They are the one-stop-shopping places for learning all about the nature and variety of stars in the Universe. They're unique, because in clusters, all the stars were born at about the same time, from the same material and all are at the same approximate distance from Earth. This means we can be sure that any differences among them are due to their true natures and not distorted by different distances from Earth and other factors. In this episode, two kinds of star clusters in the galaxy are explored. "Open Clusters" are young, live in the spiral arms of the galaxy and give us insight into the birth and formation of stars. "Globular Clusters" are old, live in the outskirts of the galaxy and could be nearly as old as the Universe itself. In addition, explore Galaxy clusters to reveal the large-scale structure of the Universe, which is expanding so fast that eventually all other galaxies, except for our own, will literally disappear from our sight.

Source: History channel