M3: Globular Cluster

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). M3 is a beautiful cluster. It is harder to find than the easy M13, and is smaller, but is dense and very pretty in a scope capable of resolving it.

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 M3

Start by finding the Big Dipper in the Northern summer sky.
Next, find the bright yellow star Arcturus, high in the sky toward the West.
The easy way to find Arcturus is to trace the arc of the Big Dipper's handle, and imagine that line continuing to Arcturus. Remember the phrase "follow the arc to arcturus".
Next, find Cor Caroli. This is a dimmer star, but is the brightest star in the neighbourhood, underneath the arc of the Big Dipper's handle. It's at a distance below the handle about the same as the length of the handle.
Draw an imaginary line from Cor Caroli to Arcturus.
Point your telescope to just past the middle of this line - that is, just slightly closer to Arcturus than the middle. If you have a Telrad, put the right side of the middle circle right on the middle of the line, as shown here.
If you have a magnifying finder, you should be able to find M3 in the edge of the field. It will be a small dense patch of light, obviously not a star because it is fuzzy. If you don't have a magnifying finder, use the widest-field, lowest-magnification eyepiece you have in your telescope.
Gently use your slow-motion controls to centre the cluster in the finder.
In your main scope, you should be able to see the dense core and surrounding stars of the cluster. This view simulates what I see in a 100 mm (4") refractor at about 40x magnification.

All the above images were generated with Starry Night Pro.