Globular Clusters

Globular Cluster NGC 6093, image from Hubble Space Telescope, NASA.

Globular clusters are roughly spherical collections of tens of thousands, and in some cases millions of stars.  By  1919 Shapley had identified most of the approximately 150 globular clusters in the Milky Way galaxy.  With the more powerful telescopes and spectroscopes of the late 20th century it has been possible to locate these more precisely in space, to find globular clusters in other galaxies and to study the nature of the stars they contain.  In brief, the stars that make up the bulk of globular clusters are older than the stars in the disk of the galaxy, and have a slightly red color; whereas stars in the disk tend toward blue.  The most prominent globular cluster visible in a small telescope or binoculars is M13 in the constellation Hercules, with several hundred thousand stars.