SEPTEMBER/OCTOBER 2019 •SKYNEWS 19
tude 6.2, M 15 is barely half a magnitude
fainter than M13, the Great Hercules Clus-
ter, and it spans roughly 12.3 arc minutes
of sky. But the devil is in the details. The
brightest members of M15 are only magni-
tude 12.6, and the vast majority are at least
three magnitudes dimmer. Moreover, the
cluster’s central region is jam-packed. In
terms of concentration, M15 is a fairly
dense class-4 globular, where 1 is the most
compact and 12 the least. Can we expect
backyard telescopes to reveal a blaze of pin-
points? Let’s find out.
Locating M15 is easy. We begin at 2.4-
magnitude Epsilon (ε) Pegasi, also known
as Enif, southwest of the Great Square of
Pegasus. From Enif, hop four degrees north -
westward to a 6.1-magnitude star (labelled
HD 204862on our chart). If you have an
optical finderscope, you might pick up M15
as a teensy fuzzball ¼ degree west of that
star. Or simply aim your telescope at the
star itself—M15 will appear in the same
low-power field.
Your telescopic field will also contain a
7.3-magnitude star ¼ degree southwest of
M15 and a 7.6-magnitude star near the
cluster’s north-northeast edge. Together
with HD 204862, these stars form a triangle
roughly framing M15. The triangle stars are
handy for ensuring your scope is sharply
focused. Admittedly, the two brightest stars
are somewhat distracting, but they’ll be-
come excluded from the field of view as you
crank up the power. At high magnification,
only the unobtrusive 7.6-magnitude star
will be visible as you scrutinize the cluster.
When I use 22×on my 4¼-inch f/6 re-
flector telescope, M15 is a well-defined cir-
cular haze, perhaps two arc minutes across,
growing brighter toward the centre. At 54×,
I can detect the cluster’s tenuous outskirts
as a dim halo, provided I look slightly to
one side, allowing the cluster’s light to fall
on the most sensitive portion of my eye.
At 72×, averted vision lets me pick up a
few barely perceptible stars in the halo. A
long look at 100×picks up several more. Soeven in small scopes, the cluster is enticing.
Larger scopes elevate that enticement.
Working with an 8-inch f/10 Schmidt-
Cassegrain telescope at 120× last fall, I
perceived M15 as a spherical mist of faint
stars enveloping a brilliant nucleus. At
226 ×, the cluster was impressively larger
(albeit dimmer), with lots of dots scattered
all around. Viewed in my 10-inch f/5.5
Dobsonian at similar magnifications, M15
morphs into a veritable pincushion of stars
—except for its compact core, which re-
mains stubbornly unresolved.
Needless to say, that ultracrowded cen-
tre has caught the attention of professional
astronomers. Indeed, they theorize that a
massive black hole lurks deep inside M15.
As you observe this exceptional globular,
think of it as the radiant cluster with the
dark heart. ✦SkyNewsassociate editor Ken Hewitt-White
loves globular clusters, with or without black
holes.http://www.All-StarTelescope.com
1-866-310-8844
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