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Giuseppe Piazzi showed, based on
repeated observations, that 61 Cygni
was moving noticeably against the back-
ground of the other stars in the Swan.
Because of its high proper motion, he
christened it the Flying Star. Three
decades later, Friedrich Bessel reasoned
that 61 Cygni must be relatively close to
Earth. After years of observations, he
was able to make the first measurement
of stellar parallax, using the double star’s
motion against the more distant, station-
ary background to determine it is only
10.4 light-years away. Modern observa-
tions place it just 1 light-year farther.
Next, let’s move on to Virgo. The
Maiden looks like a giant letter Y.
Gamma Virginis, also known as
Porrima, is the star at the joint of this
Y. On a spring night, turn a tele-
scope on Gamma; the
two stars of this
system are almost
equally bright at
magnitude 3.65
and 3.56. They
look like twotiny headlights in space.
This particular double star helped
confirm the universal nature of Newton’s
theory of gravity. Early 19th-century
astronomers, including Herschel, spent
years refining their observations of
this fascinating star system. The two
stars have an orbital period of almost
169 years. At their greatest angular sepa-
ration, they are nearly 6" apart. At their
closest, they become difficult to split in a
large-aperture telescope. This disappear-
ing act occurred in 1836, and by chartingAlcor Mizar A & BMizar (right) is itself a binary star, with its two components 14" apart. Alcor, visible at left, is not in a system
with Mizar, although the two share the same proper motion. GIUSEPPE DONATIELLO
Seeing double
Mizar and Alcor have long been a test
for good eyesight. Called the horse and
rider, these stars (and hence the ability to
separate them) are even referred to early
Arabic texts as “the test.” But skywatchers
had no idea that they were seeing just a
chance alignment. Mizar and Alcor now
appear to be part of a group of stars mov-
ing together through the galaxy, but they
are not a true binary system bound by
gravity. They are instead called a com-
mon proper motion pair.
Less than 10 years after Galileo
turned his telescope to the sky, his friend
Benedetto Castelli discovered that Mizar
has a nearby companion — Mizar B —
of almost equal brightness. Mizar is easy
to split in a small telescope. Castelli also
noted the nearby companion of Beta (β)
Scorpii, another favorite for small tele-
scopes. In the mid-17th century, the
English scientist Robert Hooke split
Gamma (γ) Arietis in the Ram. Yet none
of these observers could determine
whether the stars were physically related.
In 1849, Sir John Herschel wrote in
his book Outlines of Astronomy, “Many
of the stars, when examined with a
telescope, are found to be double. ...
This might be attributed to accidental
proximity, did it occur only in a few
instances.” He goes on to write that “the
frequency of this companionship [and]
the extreme closeness,” means many
must be physically related.
Anyone who has swept a star-filled
sky with binoculars or a small telescope
has seen the profusion of what appear
to be closely related stars. But the true
relationship of double stars is more
complex than Herschel’s straightfor-
ward statement. If you have a modest
backyard telescope, you can follow the
trail that led to our understanding of
the true nature of double stars.
Follow the stars
Let’s start in the constellation Cygnus.
The Swan is full of wonderful objects,
including two fascinating double stars.
One is 61 Cygni, an easy target for a
small telescope. To find it, draw a line
from the tail of the Swan, Deneb, to
the tip of its right wing, Zeta (ζ) Cygni.
61 Cygni lies about halfway between
these two stars. You will need a dark
sky to see the primary, at magnitude
5.2, with the unaided eye. A good pair
of binoculars will split the two widely
separated stars. The companion shines
at about 6th magnitude.
In 1804, the astronomer priestA sketch made with a 6-inch f/8 Newtonian
scope resolves the two stars that make up
Porrima in Virgo the Maiden. JEREMY PEREZCastor is Gemini’s alpha
star and garnered the
attention of both William and John
Herschel. This sketch shows the
system’s two brightest stars through an
8-inch f/6 Newtonian at 240x. JEREMY PEREZCastor
Gamma Virginis (Porrima)
PERFECT
EYESIGHT NOT
REQUIRED
Presenters at star parties often say that
in ancient times, the ability to see both
Mizar and Alcor would qualify you for
the Roman army. I have searched
Roman literature by Cicero, Pliny the
Elder, Seneca, and, most importantly,
De Re Militari by Vegetius. There is no
mention of any such test of eyesight
either for foot soldiers or archers. Early
Arabic literature does mention Alcor as
a test for visual acuity, but as far as the
Romans were concerned, if you could
pick up a sword and shield, they signed
you up for active duty. — R.S.