Globular star clusters are among the oldest objects in our galaxy. Their beauty is easily discerned through amateur telescopes that resolve tightly packed swarms of glistening stars, suspended in the night sky like Christmas ornaments. More than 150 globular star clusters are known to be associated with the Milky Way Galaxy. Each cluster contains hundreds of thousands to a million stars within a volume of 10 to 30 light-years across.
In 1918, Harlow Shapley recognized the existence and structure of globular clusters. By studying the clusters' distribution in the sky and by measuring their distances, he was able to deduce the location of the center of the Milky Way Galaxy and the Sun's distance from it. In the 1930s, Edwin P. Hubble discovered globular clusters in the neighboring Andromeda Galaxy, and since then globular star clusters have been found surrounding many other galaxies.
Globular clusters reside within a spherical volume of space called the "galactic halo," which surrounds the disk of our galaxy. The clusters orbit around the galactic center, taking millions of years to complete their highly elongated, randomly oriented orbits. Most globular clusters wander as far as 90,000 to 120,000 light-years from the galactic center, and some extend as far as 300,000 light-years out. The motions of these distant objects, influenced by the gravitational pull of the entire galaxy, allow astronomers to calculate the amount of mass in the galaxy. Some recent estimates reveal that the galaxy is 500 billion times the mass of the Sun. This estimate is significantly higher than the mass contributed by visible stars and nebulae alone, indicating that there is a great amount of unseen dark matter in the galaxy.
When compared to the Sun and other stars of the galactic disk, globular cluster stars appear to be deficient in heavy elements. This indicates that they are ancient objects, made from the pristine gas that condensed to form the galaxy long ago. Although chemical composition differs from one cluster to the next, all stars within a given cluster have similar compositions, indicating that they were born from the same cloud. This provides a unique opportunity for the study of stellar evolution. Yet each star began life with a different mass. By observing the luminosity and temperatures of their current states, astronomers are learning a great deal about the life cycles of stars.
Globular clusters contain mostly low-mass stars that are so tightly packed together that the density of stars near the center is about two stars per cubic light-year. In comparison, our solar neighborhood has about one star per 300 cubic light-years. If you were looking into the sky from a hypothetical planet in the middle of a globular cluster, like M4, you would be surrounded in a perpetual twilight cast by the light of thousands of nearby stars.