Galactic clusters are found inside the spiral arms of the Milky Way and are among the youngest collections of stars in the galaxy. A few of the oldest galactic clusters such as NGC 188 are several billion years old, but these are the exceptions.
Most galactic clusters are only a few hundred million years old or less and have formed recently.
They are rich in the youngest and most heavy element-rich stars currently being formed in the universe. Over millions of years, the tidal gravitational forces in the Milky Way tend to shred these clusters of thousands of stars, scattering the stars into the general population of stars wandering interstellar space. The Sun was probably formed in such a cluster 4.5 billion years ago, but the rest of the stars in its family have long since been dispersed. Globular clusters are found outside the Milky Way in the halo of the galaxy, and these contain upwards of 100,000 stars which move together as a single unit through space in the form of a roundish ball of stars. These clusters were formed between 5 and 15 billion years ago and contain the oldest known stars in the Milky Way.