Seeing wasn't that great on May 5 and 6, but this turned out better than I expected.
M3 in Canes Venatici is one of the brightest clusters observable from the northern hemisphere. Discovered by Messier in 1764, it was first resolved by Herschel, who described it as "one of the globular clusters, very brilliant and beautiful." At magnitude 6.2, it appears as a faint fuzzy patch in binoculars. Through larger telescopes, it shows "an incredible swarm of countless star images, massing to a wonderful central blaze, with glittering streams of stars running out on all sides." [Burnham]
About 34,000 light years away, M3 contains over 500,000 stars, and is notable for its large number of variable stars. Most people here are probably already familiar with the period-luminosity relationship for certain variables, discovered by Henrietta Swan Leavitt of the Harvard observatory. While performing a survey of variables in the Magellanic Clouds, Leavitt noted that "the brighter variables have the longer periods." Further observations confirmed a relationship between the magnitude and period of 25 variables. As Leavitt put it in a 1912 paper, "Since the variables are probably at nearly the same distance from the Earth, their periods are apparently associated with their actual emission of light, as determined by their mass, density, and surface brightness." Once you know the absolute magnitude of a star, you can determine its distance from its apparent brightness.
Here's a link to Leavitt's 1912 paper:
https://articles.adsabs.harvard.edu/pdf/1912HarCi.173....1L
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