Same age stars is a mark of a galaxy formed by collapse.
<u>Explanation:
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Before a galaxy is formed, the building material of the galaxy in a protogalactic cloud that is rotating at a great speed rapidly accumulates and collapses in itself owing to its own gravitational pull.
With this collapse, globular clusters are formed away from the nucleus of the galaxy.
These clusters give birth to halo stars more or less at the same time. Thus, the stars that are formed of the same age.
Sometimes, the reference of the age of the stars is taken to determine the age of the entire galaxy.
