Have you seen the Propeller in M13?

Discovered in 1714 by Edmond Halley (of Halley’s Comet fame) the globular cluster M13 is a favorite of backyard astronomers – but did you know it has a secret?

What is M13?

M13 is the brightest, and many observers say the finest, globular cluster in the northern skies.  This beautiful tightly packed ball of 200,000 stars sits above the plane of the Milky Way galaxy, and is about 145 light years in diameter.  It’s also known as the Great Cluster in Hercules.

When you look at it, the light that hits your eyes left on its journey 25,000 years ago, because it is 25,000 light years distant. It’s believed globular clusters are very ancient, with some stars 12 to 13 billions years old.

How to find M13?

M13 is well placed for viewing in summer skies. You can find it about one third of the way from Vega to Arcturus, about 70% of the distance from Zeta Herculis to Eta Herculis in the Keystone of Hercules. 

It is visible with the naked eye from a dark site, appearing as a faint fuzzy object.  That’s in fact why Charles Messier (1730-1817) included it in his Messier catalog.  He didn’t want to confuse it with a comet.

It’s an easy target in binoculars from anywhere.  But a only backyard telescope reveals its secret.

An interesting optical phenomenon of M13 is visible in small telescopes and short time-exposure photographs: the Propeller.  I had observed M13 for 30 years before I heard about it.

Below are two photos of M13 I took with an iPhone11 through my TEC140ED refractor.  Look just northeast of the core for 3 dark lanes radiating from a central point, making the familiar propeller shape.  Haven’t spotted it yet? Look at the second photo then try again.

When observing through a telescope, it may help to use a high power view, averted vision and relax your gaze. Remember you are looking for the absence of stars.  It is unusual to see straight lines in space!

Now that you know where to look, check the original photo in this article. Do you see it?  Note that due to the differing optical properties of refractors and reflectors, the propeller is on the left side in the first image.

Next time you see a photo of M13, look for the propeller.  Or, if you have a telescope at a dark site or at least an 8” aperture telescope from the suburbs, give it a try!  

Mark Johnston / Instagram: @azastroguy

References:

1. Photo of M13 https://commons.wikimedia.org/wiki/File:M13_taken_with_14%22_Telescope_and_a_0.7x_reducer_as_imaged_on_6_May_2020.jpg
2. Finder chart for M13 https://stellarium-web.org/
3. M13 imaged with iPhone11 and TEC140ED with Mod3C image intensifier and 2.5x powermate barlow April 22 2020 from Cave Creek AZ. Image courtesy Mark Johnston (Instagram: @azastroguy)