Sky & Telescope - May 2019 - 22

SEEING DOUBLE

by Phillip Kane

Binary
Orbits
Explored
Follow the author on a celestial tour
of some of his favorite binary pairs.

M AY 2 0 1 9 * S K Y & T E L E S C O P E

p DOUBLES GALORE Epsilon Lyrae is fondly known as the Double
Double since it's a binary star in which each component is a binary star
in its own right. Join the author on a tour of some beloved binary pairs.

double in appearance, but must be allowed to be real binary
combinations of two stars, intimately held together by the
bonds of mutual attraction." Robert Grant Aitken, in his
classic book The Binary Stars, states Herschel's conclusions
were "incontrovertible."
Herschel's historic paper detailing this series of observations began with the double star Castor. As Robert Burnham
noted when he summed up Herschel's contribution in the
second volume of his Celestial Handbook, "Castor . . . was the
ﬁrst true physical binary to be recognized, and the ﬁrst object
beyond our own Solar System in which the force of gravitation
was shown to be operating . . . ."
All (seven) of the orbital elements necessary to calculate
at least a preliminary orbit have been determined for about
2,600 double stars. A list of the elements that can be used
for plotting binary orbits is available from the U.S. Naval
Observatory in the Sixth Catalog of Orbits of Visual Binary
Stars (https://is.gd/wdsc6). Several of these 2,600 conﬁrmed
binaries would probably be on any double-star enthusiast's
list of favorite doubles. Here are nine of my favorites.

EPSILON LY R A E: POSS-II / CA LTECH / STSCI / PA LO M A R OBSERVATORY;
A LL BIN A RY PLOTS: PHILLIP K A NE; A LL CONSTELL ATION IM AG ES: A K IR A FUJII

e now accept the premise that most close double
stars are not random alignments but binary pairs
gravitationally bound in orbit about each other. But
this realization did not come easily.
Prior to about 1800, astronomers mostly regarded double
stars as curiosities, the chance positioning of two stars at
different distances. (One exception was John Michell who,
in 1767, suggested that most doubles were physical pairs.)
In 1779, the great astronomer William Herschel began a
systematic study of these "accidental doubles." He saw a
clever use for them - as a way of measuring stellar parallaxes (the astronomical unit was fairly well established by
then). His hypothesis, a concept ﬁrst considered by Galileo,
was simple. If he regarded the fainter star as very distant
and "ﬁ xed," he could obtain a ﬁrst-approximation measure
of the parallax of the brighter, closer component since the
two stars would shift relative positions as the Earth made its
annual orbit around the Sun. But in his 25 years of observing binary stars, Herschel didn't determine parallaxes at all.
Instead, he realized his records clearly showed some pairs of
stars in orbital motion.
On June 9, 1803, William Herschel presented a groundbreaking paper to the Royal Society in London in which
he said he would "prove that many of them are not merely

https://www.is.gd/wdsc6

Sky & Telescope - May 2019

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