According to the ancient Greek philosophers, the stars were “fixed and immutable,” the constant, changeless, and immortal inhabitants of the heavens, appearing nightly in all their brilliance and beauty, yet forever unknowable. But the stars are, in fact, neither fixed, immutable, nor unknowable. Like humans, they are born, they live and change, they grow old, and they die – it just takes them a long time to do so. Some stars have life spans of millions of years, others hundreds of millions, and still others billions of years. Inevitably, however, they all burn out and shine no longer.
Stars are created when vast interstellar clouds of gas and dust come together through gravitational attraction. As this matter coalesces, a protostar – a loose, globular mass of the stuff – results. Gravitational forces cause the protostar to condense and heat up. Eventually, the contraction causes tremendous pressure and heat in the core of the protostar, igniting a process of fusion reaction in which hydrogen, the most abundant element in the protostar, is converted to helium. This process causes the protostar to “switch on,” to begin radiating energy – to shine.
When its energy output equals the pressure exerted on it by gravity, a balance is achieved and the protostar stabilizes and stops contracting. It is now a star; in effect, a gigantic thermonuclear reactor in outer space. It now enters what is known as the main sequence, the prime of a star’s life, burning hydrogen and radiating energy steadily. A star remains in its main sequence for about 80 percent of its lifetime. Our sun, which is an estimated 5 billion years old, is right in the middle of its main sequence.
Inevitably, however, a star exhausts its supply of hydrogen fuel. Its outer layers begin to cool, and the star swells tremendously and turns red, becoming what is known as a red giant. Star death comes in a variety of forms: the red giant may become unstable and explode violently in a supernova; or it may simply collapse, becoming a kind of fading stellar cinder, known as a white dwarf, or a small, cold, burned-out black dwarf.