Sky and Telescope - September 2017 - 54

SEPTEMBER 2017 OBSERVING
Deep-Sky Wonders by Sue French

Mu Boötis

Showpiece Doubles
Point your telescope toward these gems of the late-summer sky.
N

BOÖTES

μ
γ
δ

CORONA
BOREALIS

α
η

p The name Boötes, meaning "Herdsman" or "Plowman" in archaic Greek, can be traced as far
back as Homer, whose hero Odysseus used the constellation to guide his night journey away from
Ogygia, the island of Calypso. Izar, the common name for Epsilon Boötis, translates to "girdle" or
"loin cloth" and marks the waistline of the celestial keeper of cattle.

S E P T E M B E R 2 0 1 7 * SK Y & TELESCOPE

p Left: Fourth-magnitude Mu1 Boötis is
sometimes called Alkalurops, a mellifluous
name that derives from the Arabic adaptation
of the Greek kalaurops, meaning "a shepherd's crook" or "staff." The above sketch
shows Mu1 and the Mu2 pair as viewed at
240× through a 6-inch Newtonian.

constellation ﬁgure. William Herschel
discovered Izar's dual nature in 1779,
describing it as "very beautiful." In his
1867 double-star catalog, the Reverend
William Rutter Dawes called Izar "one
of the most beautiful stars in the heavens," but his contemporary Friedrich
Georg Wilhelm von Struve is generally
credited with bestowing the name Pulcherrima (most beautiful) on this pair.
Izar is nicely split through my 105mm refractor at 122×. The brilliant,
golden primary watches over a snowy
white companion to the north-northwest that shines 2.2 magnitudes dimmer. Many observers see the companion
as blue or green, a common colorcontrast illusion that can sometimes be
dispelled by using enough magniﬁcation
to widely separate the stars.
Izar's primary is a K0 giant that has
exhausted the hydrogen fuel in its core.
Its outer layers have expanded, turning
the star into an immense, amber light
bulb 33 times as big across and 400
times as bright as our Sun. The white
star is an A2 dwarf - a main-sequence
star that's still burning hydrogen in its
core. It's about twice as big across and
27 times as bright as our Sun. According
to Kaler, the pair's orbital period must
be well over a thousand years.
Farther north in Boötes we ﬁnd Mu1
1
(μ ) and Mu2 (μ2) Boötis. Mu1 also
bears the name Alkalurops (uhl-kuhLOO-rops), which in the course of a

A K IR A FUJII; SK E TCH: JEREM Y PERE Z

among what I affectionately call Kaler's
stars, stars with fascinating bios on
Jim Kaler's Stars website (https://is.gd/
kalerstars), which I'm pleased to credit
for many of the details here.
We'll begin with the highly praised
double Epsilon (ε) Boötis, commonly
known as Izar (EYE-zar). The name
means "girdle" or "loin cloth" and
refers to the star's position within its

bserving multiple stars is a joy.
They come in varied combinations of brightness and color, they often
have tantalizing qualities to spark the
imagination, and you don't need dark
skies or a big telescope to admire them.
Let's immerse ourselves in the wonders
some of the best and brightest offer,
each labeled on the all-sky chart at the
center of this magazine. They're also

https://www.is.gd/kalerstars https://www.is.gd/kalerstars

Sky and Telescope - September 2017

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