A star cluster is a grouping of gravitationally bound stars. There are two main
types of star clusters: open clusters, and globular clusters. An open cluster
consists of loosely bound stars, which are usually young, and disperse over
short periods of time. A globular cluster consists of hundreds of thousands of
densely-packed stars, which are billions of years old.
The Pleiades are perhaps the best-known star cluster, due to their visibility. They are one of the few deep-sky objects that have no 'discovery' date, as they are easily visible to the naked eye!
The Double Cluster in Perseus is a pair of open clusters, and has been visible in the night sky since before written records. It is one of the most famous of open clusters, and is remarkably young for an astronomical object.
The last dinosaurs on Earth died out before these stars were born!
Arguably the most famous globular cluster, M13 gives great views to both astrophotographers and observational astronomers alike! Under ideal skies, it may be possible to see this object with the naked eye. But it definitely won't look anything like this!
Messier 52 is an open cluster located near the band of the Milky Way in the constellation Cassiopeia. It is easily observable through binoculars and small telescopes, and makes an excellent pair with the nearby Bubble Nebula, which glows red from the energy of a hot, bright star at its centre!
A bright, beautiful, and large cluster, Messier 5 is an easy target for both visual observers and astrophotographers alike! Like many globulars, this object provides a stark contrast between the density of some astronomical objects and the empty space beyond!