Astronomy – October 2019

64 ASTRONOMY • OCTOBER 2019

SECRET SKY

In the mid-19th century, the observing assis-

tants of William Parsons, Third Earl of Rosse,

noticed three dark rifts crossing the interior

of the Hercules Cluster (M13). They spied the feature

through Lord Rosse’s 72-inch Leviathan

ref lector at Birr Castle in Ireland, but the

feature — now popularly known as “the

propeller” — can be seen through much

smaller instruments. Yet the elusive detail

continues to challenge and mystify visual

observers today.

Dissecting an enigma

A sketch by Rosse’s assistant, Bindon Stoney,

shows the propeller’s position, which at a

glance appears to lie near the cluster’s center.

But does it?

In 1887, Mark W. Harrington spent a month visually

studying these dark lanes through the 6- and 12-inch

refractors at the Detroit Observatory in Ann Arbor,

Michigan. The astronomer did so with the aid of H.C.

Markham, an artist whose sight Harrington found to

be remarkably keen.

At first, they found the rifts “somewhat difficult

objects.” As Harrington notes in an 1887 issue of The

Astronomical Journal, “They are so elusive that I some-

times almost doubted their existence, but I found that

with patience I could always see them.” High powers

(500x to 600x) produced the best results.

But Harrington noted a curiosity: In Stoney’s draw-

ing, the radiating point of the rifts is nearly central; in

Markham’s, it is southeast of the cluster’s core. “Whatever

the rifts are,” Harrington concluded, “it seems certain

that they have shifted their position slightly” since

Stoney’s drawing was made. But did they actually move?

Studies over the following decades revealed no evi-

dence of further shifting. That’s because the shift is

illusory. If we look at Stoney’s and Markham’s drawings

carefully, we’ll see that the propeller’s position is essen-

tially the same relative to a “hook” of stars

to the south. What’s different in the two

drawings is not the propeller’s location, but

the intensity and number of recorded stars

to the east and northwest of the propeller in

the cluster’s core region.

No astronomical artist accurately plots

the positions of tens of thousands of stars in

a globular cluster’s core. Instead, we see an

artist’s interpretation of the view. In Stoney’s

representation, the stars east of the propel-

ler’s center are brighter and more numerous

than those in the Markham drawing. Furthermore, the

outer stars in the region northwest of the propeller’s

radiant are fewer in Stoney’s representation than those

depicted in the Markham drawing.

Starlight can play tricks on a dark figure in Hercules.

The propeller’s

mysterious move

ABOVE: The three

dark rifts of M13

are quite apparent

in this sketch by

19th-century observer

Bindon Stoney. He

used the 72-inch

Leviathan reflector

at Birr Castle in

Ireland. North is up.

STEPHEN JAMES O’MEARA

ABOVE RIGHT:

The three dark rifts

in M13 were drawn

by H.C. Markham

in 1887, based on

observations made

through 6- and

12-inch refractors

at powers between

500x and 600x.

STEPHEN JAMES O’MEARA

Look for the

hook of

stars to the

south of the

core and

work from

there.