Globular cluster 47 Tucanae

This NASA/ESA Hubble Space Telescope image shows a globular cluster known as NGC 104 — or, more commonly, 47 Tucanae, since it is part of the constellation of Tucana (The Toucan) in the southern sky. After Omega Centauri it is the brightest globular cluster in the night sky, hosting tens of thousands of stars. 

NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration

Scientists using Hubble observed the white dwarfs in the cluster. These dying stars migrate from the crowded centre of the cluster to its outskirts: it is a 40-million-year migration. Whilst astronomers knew about this process they had never seen it in action, until the detailed study of 47 Tucanae.

In globular clusters, lower mass stars rob momentum from more massive stars. Heavier stars sink to the cluster’s core as their orbits slow, while lighter stars pick up speed and move across the cluster to the edge.