NGC 6638

This star-studded image from the NASA/ESA 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 groups of tens of thousands to millions of stars. To capture the data in this image, Hubble used two of its cutting-edge astronomical instruments: Wide Field Camera 3 and the Advanced Camera for Surveys.

Hubble revolutionized the study of globular clusters. The distortion caused by Earth’s atmosphere makes it nearly impossible to clearly distinguish stars in the cores of globular clusters with ground-based telescopes. Orbiting some 340 miles (550 km) above Earth, Hubble can study what kind of stars make up globular clusters, how they evolve, and the role of gravity in these dense systems without Earth’s atmosphere posing a problem.