Why are star clusters useful for understanding the evolution of stars?
a) They allow comparison of many stars that differ only in mass.
b) Determining their age is relatively straightforward.
c) All of the stars formed at nearly the same time, from the same material.
d) Stars at different stages of their evolution must be from different clusters that merged, allowing us to compare stars of different ages.
e) The surface composition of each star reveals its age.
