Sky & Telescope - October 2020 - 33

M 92: STEFA N BINNE WIES / JOSEF PÖPSEL; M10: RON BRECHER; M12: STEFA N
BINNE WIES / R AINER SPA RENBERG / JOSEF PÖPSEL; M55: JOSEF PÖPSEL

S For Northern Hemisphere observers Messier 92 is one of the most satisfying
and distinctive globulars for moderate-sized scopes. With an 8-inch M92 is far
superior to M5, for example. At 116× M92 presents a 3:1 rectangular outer halo
running north-south, within which nestle a round inner halo and brilliant core,
also round. At 244× there are 35 fairly bright stars, plus probably as many very
faint stars that fade into and out of view as the seeing changes. The inner halo
is 2′ in diameter, and the unresolved underlying haze is splotchy. The brightness
jumps suddenly at the edge of the 40″-diameter core and then remains fairly
constant across the star-studded core.
A 12-inch SCT at 220× added spiraling star streamers. The 16-inch at 366×
unveiled hundreds of stars, and M92 no longer looked rectangular.

S Messier 55 is the best of the unconcentrated globulars. In a 13-inch Dob
in Kentucky this cluster was huge, loose, and well-resolved with a multitude of
sparkles on a prominent background glow.

S Neighbors Messier 10 and Messier 12, only a
little more than 3° apart, are usually viewed in succession. The sum is greater than its parts, since M12
wouldn't make my list by itself.
M10 (above) is a rewarding globular through my
8-inch at 244×. Through careful examination of star
densities as M10 drifts across the field I can discern
its full 15′-diameter outer halo, where some 50 stars
are visible, including five long but sparse chains.
I resolved another 40 stars in the inner halo and
right across the 1.5′ core. The core brightness only
increases gradually.
My 16-inch at 174× showed two long curved star
chains, one north of the core and the other to the
south, that jointly form a flattened reverse S. I easily
resolved bright stars across the core, on a granular
background. At 261× both
the core and the halo showed
resolved stars that all seemed
to be magnitude 12 and 13,
but there was a curious lack
of fainter stars. Upping the
power to 348× brought in the
magnitude-15 sparks when the
seeing allowed; but I just didn't
spot any magnitude-14 stars!
In looser M12 (right) the
brightest stars are arranged
in pairs or trios, giving it a
strange appearance, like
an open cluster overlying a globular. In my 16-inch
at 229× M12 is well resolved right through the only
slightly concentrated core, with about 50 bright stars
on many faint ones. The halo consists of seven ragged
star chains: three on the northern side, two on the
western, and two on the southern. The core is elliptical
in a ratio of 2:1, aligned east-west, and lies entirely on
the northern side of the center of the halo. An orange
star decorates the southern edge of M12's core.
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Sky & Telescope - October 2020

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