Final answer:
Cosmologists use the age of old nearby stars, such as those in the Galaxy's globular clusters, to determine the age of the universe, which is estimated to be around 13.8 billion years old.
Step-by-step explanation:
Cosmologists determine the age of the universe by examining the oldest objects in it. Specifically, they study the age of old nearby stars in our own Galaxy, such as the globular clusters. The oldest globular clusters contain stars that are at least 13 billion years old. The universe must be at least as old as these ancient stars. When we look at distant galaxies, we observe conditions when the universe was younger and compare those observations with the properties of much older stars. This allows cosmologists to confirm that the age of the universe is currently estimated to be about 13.8 billion years.
Galaxies that contain only old stars tend to be yellowish red in color, while younger, blue galaxies contain a lot of hot, massive, young stars. By studying the oldest stars, such as those in globular clusters, and comparing the information with observations from the universe's expansion, we gain a consistent understanding of the cosmos's age.