Why are globular clusters among the oldest objects in the Milky Way?
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 are considered among the oldest objects in the Milky Way because they are composed of Population II stars, which are inherently ancient. Scientific estimates, derived from comparing their stellar populations with evolutionary models, place their ages between 11 billion and 13 billion years.1
These clusters likely formed during the very early stages of the Galaxy's existence, before the primordial gaseous material—composed primarily of hydrogen and helium—collapsed and flattened into the present thin disk of the Milky Way. This early origin is further supported by their chemical composition; globular clusters contain significantly lower amounts of heavy elements (often referred to as "metals" by astronomers) compared to younger stars like the Sun. While the interstellar gas in the solar neighborhood contains approximately 2 percent heavy elements by mass, globular clusters contain as little as 0.02 percent, reflecting the chemical state of the Galaxy shortly after its birth. Consequently, these clusters have survived relatively unchanged for billions of years, serving as relics of the Galaxy's earliest history.21