M92: The “Other” Hercules Cluster
Season Visible in Evening
Compact, dense globular cluster. Very attractive, but tends to be overlooked because of proximity to M13.
I like showing Globular Clusters to visitors to the ‘scope because they are beautiful and their appearance matches the visitors’ expectations (unlike, say, galaxies, which often disappoint an unprepared visitor). M92 is compact and bright, and would be a very popular globular if it were somewhere else in the sky. But, being so close to the spectacular M13, M92 tends to be overlooked.
Like open clusters, globular clusters are easy to find and observe in small telescopes, pretty, and scientifically important. Globular clusters are made up of 100,000s of very old stars – possibly the oldest stars in the galaxy. (We know this by measuring the content of heavy elements in the stars by analysing the spectra of their light. Heavy elements are made in stars, ejected into space by the death of those stars, and then incorporated into new stars that form. So very old stars have very few heavy elements.) Globulars are located and move in an unusual manner – they are not part of the main disk of the galaxy, and they do not rotate with the spiral arms. Instead, they orbit the core of the galaxy in their own orbits, unrelated to the disk and spiral arms. Explaining how these very old collections of stars came to exist, and explaining their unusual orbits, is a critical test for any proposed model for the formation of galaxies.
Find Arcturus, the brightest star in Bootes. To find Arcturus, trace the curve of the big dipper’s handle, and continue on that curved path. Say “”Follow the Arc to Arcturus””.
Find Vega, the brightest star in Lyra. From Arcturus, sweep your gaze up until you are looking straight up. Continue sweeping in that direction, to a very bright star on the “other side” of straight up. That’s Vega. To confirm, it’s next door to a distinctive diamond of 4 stars.
Look for the “square of Hercules”. This is a lopsided rectangle of 4 stars about halfway between Arcturus and Vega – about 10% closer to Vega than the halfway point.
Here is that section of sky again, without the constellation lines. Find Arcturus, then Vega, then the Square of Hercules.
Stand so Arcturus is on your right and Vega on your left. Find the side of the square of Hercules that is made of the two stars closest to Arcturus. One of these two stars is closer to Polaris — call that the “top”.
As a reminder, M13 (not presently our target) is located on the right edge of the square.
M92 is in the space between the arms, above the top of the square. Here are details on how to find it:
We’ll be using the square in the centre of Hercules, and nearby Lyra, as references.
Note the line across the top of the square.
Imagine turning that line exactly 90 degrees, so it sticks straight up from the left corner. Keep the length the same.
M92 is just to the right of the top of this line.
How far to the right?
The short side of the diamond shape in Lyra gives the correct distance.
Move right by that amount,
And point your telescope there.
Mentally practice on this enhanced star field.
And now practice on his unenhanced star field.
In your magnifying finder, M92 will be a small, very compact, fuzzy ball of light.
In your main telescope, start with a very low magnification for the brightest view.
This simulates about 25x magnification.
Because M92 is dense and bright, it can stand some magnification, so be sure to try one of your shorter focal-length eyepieces.
This image simulates a 4mm eyepiece in a 650mm scope, for about 160x magnification.
All the above images were generated with Starry Night Pro.