Sky & Telescope - September 2022 - 45

Evenings with the Stars by Fred Schaaf
Visiting the
House of
Cepheus
This circumpolar constellation
is full of fascinating finds.
O
n September evenings there's a
temptation to begin a night of
stargazing with the bright constellations
of summer that still linger near the
meridian. But let's not be hasty. If you
turn your attention toward the north,
your eye will doubtlessly be drawn to
the distinctive W of Cassiopeia, the
Queen, rising in the northeast. But it's
her mythological consort, Cepheus, the
King, that's positioned higher in the
evening sky. And although Cepheus is
much dimmer, it's a truly varied and
fascinating constellation.
My favorite path to Cepheus at this
time of year is via the bright Cygnus
Milky Way. In last month's column I
mentioned the big, dark notch in the
Milky Way centered about 7° northnortheast
of Deneb. If you proceed
northward from this feature, you'll
arrive at a dim, little triangle of stars
that includes the outstandingly important
variable star, Delta (δ) Cephei.
(More about Delta later.) This triangle
lies almost exactly halfway between
Deneb and Cassiopeia and is only a
small part of Cepheus.
The basic pattern of Cepheus
resembles a simple house with a steep
roof. The roof's peak is marked by
3.2-magnitude Gamma (γ) Cephei,
also known as Errai. In 1988 Gamma
became famous as the first star around
which an exoplanet was detected. However,
the existence of that planet was
only confirmed in 2002 - and by then,
dozens of other worlds had been found
orbiting other stars.
But Errai has another claim to fame.
With the exception of Polaris, it's the
KING OF THE NORTH The view presented in this photo is facing north. The brightest
star (near the top right of the frame) is Deneb, in Cygnus, while the eye-catching W of
Cassiopeia is found at lower right. Use the Northern Hemisphere Sky Chart on pages 42
and 43 to locate Cepheus, the King, in the center right of this image.
brightest star within 15° of the north
celestial pole. Thanks to the precession
of Earth's axis, it will become the next
major North Star, shining near the celestial
pole about 2,000 years from now.
The rest of the main pattern of
Cepheus is the square of the house
itself. At its northwestern corner is Beta
Cephei, commonly known as Alfirk. It
shines at about the same brightness as
Errai, as does Zeta (ζ) Cephei, which
occupies the house's southeastern corner.
Holding down the northeast corner
is 3.5-magnitude Iota (ι) Cephei, which
is by a very slight margin the house's
faintest star. At the opposite (southwestern)
corner is the constellation's
brightest star, 2.5-magnitude Alpha (α)
Cephei, also known as Alderamin.
The southeastern corner is notable
not just for Zeta, but also for Epsilon
(ε) and Delta - the two other stars
that join Zeta to complete the compact
triangle mentioned earlier. Delta
is well known as the archetype of the
Cepheid class of variable stars. Remarkably,
the length of a Cepheid's period
is proportional to its luminosity - the
longer the period, the more luminous
the star. First noted by Henrietta Swan
Leavitt, this period-luminosity relation
makes Cepheids crucial yardsticks for
measuring distances (S&T: Dec. 2021,
p. 12). Delta Cephei itself dims from
3.5 to 4.4 in about four days, then
returns to maximum again in about a
day and a half.
Another notable variable star in
Cepheus is the vast, cool supergiant
Mu (μ) Cephei. Its brightness ranges
typically from about 3.9 to 4.5, with
extremes of 3.4 and 5.1. The changes
are irregular over an average period of
835 days. Binoculars reveal Mu's very
red hue, which earned it the title of
Herschel's Garnet Star.
¢ FRED SCHAAF is now beginning his
17th year teaching astronomy at Rowan
University in Glassboro, New Jersey.
sk yand tele scope .o r g * SEPTEMBER 2022 45
AKIRA FUJII
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Sky & Telescope - September 2022

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