1665

Globular Clusters

Johann Ihle (c. 1627–1699)

Stars form from the gravitational collapse of immense clouds of gas and dust. Astronomers have found that these clouds are often so large that many stars can form from a single cloud or closely associated collections of clouds, leading to the creation of multiple star systems and particular concentrations of enhanced star formation, such as spiral galaxy arms. Some interstellar clouds, especially in the early universe, appear to have been so massive that they led to the formation of literally hundreds of thousands of stars in relatively close proximity. When gravitational interactions between these nearby stars pull them together into a spherical mass, all orbiting a common center of gravity (and perhaps a black hole), the resulting collection of stars is called a globular cluster.

The first reported observation of a globular cluster was in 1665, from telescopic observations by a German post office official and avid amateur astronomer named Johann Ihle. Ihle observed a dense cluster of stars that is now known in the catalog of astronomer Charles Messier as M22, or the Sagittarius Cluster. M22 is visible to the naked eye as a faint 5th Magnitude smudge; Ihle and other seventeenth-century astronomers were able to use their astronomical telescopes to reveal the smudge to be the collected light of a swarm of countless numbers of closely packed stars.

More than 150 bright, densely packed globular clusters like M22 have since been discovered orbiting the center of our Milky Way galaxy as part of a semispherical halo of stars and star clusters that are older than the typical stars found in the galactic disk. Other galaxies have been found to have halos of globular clusters as well—apparently, halo formation is an important early stage in the formation of galaxies in general. Globular cluster halos extend so far out from the centers of many galaxies that astronomers speculate that some galaxies actually trade star clusters when they pass by or interact with each other gravitationally.

Stars in globular clusters interact with their close neighbors much more often than stars in “normal” space, and, as a result, astronomers do not believe that globular clusters would be good places to find stable, habitable planets. However, much still remains unknown about these ancient stellar swarms.

SEE ALSO First Stars (c. 13.5 Billion BCE), Milky Way (c. 13.3 Billion BCE), Solar Nebula (c. 5 Billion BCE), Stellar Magnitude (c. 150 BCE).

Hubble Space Telescope photo of the globular star cluster known as NGC 6093, or M80 in Messier’s catalog. The cluster is about 28,000 light-years away and contains hundreds of thousands of stars, all bound together by their mutual gravitational attraction.