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Cosmic Challenge: M13's propeller

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Cosmic Challenge: M13's propeller

July 2017

Phil Harrington

This month's suggested aperture range

10- to 14-inch (25-36 cm) scopes








M13's propeller

Globular cluster

16 41.7

+36 27.6




In last month's column, we paid a visit to the Moon and the crater pair of Messier and Messier A.  This month, we head back out into deep space to examine one of the most spectacular entries in Charles Messier's catalog: M13, the Great Hercules Globular Cluster.

Above: Summer star map from Star Watch by Phil Harrington.


Above: Finder chart for this month's Cosmic Challenge.

Chart adapted from Cosmic Challenge by Phil Harrington.
Click on the chart to open a printable PDF version in a new window.

There are few deep-sky objects more spectacular through a large telescope than globular clusters. Each globular cluster may contain hundreds of thousands to several million stars, all swarming together around a core so densely packed that seeing individual points may defy resolution. But the view is spectacular nonetheless.

To the casual eye, all globular clusters may look the same. Just a big ball of stars, right? Nothing could be further from the truth. Each has a personality all its own, often with hidden treasures lying within, if you take the time to look.

M13, the grand globular cluster in mighty Hercules, is the perfect example. It doesn't take a double-digit aperture to see that the stars of M13 are distributed asymmetrically. An 8-inch (20cm), and even smaller instruments, will show how the stars appear to be arranged into curves or rows. Many have likened the cluster's appearance to that of a spider. John Herschel described the cluster as exhibiting "hairy-looking, curvilinear branches." Later, Lord Rosse saw M13 as "more distinctly separated and brighter than anticipated; singularly fringed appendages to the globular figure branching out into the surrounding space."

Observing notes made while I was viewing through my 10-inch telescope at 58x recall irregular strings of stars streaming out from the cluster's dense core. Two thin threads curving away toward the west strike me as particularly conspicuous. These star-strings give the impression that M13 is hurtling through space so quickly that is it leaving a trail of stars behind in its wake.

Increasing the 10-inch's magnification to 181x reveals a surprise that goes unsuspected at lower values. The star streamers are still evident, but hidden within the core, southeast of the exact center, are three subtle dark lanes that seemingly join together to form the letter "Y." The combined effect is nicknamed the M13 propeller.

Above: A sketch of M13 and its propeller through my 10-inch (25cm) Newtonian.

These unusual lanes, or propeller as many call them, were first discovered by Bindon Stoney in about 1850. At the time, Stoney was an astronomer working for Lord Rosse at Birr Castle in Parsontown, Ireland. After Stoney's initial sighting became known, many other observers confirmed the existence of these unique dark rifts through instruments as small as 6 inches (15cm) aperture. But as photography diminished the need for accurate visual observations, the M13 propeller became lost in the glow of the intense core.

Above: Stoney's sketch of M13. The breadth of the blades is greatly exaggerated, but the overall shape is correct.


Walter Scott Houston resurrected Stoney's dark lanes in his Deep-Sky Wonders column back in the July 1953 issue of Sky & Telescope magazine. His mention garnered little response at the time, however.  But thanks to his persistence, along with the Dobsonian revolution, the propeller started to become a popular challenge in the 1980s. Today, another three decades later, many of today's amateurs have seen the M13 propeller.

Most photographs of M13 do not show the propeller, however, because the cluster's core is usually so overexposed that they are absorbed into the glow. Yet by using a shorter exposure geared to resolve the core, the propeller can be seen.  I took the photograph here several years ago through a 100mm apochromatic refractor that I was reviewing for Astronomy magazine at the time.

Above: M13's propeller is visible in this image taken by the author through a 100mm apochromatic refractor.

For a successful visual sighting, magnification is key. Too low and the lanes will remain hidden from view.  To see the lanes for yourself, wait until the cluster is high in the sky, away from any haze and light pollution, which can stifle them. Under ideal conditions, the dark lanes are evident through a 12-inch, joined at their ends to resemble the corporate logo of a famous German auto maker -- proving once again that M13 is the Mercedes-Benz of globular clusters.

Have a favorite challenge object of your own?  I'd love to hear about it, as well as how you did with this month's test.  Contact me through my web site or post to this e-column's discussion forum.

Remember, half of the fun is the thrill of the chase.  Game on!

About the Author:

Phil Harrington writes the monthly Binocular Universe column in Astronomy magazine and is the author of 9 books on astronomy.  Visit his web site at www.philharrington.net to learn more.

Phil Harrington's Cosmic Challenge is copyright 2017 by Philip S. Harrington.  All rights reserved.  No reproduction, in whole or in part, beyond single copies for use by an individual, is permitted without written permission of the copyright holder.


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