From the Outside In

We tend to think of stars as being stable and ancient structures—the constants of the sky. Even in a long human lifetime of 100 years, the heavens hardly change: the constellations are the same, the Sun is just as hot and luminous in the summer months, and the day is still 24 h long. This constancy is a consequence of our perception of time. The Sun may seem constant to our senses and memory, but it is undergoing continuous change and struggle. Each second, its composition alters ever so slightly (as we shall see in Chap. 4), and it fights against the ever-present threat of gravitational collapse.