Globular clusters are densely packed groups of stars which orbit a galactic core. In the Milky Way, over 100 globular clusters have been identified, and at one point, there were many more. These structures are the oldest structures in the galaxy, with materials which may provide information about the age of the universe in addition to adding understanding to human knowledge of the Milky Way. Some very lovely images of globular clusters taken with advanced telescopes can be found on astronomy websites.
The composition of a globular cluster can include as many as one million stars, and the structure may stretch 200 light years in diameter. The stars in the cluster tend to be most densely packed in the middle. Over time, globular clusters break up because they encounter the galactic core. Studies on these structures also seem to suggest that they are unlikely to support planetary systems, because the stars are close together enough that they could pull orbiting planets out of orbit.
Within a globular cluster, there is minimal interstellar dust. The composition of the stars themselves also shows that they are extremely old. Information about the composition of the cluster and its contents can be obtained with imaging methods including the use of telescopes which are sensitive to x-ray, infrared, and other types of non-visible radiation. These types of observations can provide valuable data about many other objects in the universe, in addition to generating stunning visual images which often pique the public's interest in science, which is why they are used for promotional tools in programs which support scientific research.
The first recorded sighting of a globular cluster occurred in the mid 1600s, although it took longer for people to recognize that these tight points of light actually contained thousands of stars. As the resolution powers of telescopes improved, people learned more about distant objects which had been recorded and recognized, but not fully understood, by earlier generations of astronomers. Better telescopes also allowed scientists to identify more structures which had been hidden before.
Globular clusters have been recognized in other galaxies, as well. Researchers have also identified younger globular clusters, which allows them to learn more about how these structures form and how they vary over time. All of this information can be tremendously helpful for people who are trying to learn more about the origins of the universe, or who are simply interested in explaining existing phenomena in the universe.