Stars vary widely in age, ranging from very young (a few million years old) to nearly as old as the universe itself, about 13.8 billion years. Some of the oldest known stars have ages estimated close to 13.7 billion years, just slightly younger than the universe's age. Most stars spend billions of years in a stable phase called the main sequence, like our Sun, which is roughly 4.6 billion years old. The age of a star is determined by several factors, including its mass, brightness, color, and especially by observing stars that are nearing the end of their lives. Large stars burn their fuel faster and live shorter lives, while smaller stars burn slower and can live for tens of billions of years. Astronomers also estimate stellar ages by grouping stars in clusters and using models of stellar evolution calibrated with precise measurements like the star's rotation rate or chemical composition. The light from stars takes time to reach us, meaning when we look at stars, we see them as they were in the past, effectively looking back in time.