Transcript: Ancient astronomers once thought that stars were eternal and unchanging. We now know that stars evolve. They are born, they live, and they die. Well with stellar lifetimes so much longer than a human lifetime how do we actually know that stars evolve? The Sun is 4.6 billion years old, and astronomers have evidence that the system of stars in which the Sun sits, the Milky Way, is much older, perhaps ten or eleven billion years old. Thus the Sun is not the first generation of stars to live in the Milky Way. We also know from the theory of stellar evolution that the most massive stars must have short lives. So the most massive stars we see in the sky cannot have lived for very long, and they will not live for long. The most massive stars have total main sequence lifetimes that are less than the span of the human species so far.