Our universe is 13.8 billion years old, a timescale much longer than the more relatable spans of hundreds or thousands of years that impact our lived experiences. So how do astronomers arrive at such an enormous number?
The universe, quite simply, must at least be as old as the oldest thing we can find in it. Thus, a direct test of the age of the universe is to go hunting for ancient stars.
Stars in clusters, or agglomerations of stars, all born at the same time, can be most accurately age-dated by looking for what is called the “main sequence turn off” for the cluster. The longest stage of a star’s life is spent burning hydrogen. During this phase, stars follow a relation between their temperature and brightness known as the main sequence. In other words, hotter stars shine brighter.