NGC 6638: A Star-Studded Skyfield

This star-studded image from the Hubble Space Telescope shows the heart of the globular cluster NGC 6638 in the constellation Sagittarius. The star-strewn observation highlights the density of stars at the heart of globular clusters, which are stable, tightly bound clusters of tens of thousands to millions of stars. Hubble revolutionized the study of globular clusters, since it is almost impossible to clearly distinguish the stars in globular clusters with ground-based telescopes. The blurring caused by Earth’s atmosphere makes it impossible to tell one star from another, but from Hubble’s location in low-Earth orbit the atmosphere no longer poses a problem. As a result, Hubble has been used to study what kind of stars globular clusters are made up of, how they evolve, and the role of gravity in these dense systems.