- Contain ~10 — ~1000 stars.
- Radii between ~0.1 and ~1 pc.
- Between 70% and 90% of mass is in gas.
- Clusters last ~5 and ~10 Myr. (Lada & Lada 2003)
- Gas is removed by supernovae and strong stellar winds.
- Very fast once these processes turn on (i.e. when the massive stars get old).
- Causes cluster to explode.
