M13 – The Reference Point 

Messier 13 is the benchmark object. We compare every other northern globular cluster to it. Located 25,000 light-years away, it is bright enough (mag 5.8) to be seen with the naked eye under dark skies as a “fuzzy patch.” However, only astrophotography reveals its true nature—a dense ball comprising several hundred thousand stars.

Structure: “The Propeller”

Looking at my image, it’s worth noting a subtle detail known to cluster hunters. In the southwestern part of M13, there is a distinctive dark structure shaped like the letter “Y” or a three-bladed propeller (similar to the Mercedes logo).

• This is not a hole in the cluster, but a lane of dark dust obscuring the glow of the stars behind it.

• Capturing “The Propeller” is a test of your setup’s resolution and seeing quality, as it tends to blend into the core’s glare during longer exposures.

Density and Collisions

M13 spans about 145 light-years. The stars in its center are packed so tightly that if Earth were located inside M13, our sky would never know night—it would be filled with thousands of stars brighter than Venus. This density has consequences. Stellar collisions occur here, creating so-called Blue Stragglers—hot, blue stars that theoretically shouldn’t exist in such an old population but have been “reborn” through matter fusion with neighbors.

Historical Context: The Arecibo Message

M13 also holds cultural significance. It was the target of the famous 1974 radio message sent by the Arecibo telescope, containing information about humanity, DNA, and the Solar System. Why there? Because the high density of stars in the antenna’s beam provided the highest statistical chance that the signal would encounter a civilization. However, it will not arrive there for another 25,000 years.

Photographs taken on: March 2019.

Composition: APP (10 best frames and Flat and Dark + Bias frames)
Processing: GIMP v2.10.14 + add-ons (Linux)
Lights: 10 x 57[s], ISO-1600;

Comparative photo. Left shows new processing with a new method that I use today. Right, older, processed by the old method.