Sky & Telescope - October 2019 - 30

Herschel's Garnet Star. William Herschel described the star
as having "a very ﬁne deep garnet colour" in a 1783 paper,
and it's indeed one of reddest stars visible to the naked eye.
Mu Cephei is also one of the largest stars ever discovered.
With a radius between 1,200 and 1,600 times that of our
Sun, the Garnet Star would engulf the orbit of Jupiter if
placed at the center of our solar system. This supergiant star
is fusing helium to carbon, and in a few hundred thousand
years it will blow itself apart in a supernova.
From death to birth: 1.5° south of Mu Cephei you'll ﬁnd
Trumpler 37, a very young star cluster. This sprawling ﬁeld
of suns is only 3 to 4 million years old, which means that
our ancestors were making stone tools in Africa's Great Rift
Valley when these stars were forming. At the center of the
cluster, 6th-magnitude HD 206267 and 8th-magnitude HD
239729 form a nice binocular double. If you're under very
clear, dark skies, see if you can pick out the gigantic but faint
nebula IC 1396, in which Trumpler 37 is embedded.

q A SIMPLE STRUCTURE Though meant to represent a king, the stars
of Cepheus could just as easily be used to build a house with a gabled
roof. The riches here - Mu Cephei, Trumpler 37, NGC 7160, NGC 7235,
and Delta Cephei - all reside in the lower story or cellar of this celestial
domicile.

Cepheus
γ

β
The King's Gift

4
M52

7160

ζ
7235

μ
Tr 37

OCTOBE R 2 019 * SK Y & TELESCOPE

Scan from Trumpler 37 back past Mu Cephei and on for
another 4° to ﬁnd a tight, tiny cluster of stars: NGC 7160.
The cluster is bright, at 6th magnitude, but very compact,
with a diameter of just 5′ (arcminutes), so the individual stars
are tricky to resolve even at 15×. So instead of zooming in,
broaden out, and survey the sweep of bright stars scattered
between Alpha (α) and Delta (δ) Cephei. This is the Cepheus
OB2 Association, of which NGC 7160 is a part. Like other
OB associations, Cepheus OB2 is full of hot, young, massive
stars. The spiral arms of our galaxy are star factories, and
Trumpler 37, NGC 7160, and the rest of Cepheus OB2 are the
output of that ongoing process.
NGC 7160 lies about 2,600 light-years away, which is
about the average for the brighter Messier, Caldwell, and
NGC clusters. Our next target takes us farther out. Look
just west of the line between Zeta (ζ) Cephei and Epsilon
(ε) Cephei to ﬁnd NGC 7235. NGC 7235 is about the same
apparent size as NGC 7160 but much less bright, in part
because it's about four times as distant at around 10,500
light-years. Whereas NGC 7160 - along with our own solar
system - is part of the Orion Spur, NGC 7235 lies beyond,
in the great Perseus Arm of the Milky Way. In comparing the
two clusters, you're getting a glimpse of the larger structure
of our galaxy.
While you're in the neighborhood and thinking about
galactic structure, check out Delta Cephei, just 2.5° eastnortheast of NGC 7235. Delta Cephei is a variable star, the
prototype and namesake of the Cepheid variable class. As
early as 1908, Henrietta Leavitt of Harvard College Observatory noted that Cepheid variables have a period-luminosity
relationship, such that those with longer variability periods
are intrinsically brighter than those with shorter periods.
When Edwin Hubble observed Cepheid variables in the
Andromeda Galaxy, this property allowed him to ﬁ x Andromeda's distance and thus establish that "spiral nebulae" are
other galaxies, and not features inside the Milky Way. Delta
Cephei is also of interest as a charming double star for
binoculars and small telescopes. The secondary, a blue-white
main-sequence star, lies 41″ (arcseconds) south-southwest
of the yellow supergiant primary - a tight split for anything
under 15×.
The next stop on the tour has no formal name that I've
been able to discover, but in my notes I call it "the King's
Gift". Halfway between Iota (ι) Cephei and 4 Cassiopeiae
you'll ﬁnd a dense stream of stars running northeast to
southwest, crowding an area about 5° long and 2° wide. In
reality this is a group of unrelated stars scattered between
100 and 1,500 light-years away, but in mind it's a trove of
jewels that Cepheus is presenting to Cassiopeia, hence my
nickname for it.
It's time for us to attend the Queen as well, and our ﬁrst
stop in Cassiopeia is M52. This charming 7th-magnitude
open cluster stands out as a distinctly bright patch in the
dense Milky Way starﬁeld. See if you can pick out a bright
yellow F-type star near the western margin of the cluster.

A K IR A FUJII

Bino Treasure

Sky & Telescope - October 2019

Table of Contents for the Digital Edition of Sky & Telescope - October 2019