Sky and Telescope - July 2018 - 55

3° above my horizon, it's too low to
proﬁtably observe. Arbitrarily disqualifying NGC 6541 gives seventh place to
Messier 55 in Sagittarius. Through the
refractor at 48×, M55 is a large, ghostly
glow with a dash of very faint stars
overlaying its outer reaches. This 11′
cluster is delicately pretty at 102×, with
a score of stars scattered across its hazy,
ashen surface. A more conspicuous star
tacks the halo's south-southeastern
edge. With the reﬂector at 68×, M55's
irregular form is nicely speckled with
stars. It's well resolved at 187× and
shows almost no concentration toward
the center. Prominent star chains
crisscross the cluster and darkling veins
decorate its face. A relative paucity of
stars creates a murky bay that dents
into M55's southeastern side.
For sixth place we turn to Messier
15 in Pegasus. This pretty cluster grows
ever more brilliant toward the center in
the refractor at 37×. M15 takes magniﬁcation well, and at 234× it shows some
resolution right down to a very small,
intense core that hosts a tiny, dazzling
center. Wide spikes of stars stick out
from the cluster, and the sparsely populated outer halo spreads to a diameter
of roughly 8′. M15 is gorgeous in the
reﬂector at 187×. Many of the halo stars
gather into starﬁshy arms, which yields
an irregular diameter of as much as 10′.
Even the brilliant core surrenders some
resolution into stars. At 299× they show
a large range in brightness, and apparent outliers stretch the group to 11′.
Messier 3 in Canes Venatici holds
ﬁfth place, sporting a bright core and
fairly bright, 9½′ halo when viewed

through the refractor at 37×. At 164×
the halo and core are rich in pinpoint
stars, the core brightly backdropped
with mist. A scattering of brighter stars
stands out from the rest, most notably
an 11.7-magnitude star on the western side. At 234× this beautiful group
shows stars right across the core's 2½′
glow. Some dusky areas shadow the
northeastern part of the outer core, and
the cluster's brightest region extends
northeast-southwest. In the reﬂector at
187×, the 7′ outer core is irregular and
vaguely boxy with a ﬂat eastern side.
M3's thinly populated halo swells the
cluster to about 14½′.

Fourth place goes to magniﬁcent
Messier 13 in Hercules. At 37× the
refractor bares a large, bright core with
a very bright center and a 10′ halo
dosed with stars. The cluster is beautifully resolved at 102×, with its signature, drooping arcs of stars giving it a
tarantula-like mien. There's even some
resolution across the core. At 164× M13
is well resolved and at least a dozen
stars ﬂeck the center. Outliers stretch
the group to about 11′. In the reﬂector at 187×, M13 presents a wonderful ﬁreworks display frozen in time
for our prolonged enjoyment. A dusky
three-bladed propeller garnishes the

Summer Globular Clusters
Object

Mag(v)

Size

Vtip

Vhb

Dist (l-y)

Dec.

6.47

16′

13.1

16.05

38,000

21h 33.5m

-00° 49′

M62

6.45

15′

13.5

16.25

22,000

17h 01.2m

-30° 07′

17.1m

+43° 08′
-30° 58′

M92

6.44

14′

12.1

15.10

27,000

17h

M55

6.32

19′

11.2

14.40

18,000

19h 40.0m

M15

6.20

18′

12.6

15.83

34,000

21h

30.0m

6.19

18′

12.7

15.64

33,000

13h 42.2m

+28° 23′

+12° 10′

M13

5.78

20′

11.9

14.90

23,000

16h 41.7m

+36° 28′

18.6m

+02° 05′

5.65

23′

12.2

15.07

24,000

15h

5.63

36′

10.8

13.45

7,200

16h 23.6m

-26° 32′

10,000

18h

-23° 54′

M22

5.10

32′

10.7

14.15

36.4m

Angular sizes and separations are from recent catalogs. Visually, an object's size is often smaller than
the cataloged value and varies according to the aperture and magnification of the viewing instrument.
Right ascension and declination are for equinox 2000.0.

M55: HUNTER WILSON

M55 takes magniﬁcation well. As you pump up the power, its irregular form should reveal
more and more individual stars. Unlike M2, M55 shows no bright core at its center.

s k y a n d t e l e s c o p e . c o m * J U LY 2 0 1 8

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Sky and Telescope - July 2018

Table of Contents for the Digital Edition of Sky and Telescope - July 2018