NGC 6791: An Old, Large Open Cluster

Explanation: NGC 6791 is one of the oldest and largest open clusters of stars known. But how did it get so dirty? Open star clusters usually contain a few hundred stars each less than a billion years old. Open star cluster NGC 6791 , however, contains thousands of stars recently measured to be about 8 billion years old. What's really confusing, though, is that the stars of NGC 6791 are relatively dirty - the minuscule amounts of heavy elements (generically called metals) are high relative to most other star clusters . Older stars are supposed to be metal poor, since metals have only been slowly accumulating in our Milky Way Galaxy . This enigma makes NGC 6791, pictured above , one of the most studied open clusters and a possible example of how stars might evolve in the centers of galaxies.