Sky & Telescope - May 2019 - 25

-4

1298
70 Oph

578

40 a.u.

Sun's mag: 10.0

-2

Sun

Distance from
Sun to Pluto

2018
E

A
-4

-2

3458

Ophiuchus

2738
4

Alpha Herculis
Moving about 5 hours of RA east and some 10° north
of Gamma Vir will center the view on Alpha Ophiuchi. Flanking this conspicuous 2nd-magnitude star
are two impressive binaries, but impressive for different reasons. Off to the northwest is Alpha Herculis.
Besides its strikingly orange primary star, it's the
shape of Alpha Her's apparent orbit that also stands
out. It plots as near circular (but not quite), because it
not only happens to be oriented close to face-on from
our point of view, but its orbit also has an eccentricity of zero. The secondary's distance from the primary
varies from only 4.3″ to 4.7″ over a 3,600-year period
of revolution! Herschel ﬁrst noticed the secondary in
1779; it was identiﬁed much later to be a spectroscopic
binary. The primary is a luminous red giant whose
magnitude varies irregularly between 2.7 and 4.0 over
a three-month period, averaging at magnitude 3.5.
Herschel discovered Alpha Her's variability in 1795, 16
years after he recognized its binary nature. (Can you
imagine his dedication to ﬁ nd this out!)
We should wish Alpha Her
were closer. Of the 18 stars in
the nine pairs here, Alpha
Her A has the highest luminosity. Its absolute magnitude
varies from -2.5 (similar
to Jupiter) to -1.2 (slightly
fainter than Sirius), which
is what its apparent magnitude would be were it about
ten times closer. It would
also be a deeper orange than
Betelgeuse. The B companion
α
would be no slouch - in a
telescope it would look similar
Hercules
to Arcturus, just 50″ from A.

70 Ophiuchi
Southeast of Alpha Ophiuchi is one of the most studied
binaries, 70 Ophiuchi. It's only 16.6 light-years away
and has a rapid 88-year revolution, having made almost
three revolutions since Herschel discovered it in 1779.
The pair will be widest in 2025 (7.74″) and closest just
53 years later in 2078 (1.54″). Some reading this will be
alive to follow B from its widest separation from A all
the way to its closest approach.
Of the nine binaries here, 70 Oph is impressive
because the changing positions of its components are
the easiest to follow, even by casual observers. Inspired
by Burnham's 1978 plot of 70 Oph, I occasionally
plotted B's position around A from 1990 to 2008. I
used a red ﬁeld star 30′ to the east to judge B's PA and
nearby 59 Serpentis (a 3.9″ ﬁ xed double) to estimate the
AB distance. My sketch shows AB widened by 4″ and
changed its PA by 110°. Observing real celestial movement like this in a binary pair is ethereal!
The movements of 70 Oph's components have some
odd orbital anomalies that suggest exoplanets may be
present, but none have been conﬁrmed. Irish astronomer Agnes Mary Clerke summed it up well, referring to
70 Oph as far back as 1890: "The stars have . . . persistently refused to keep to their predicted places . . ." This
pair obviously needs even more study.
-5
10 a.u.
Distance from
Sun to Saturn

2000

2018

-3

2036
-1

A
-3

2072

-1

1
1

E 3
2054

Sun
Sun's mag: 3.3

s k y a n d t e l e s c o p e . c o m * M AY 2 0 1 9

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Sky & Telescope - May 2019

Table of Contents for the Digital Edition of Sky & Telescope - May 2019