The Milky Way Galaxy has a system of roughly 150 globular clusters, which are all older than 9 billion years. Many of these are metal-poor, meaning they consist mostly of hydrogen, with some helium and only a sprinkling of heavy elements such as iron. Our current limited understanding of the formation of the Milky Way suggests that it formed about 11 billion years ago by mergers of fairly massive clumps of first-generation stars and gas. Subsequent accretion of even more clumps collapsed along filaments into the massive protogalaxy core.Astronomy magazine subscribers can read the full answer for free. Just make sure you're registered with the website.