101 SKY OBJECTS
75 M15
While most of the Milky Way’s globular
clusters are found in the summer sky
hovering around the galactic center,
a few stragglers reside off the beaten
path. One such cluster is M15, located
near the nose of the autumn constella-
tion Pegasus the Winged Horse.
M15 was first spotted in 1746
by Jean-Dominique Maraldi, who
described it as “a quite bright, nebu-
lous star, which is composed of many
stars.” Charles Messier rediscovered
it 18 years later, but because his tele-
scope couldn't resolve the cluster, he
described it as a “nebula without star.”
Binoculars reveal M15 as a small,
misty glow accented by a brighter
core about 4° northwest of Enif
(Epsilon [ε] Pegasi). A 4-inch telescope
can resolve some of the stars around
the fringe of this globular. For the best
views, try a moderate- to high-power
eyepiece greater than 100x, if seeing
conditions permit.
M15, located some 34,000 light-years
from Earth, crams more than 100,000
stars into a space about 175 light-years
across. Nestled among those stars are
some unusual suspects, too, including
eight pulsars, a double neutron star
named M15-C, and more than 100
variable stars.
Another unique resident of M15 is
the first planetary nebula found in a
globular. German astronomer Friedrich
Küstner first cataloged this planetary
in 1921, although he erroneously listed
it as just another cluster star. However,
subsequent studies by Francis Pease
with the 100-inch (2.5-meter) Hooker
reflector at Mount Wilson Observatory
revealed that Küstner’s star was not
a star at all, but instead a planetary
nebula. Today, we know it as Pease 1.
To visually spot Pease 1, you’d need
at least a 15-inch telescope, 300x or
greater, and an OIII filter. Pristinely
dark skies are not necessary and
it can be seen in an 18-inch scope
through suburban light pollution.
However, your sky must be exception-
ally transparent and steady. — P. H.74 IC 1396
Cepheus is a rich region of the Milky Way with many
emission nebulae. One of the largest, IC 1396, could be
called the Northern Rosette (see The Rosette Nebula,
#6) because it resembles the more famous nebula in
Monoceros. IC 1396 is about 3° wide, with a hollow interior
showing minimal nebulosity. This fascinating object sits
some 2,400 to 3,000 light-years away.
The nebula is illuminated by hot stars in its gas-poor
interior, where the radiation has physically pushed the gas
away. This star cluster is less conspicuous than NGC 2244
in the true Rosette Nebula. The brightest parts of IC 1396
are its northwest and eastern edges. Photographs show a
complex of dark nebulae threaded throughout the perime-
ter. Many of the dust structures are aligned so they appear
to radiate away from the stars in the nebula’s core. Those
that don’t are likely on our side of the roughly spherical
nebular shell. Six of the dark nebulae were discovered
by Edward Emerson Barnard: In order of decreasing size,
they are B160, B161, B365, B163, B162, and B367.
IC 1396’s most famous feature is IC 1396A, better known
as the Elephant Trunk Nebula. The trunk of this dark nebula
is formed by an irregular pillar of dust many light-years long.
Some observers find it easier to spot than the Horsehead
Nebula (see #28) — another (arguably more famous) dark
nebula — because the Elephant Trunk is larger.
On IC 1396’s northern edge is Herschel’s Garnet Star (Mu
[μ] Cephei). This supergiant star is similar to Mira and is the
archetype of a class of semi-irregular variables called Mu
Cepheids (not to be confused with Cepheids, named after
Delta [δ] Cephei). Mu varies between magnitude 3.4 and 5.1
over the course of two to two and a half years. This spectral
type M2 star is a colorful contrast to the gray nebula.
Some observers report the nebula is visible as a
magnitude 5.6 gray patch with naked-eye averted vision.
Others instead give its surface brightness, or brightness per
unit of area, as about 14 magnitudes per square arcminute.
If you observe under exceptionally dark skies, add it to your
ANDREI PLESKATSEVICH list of targets. You can decide how bright it appears. — A.G.DAN CROWSON