M92! This is an interesting visual "trick", usually best noted
when conditions are the worst. I've yet to hear it convincingly explained.)
--
What's more, with the poorer contrast an even more striking feature (one I
have never before noticed) became apparent: I quickly noted what I can only
describe as "ramifying waves", or outward-falling cascades of well-resolved
stars, stretching like ripples from the NE corner of the innermost region of
the globular. I commented to Steve Clougherty that "features" like this make
it clear how readily the human eye and brain can perceive "patterns" in what
is essentially a random scattering of stars: and yet interestingly enough, I
have since seen this very feature noted by other well-known observers on the
Web, and even hinted at in Harlow Shapley's description of this globular as
having an "elongation"! However, Shapley records the elongation as having a
PA of 45 degrees (i.e., NE). See Hartmut Frommert's wonderful page about M56
in his SEDS Messier Database: http://www.seds.org/messier/m/m056.html
--
So are these "slices" and "ripples" somehow true features, related perhaps
to the orbital dynamics of certain globulars? Or are they just "Rorschach
Tests" for amateurs to ponder, amidst truly random stellar distributions?
Such "curious questions" are among the things I enjoy most about deep-sky!
--
Some prior IAAC logs of bright Globulars where "dark" features were noted:
http://www.visualdeepsky.org/msg01407.html
http://www.visualdeepsky.org/msg00050.html
http://www.visualdeepsky.org/msg00049.html
http://www.visualdeepsky.org/msg00587.html
http://www.visualdeepsky.org/msg00258.html
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