Globular Cluster Messier 68 The NASA/ESA Hubble Space Telescope
offers this delightful view of the crowded stellar encampment
called Messier 68, a spherical, star-filled region of space
known as a globular cluster. Mutual gravitational attraction
among a cluster's hundreds of thousands or even millions of
stars keeps stellar members in check, allowing globular clusters
to hang together for many billions of years. Astronomers can
measure the ages of globular clusters by looking at the light of
their constituent stars. The chemical elements leave signatures
in this light, and the starlight reveals that stars of globular
clusters typically contain fewer heavy elements, such as carbon,
oxygen and iron, than stars like the Sun. Since successive
generations of stars gradually create these elements through
nuclear fusion, stars having fewer of them are relics of earlier
epochs in the Universe. Indeed, the stars in globular clusters
rank among the oldest on record, dating back more than 10
billion years. More than 15