English: French astronomer Charles Messier discovered the globular cluster M3 in 1764, mistaking it for a nebula without any stars. This misunderstanding of M3’s nature was corrected in 1784 when William Herschel was able to resolve the cluster’s individual stars. Today it is known to contain over 500,000 stars.

M3 is notable for containing more variable stars than any other known cluster. As their name implies, the brightness of a variable star fluctuates with time. For some variable stars, their period relates to their intrinsic luminosity, so astronomers can use those stars’ brightness fluctuations to estimate their distances. This makes them extremely useful for measuring the general distances to the clusters or even galaxies in which they reside. M3 contains at least 274 variable stars.

For more information, visit: www.nasa.gov/feature/goddard/2017/messier-3

Credit: NASA, ESA, STScI, and A. Sarajedini (University of Florida)