NASA Astronomy Picture of the Day Collection

NGC 6791: An Old, Large Open Cluster

NGC 6791 is one of the oldest and largest open clusters of stars [ http://antwrp.gsfc.…] known. But how did it get so dirty? Open star clusters [ http://www.seds.org…] usually contain a few hundred stars each less than a billion years old. Open star cluster [ http://antwrp.gsfc.…] NGC 6791 [ http://xxx.lanl.gov…], 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 [ http://xxx.lanl.gov…] are relatively dirty - the minuscule amounts of heavy elements [ http://pearl1.lanl.…] (generically called metals) are high relative to most other star clusters [ http://antwrp.gsfc.…]. Older stars are supposed to be metal poor, since metals have only been slowly accumulating in our Milky Way Galaxy [ http://www.damtp.ca…]. This enigma makes NGC 6791, pictured above [ http://www.astrouw.…], one of the most studied open clusters [ http://cfa-www.harv…] and a possible example of how stars might evolve in the centers of galaxies.

Barbara J. Mochejska [ http://www.astrouw.…] (CAMK [ http://www.camk.edu…]) et al., 2.1-m Telescope [ http://www.noao.edu…], KPNO [ http://www.noao.edu…], NOAO [ http://www.noao.edu/], NSF [ http://www.nsf.gov/]

open cluster

NGC 6791

stars

Milky Way Galaxy

Goddard Space Flight Center (GSFC)

open cluster

New General Catalogue (NGC)

http://antwrp.gsfc.…

SPD-APOD-ap000112