PX158 - I4 - star clusters

• star clusters contain stars of the same age and composition, so they are useful to study stars as they

open clusters

• contain $100\to 1000$ stars
• found in the disk of the milkyway
• all the stars are very young ($1Myr\to 1Gyr$)
• it is speculated that the sun formed in such a cluster and drifted away
  

globular clusters

• contain ${10}^5\to {10}^6$ stars
• distributed all over the galaxy
• ${10}^2\to {10}^{3}starsperp{c}^3$
• very old: $\sim 12Gyr$