M92: A globular cluster is a group of ancient stars (M92 is thought to be about 12 billion years old), gravitationally bound into a spherical shape; by comparison, our sun is about 4.6 billion years old; our galaxy is about 13.5 billion years old, and the universe is thought to be only slightly older than our galaxy), orbiting the core of its associated galaxy.
M13 is about 25,000 light years from Earth, and is about two-thirds of the angular size of the full moon when viewed from very dark skies. It is roughly 150 light years across. It shines at
magnitude 5.8 (barely visible, in very dark skies, to the naked eye, but easily seen with binoculars). It has a mass equal to about 550,000 times that of our sun (for comparison, there are
estimated to be no more than 4,000 stars within 75 light years of earth, which is a similar-sized bubble); it would be a very bright "night sky" on a planet in the midst of that cluster! M13 is the
largest and brightest globular cluster in the norther sky.
Because all the stars in a globular cluster are essentially the same distance from us (relative to the absolute distance from us), it is interesting to create a Hertzsprung-Russell diagram ("HR Diagram") of the members of the cluster. To learn about HR Diagrams, click here. I have done just that for this ancient cluster; those
charts are here (absolute magnitude) and here
(apparent magnitude). In addition to the interesting science, I think that this is a useful way to validate (or not) the colors the software created for the cluster from my data. And it
vividly shows the cutoff in apparent magnitude even a fairly large telescope, with a sensitive camera taking long exposures, can capture.
Globular clusters present a challenge to photograph; the large dynamic range between the very bright core and the very dim outer stars makes it very difficult to present a realistic view of the
cluster; either the outer stars will be almost invisible, the core will be burned out, or the image won't show how much brighter the core is than the rest of the cluster. I chose a middle ground--I
allowed the core to be very bright (but not quite burned out), while enhancing the outer stars, to show the remarkable size of the cluster.
This is the third time I have imaged this cluster, with the previous most recent having been 20 years earlier. To see what a combination of better equipment, better skies, better software, and better skills does, click here (one of my very first images ever, from my yard with a 10" Meade SCT, in 2001), and click here (from my yard with a
12" Meade coma-free SCT five years later).
Copyright 2025 Mark de Regt
