42 ASTRONOMY • DECEMBER 2021
the stars’ motions, astronomers proved
they were orbiting each other according
to Newton’s law of gravity. This, in turn,
showed gravity not only works in our
solar system with the Sun, the Moon, the
planets, and other small bodies, but also
in the depths of interstellar space!
The stars again came closest in 2005.
Currently, they are approaching 3" in
separation, making them fairly easy to
split with a small telescope. As a bonus
for observers, Porrima lies less than 3°
above the ecliptic and is occasionally
occulted by the Moon.
The winter sky gleams with bright
jewels, and one of the best is Castor in
Gemini the Twins. At magnitude 1.9,
Castor is the 25th-brightest star in the
sky. It is also a fascinating multiple-star
system. Through
a modest-sized
telescope, Castor
is relatively easy to
split into two stars,
with the companion
shining at magnitude 2.97
about 5" from the primary.
Sir William Herschel was the firstastronomer to systematically study dou-
ble stars. When he started his work in
1800, he “resolved to examine every star
in the heavens with the utmost atten-
tion” — a monumental challenge for any
observer. After several years observing
Castor and its companion, Herschel was
convinced the pair was a true binary star
system and not a chance alignment.
This was later confirmed by John
Herschel, William’s son. John continued
his father’s studies of Castor, and gath-
ered observations taken by
other astronomers over the
previous 100 years.
These allowed him to
produce detailed
elements of the
stars’ orbits, mak-
ing this the first
confirmed binary.
We now know that
Castor is even more
complex than the
Herschels could have
imagined, with a total of
six stars locked in mutual
orbits.Color vision
The night sky is punctuated with stars
that present both vivid and subtle hues.
It’s easier to see these incredible col-
ors on display in double stars because
their proximity makes them easy to
compare. During the 19th century, avid
double-star observers reported an amaz-
ing range of colors among paired stars.
Astronomer Joel Dorman Steele wrote,
“Every tint that blooms in the f lowers of
summer, f lames out in the stars at night.”
Bear in mind, though, that humans
see the full range of color best in bright
sunlight. The ability
decreases as the light
level drops. In the
dark, we are
more sensitive
to blues and
greens. This1'certainly affects the way we see color in
stars through a telescope.
Gleaming yellow and amber in the
head of Cygnus is the constellation’s other
famous pair: Albireo, the crown jewel of
double stars. Turn a small telescope on
this star and it will resolve into a golden
primary (Albireo A) and a brilliant sap-
phire blue companion (Albireo B). Some
observers see a hint of green in the fainter
star. This may be an effect of the contrast
between the two stars, whose
amazing colors arise from
their physical properties.
Albireo A is a giant
more than 14 times the
mass of the Sun, pro-
ducing light in the
yellow-orange part
of the spectrum.
Its companion is
younger and only1'One of the sky’s most
stunning double stars
is Albireo, whose
components are
brilliant sapphire
and yellow. ALAN DYERAlbireo Izar in Boötes shows off its colorful
components in this 2012 sketch made through
a 6-inch 6/8 Newtonian scope. JEREMY PEREZEpsilon Boötis (Izar)
ABOVE: Although this wide-field view of
Delphinus doesn’t split Gamma (upper
left), the star does appear slightly
greenish. ALAN DYERLEFT: This sketch, made with a
6-inch f/8 Newtonian telescope at
240x magnification, splits Gamma
Delphini into two stars. JEREMY PEREZGamma Delphini
61 Cygni is known as the Flying Star for its rapid
motion against the background stars. These
images, taken eight years apart, show the pair’s
displacement over time. INDIVIDUSOBSERVANIS/WIKIMEDIA
COMMONS61 Cygni