Why do globular clusters contain fewer heavy elements than the Sun?
Most of the brightest stars are older yellow stars, but a few young blue stars are also visible. This picture is a composite of three images taken by the Hubble Space Telescope.
Globular clusters contain significantly fewer heavy elements than the Sun because they are among the oldest objects in the Galaxy, having formed between 11 billion and 13 billion years ago.12
As Population II objects, these clusters formed before the interstellar gas in the Galaxy had been substantially enriched by heavy elements produced in the later evolutionary stages of earlier generations of stars. In contrast, the Sun is a Population I star, which formed much later from interstellar material that had already been enriched by the supernova explosions and mass loss of previous stellar generations.134
While the present interstellar gas in the solar neighbourhood contains about 2 percent heavy elements by mass, globular clusters contain as little as 0.02 percent. Because globular clusters formed so early in the history of the universe, they consist primarily of the hydrogen and helium that were available at that time, with only a trace amount of heavier elements.123