BINOCULARUNIVERSE
BY P H I L H A R R I N G TO N
A great glob
under the radar
The cluster M5 lies in an interesting neighborhood.
I
f you were asked to name
the greatest visual observer
of all time, whom would
you choose? There are
many worthy candidates,
but my vote would have to go to
the 18th-century astronomer
William Herschel. He had quite
a track record, including the
discovery of the planet Uranus
as well as thousands of deep-sky
objects. That’s a pretty good run.
Herschel did more than just
look at the sky, however. He also
tried to understand what he was
looking at. Based on his studies
of how stars appear distributed
along the hazy band of the
Milky Way, Herschel concluded
that the Milky Way is shaped
like a flattened disk. We know
today that he was exactly right.
When it came time to know-
ing his place in the Milky Way,
however, Herschel was way off.
He noticed how stars appeared
to be evenly distributed along
the Milky Way. From this, he
reasoned that the Sun must be
in the very center. Of course,
that isn’t correct. That’s
because, unbeknownst to him,
cosmic dust blocked his view
(as well as ours) along the plane
of the galaxy beyond about
6,000 light-years.
It would be another century
and a half before Harlow
Shapley discovered our solar
system’s true position in the
Milky Way. By studying the
distances to globular star clus-
ters, Shapley found that most
seemed to be distributed in a
spherical volume centered not
on the Sun, but instead on a
distant point in the direction of
the constellation Sagittarius. He
concluded that globular clusters
must hover around the core of
the Milky Way — a conclusion
we know today to be correct.
α
M5
5
Sail
ε
The author describes this asterism near cluster M5 as the “Serpent’s Sailboat.” It’s an
easy binocular target. TONY HALLAS
18
A ST R O N O M Y • MAY 2018
M5 in Serpens is one of the most brilliant globular clusters in the northern sky, and an
attractive target for binoculars. ADAM BLOCK/MOUNT LEMMON SKYCENTER/UNIVERSITY OF ARIZONA
While most of the Milky
Way’s globular clusters are
clumped in Sagittarius,
Scorpius, and Ophiuchus, a few
renegades are out there. This
month, let’s examine a loner
found in western Serpens. M5
may be off the beaten path, but
this recluse is one of the most
magnificent globulars any-
where in the sky.
Gottfried Kirch discovered
M5 on May 5, 1702, from
Berlin, Germany. His notes
described it as a “nebulous star.”
Although Kirch was Germany’s
first Astronomer Royal at the
time, those notes remained
unpublished until after Charles
Messier independently found
M5 on May 23, 1764. Messier
described it as a “beautiful neb-
ula; it does not contain any
star.” It was only after Herschel
viewed it through a larger tele-
scope 27 years later that its true
nature became clear.
To find M5, cast off from
Arcturus and slide southeast-
ward to 4th-magnitude Zeta (ζ)
Boötis. Continue from there
twice the distance along a
slightly more easterly course
until you come to a not-quite
right triangle of stars. The tri-
angle’s brightest star, positioned
at the not-quite right angle, is
3rd-magnitude Unukalhai
[pronounced oo-NOO-kul-hi
and also known as Alpha (α)
Serpentis]. Next, turn southwest
toward the 5th-magnitude star
5 Serpentis. M5 will be just to
its north-northwest. Through
binoculars, it looks like a fuzzy
star, just as Kirch described.
Estimates say that M5 is
24,500 light-years away and
may contain as many as
500,000 stars crammed into a
space about 165 light-years
across. By contrast, M13 —
summer’s Hercules Cluster,
which we visited in this col-
umn in July 2016 — holds
about 300,000 stars.
Before we bid farewell to this
month’s binocular universe,
let’s take a look at an asterism
drawn among several stars to
the south and east of M5.
Added together, they remind
me of a sailboat. The boat’s bow
is marked by 5 Serpentis, while
6 and 8 Serpentis symbolize the
deck. The hull extends to
4 Serpentis and SAO 140444,
the faintest star in the pattern.
Finally, the sails’ mast extends
east-northeast from 6 to 10
Serpentis. The sailboat looks
capsized in the evening sky, but
if you happen to pull an all-
nighter this month, come back
just before the onset of morn-
ing twilight. That’s when the
Serpent’s Sailboat asterism
will be most apparent, as it
approaches the western
horizon.
If you have a favorite bin-
ocular target that you’d like
to share with the rest of us,
I’d love to feature it in a
future column. Drop me a
line through my website,
philharrington.net. Until next
time, don’t forget that two eyes
are better than one!
Phil Harrington is a longtime
contributor to Astronomy and
the author of many books.