Astronomy

BINOCULARUNIVERSE

BY PHIL HARRINGTON

18 ASTRONOMY • MAY 2018

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 f lattened 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.

While most of the Milky

Way ’s g lobu la r clu sters a re

clumped in Sagittarius,

Scorpius, and Ophiuchus, a few

renegades are out there. This

month, let’s examine a loner

found in western Serpens. M

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unpublished until after Charles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!

A great glob

under the radar

The cluster M5 lies in an interesting neighborhood. 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

The author describes this asterism near cluster M5 as the “Serpent’s Sailboat.” It’s an

easy binocular target. TONY HALLAS

Phil Harrington is a longtime

contributor to Astronomy and

the author of many books.

ε

α

M

5

Sail