M3

Credits: Keith Turnecliff, Long Itchington with Vaonis Stellina Observatory Station

The globular cluster M3 was the first object in the Messier catalog to be discovered by Charles Messier himself. Messier spotted the cluster in 1764, mistaking it for a nebula without any stars. Today it is known to contain over 500,000 stars.
M3 is notable for containing more variable stars than any other known cluster. 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 distances to deep-sky objects.
M3 contains at least 274 variable stars.

Facts about M3 by Keith Turnecliff

M3 (or NGC 5272) is a globular cluster of stars in the northern constellation of Canes Venatici. It was discovered on May 3, 1764, and was the first Messier object to be discovered by Charles Messier himself. Messier originally mistook the object for a nebula without stars. This mistake was corrected after the stars were resolved by William Herschel around 1784. Since then, it has become one of the best-studied globular clusters. Identification of the cluster's unusually large variable star population was begun in 1913 by American astronomer Solon Irving Bailey and new variable members continue to be identified up through 2004.
Many amateur astronomers consider it one of the finest northern globular clusters, following only Messier 13. M3 has an apparent magnitude of 6.2, making it a difficult naked eye target even with dark conditions. With a moderate-sized telescope, the cluster is fully defined. It can be a challenge to locate through the technique of star hopping, but can be found by looking almost exactly halfway along an imaginary line connecting the bright star Arcturus to Cor Caroli. Using a telescope with a 25 cm (9.8 in) aperture, the cluster has a bright core with a diameter of about 6 arcminutes and spans a total of 12 arcminutes.
This cluster is one of the largest and brightest, and is made up of around 500,000 stars. It is estimated to be 11.4 billion years old. It is located at a distance of about 33,900 light-years away from Earth.
Messier 3 contains 274 known variable stars; by far the highest number found in any globular cluster. It is best viewed in May.