Finding M13

M13: The Hercules Cluster

M13; NGC 6205
Great Hercules Cluster
Globular Cluster
Season Visible in Evening
Summer; April-September
Conversation Notes
Best globular cluster visible from my latitude. Visible with naked eye from dark site, easy with binoculars.

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). M13 is my favourite object, after Saturn and Jupiter, to show people at outreach events. And it’s so easy to find that I’ve taught people how to find it who were then able to go home and pick it out with their binoculars.

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.

Finding M13

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.

If necessary, confirm it’s Vega by finding it in the Summer Triangle.

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”.
Point your finder at the line between those two stars, about 1/3 of the way down from the top star.
In the Finder, M13 will be visible as a round patch of light.
M13 is beautiful at almost any magnification. I like to start looking at very low power, to see it in context, then increase the power to a bit over 100x to study the structure. This is a simulated view at 130x.
Photography brings out detail that you can’t see with your eye at an eyepiece. This is a 5-minute exposure of M13.

All the above images were generated with Starry Night Pro.

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