Sky and Telescope - October 2017 - 56

OCTOBER 2017 OBSERVING
Deep-Sky Wonders

Cetus
ι

Mira
α

ο
τ

p Omicron Ceti, better known as Mira, is the prototype star for that class of variables. While Mira's
full period is 332 days, its climb from maximum dimness to maximum brightness takes only about
126 days. Look for it to reach about magnitude 3.4 in January 2018.

O C T O B E R 2 0 17 * S K Y & T E L E S C O P E

Cepheus

γ
ι
δ
ε

β
μ

p Delta Cepheid, the prototype star for the Cepheid variable class, has a relatively short period, cycling through its variability in just 5.366
days. Delta's apparent brightness is affected by
that of its companion, whose light contributes a
0.2-magnitude bump up to the primary.

to 4.2 because Delta's light is combined
with that of a 6.3-magnitude companion 41″ to the south. The duo separates
into a pretty yellow and blue-white pair
at low power through any telescope.
Good comparison stars for following
Delta are Iota Cephei at magnitude
3.5, Alpha (α) Lacertae at 3.8, Epsilon
Cephei at 4.2, and Beta Lacertae at 4.4.
The variables described above are
conveniently labeled on the all-sky
chart at the center of this magazine. If
you ﬁnd that watching variable stars
strikes a chord with you, you can delve
more deeply with the charts and other
resources of the American Association
of Variable Star Observers (AAVSO) at
aavso.org.
At ﬁrst enigmatic, these changeable stars have now yielded some of the
secrets behind their inconstant light to
those who studied them across the years.
But variable stars their mysteries still do
keep. They slowly blink at us as if to say,
"Watch me. Watch me and learn."
¢ Contributing Editor SUE FRENCH
welcomes your comments at scfrench@
nycap.rr.com.

TILL CREDNER (2)

Omicron (ο) Ceti, the prototype of
the Miras, is currently making its way
into our evening sky. Its common name,
Mira, means "wonderful" and was
bestowed by Johannes Hevelius in his
1662 Historiola Mirae Stellae, where he
described the star's remarkable variability. As with Algol, awareness of Mira's
changing intensity is rumored to have
ancient origins, but solid credit goes to
the German clergyman David Fabricius.
While measuring Jupiter's position in
1596, Fabricius noticed a 3rd-magnitude
star where none was seen before. He
watched it brighten and fade to invisibility over the course of two months. He
wrote several tracts about the star and
sent his observations to Tycho Brahe.
Mira should be bottoming out this
September. Its climb to maximum takes
only 38 percent of its 332-day cycle, so
the star ought to beam at maximum
light in January. Its peak can reach as
high as 2nd magnitude, but typical
maxima tip the scale at about 3.4. The
star can be stalked with a small telescope
as soon as it's high enough in the sky,

since even extreme minima only dip to
magnitude 10.1 (S&T: Oct. 2016, p. 49).
Cepheid variables hold a place among
the stars most valued by professional
astronomers. They're pulsating yellow giants that rhythmically expand
and contract, but with breaths much
shorter than those of Mira variables.
A Cepheid's luminosity and period
are closely tied. Once the relationship
was established and calibrated with
an independently determined Cepheid
distance, the distances to others could
be calculated by observing their periods
and comparing how bright they appear
to how luminous they are. Because they
are highly luminous, Cepheids can be
used to establish distances to galaxies as
much as 100 million light-years away.
Delta (δ) Cephei is the prototype of
the classical Cepheid variables. It sits
near the southernmost corner of the
stick-ﬁgure house often drawn on sky
charts to connect the constellation's
brightest stars. It varies from magnitude
3.5 to 4.4 and back over a period of
5.366 days, with the rise to maximum
taking only 1.342 days (32 hours). However, if you follow the star with your
unaided eye, the magnitude range is 3.4

http://www.aavso.org

Sky and Telescope - October 2017

Table of Contents for the Digital Edition of Sky and Telescope - October 2017