From the September 2019 issue

The propeller’s mysterious move

Starlight can play tricks on a dark figure in Hercules.
By | Published: September 10, 2019 | Last updated on May 18, 2023
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
In the mid-19th century, the observing assistants 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 reflector 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.
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

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.

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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 sometimes 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 drawing, 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?

This image of the Hercules Cluster (M13) shows the dark “propeller” feature, 
to the upper left of center in this image. 
Rodney Pommier
Studies over the following decades revealed no evidence 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 essentially 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 propeller’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.

To figure out what was going on, I used Photoshop to add stars to the Stoney drawing to enhance the core’s outer region northwest of the propeller’s radiant. Then I darkened the core region east of the propeller’s center. The result: The propeller’s position in Stoney’s drawing better resembles that in the Markham drawing. Any confusion, then, boils down to artistic impression of starlight, not the movement or misrepresentation of the dark propeller’s position.

This Hubble Space Telescope image of M13’s core region reveals the position of the propeller. 
NASA/ESA/ The Hubble Heritage Team (STScI/AURA)
How to see the propeller

The propeller is a low-contrast feature about 3′ across, whose radiant lies just southeast of the cluster’s core. Through my 8-inch reflector, the propeller shows up best when using magnifications ranging from 244x to 300x. I find the two southern blades more apparent than the northern one, owing to the increased contrast of starlight near the core.

Look for the hook of stars to the south of the core and work from there. I find using averted vision, then relaxing my gaze, works best. In other words, while using averted vision, I set my mind at ease by thinking of something pleasant. I do not focus on the stars, but rather let my eyes relax while keeping my mind alert as to where I want to look. By softening my gaze, I become less aware of minute details and more aware of greater shades of contrast. So give the propeller a whirl.

As always, report what you see, or don’t see, to