Globular clusters are dense spheroid collections of stars, often found in the outer halos of galaxies.  There are roughly 150 known globular clusters in the Milky Way, and most other observable large galaxies show an associated system of globular clusters.  The exact nature and formation of these clusters is not well understood, but spectral analysis shows that the stars they contain are some of the oldest in the galaxy, whereas stars found in the open clusters in the galactic disc are often much younger and newer. Globulars usually contain hundreds of thousands of stars, and some of the largest can contain millions of stars.