(IAAC) Obj: M5 ("Gladiator's Helmet" Globular) - Inst: 20" f/5 Dobsonian

Observer: Lew Gramer
Your skills: Intermediate (some years)
Date/time of observation: 1998-01-01/02 11:40 UT
Location of site: Deerfield Township, OH, USA (Lat 41oN, Elev 100m)
Site classification: Rural
Sky darkness: 6.5 <Limiting magnitude>
Seeing: 7 <1-10 Seeing Scale (10 best)>
Moon presence: None - moon not in sky
Instrument: 20" f/5 Dobsonian
Magnification: 70x, 210x, 420x
Filter(s): None
Object(s): M5 ("Gladiator's Helmet" Globular)
Category: Globular cluster
Class: 5
Constellation: Ser
Data: mag 5.75  size 17.4'
Position: RA 15:19  DEC +02:05
Among my more daft observing projects is a long-term effort to observe the
effects of changing sky contrast on globular clusters. I became interested
in this after one long night of observing, when dawn caught me unawares at
the eyepiece looking at M2! Since then, I've observed M3, M4, M13, and M22
against the kaleidoscopic contrasts of twilit skies. So on this particular
night, as the dewy fields and buildings of my friends' farm gradually began
to resolve, I stomped through the icy barnyard mud and swung the 20" scope
over to Unukalhai (alpha Ser). Sweeping wide and far SW to 10 Ser, and then
to 5 and yellowish 6 Ser, M5 floated into view. Well resolved at first to a
tiny, brilliant core even at 210x, the faintest outliers and well-resolved
clumps near the center all gradually began to blur in the light, and patterns
and shapes manifested themselves as the contrast changed. The haze within
the main core of the GC formed an intriguing "helmet" shape, with a nearly
linear SE edge running from the NE end of the cluster, through the center all
the way to the SW end. Poking out SE of this dark edge from the cluster's
core was a small "nub" of bright haze, almost like an extension of the core.
The overall image of the broad core, edge, and this nub was surprisingly
reminiscent of a Roman gladiator's helmet, complete with "ear flap"! To the
S and E beyond this curious helmet was a striking dark area, some 4' wide and
5' long. Just beyond this interregnum lay a lovely arc of mag. 12-14 outlier
stars, stretching from the E around to the SW. From this arc S & SE stretched
still further outliers, all the way to a fine-grained irregular scattering
some 10' out from the inner core. To N and W on the other hand, the "helmet"
ended abruptly in a fine sprinkling of field stars, much less unusual than
the field to the S, especially in the dark NW region. Even this faint halo
of unresolved stars only stretched some 7', with a lonely "peak" protruding
NNW of the core. Finally, and visible well into bright twilight, two bright,
tight, irregular clumps of unresolved stars could be seen lying symmetrically
E and N of the still-bright core. A gorgeous sight by any light!
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