The Life Cycles of Stars

Understanding how stars die and create superdense bodies requires some basic knowledge of how stars live. In the same way that people undergo an inevitable life cycle, stars are born, live out their lives, and finally die. The nursery of a typical star, including one like the Sun, is an extremely large cloud of gases and dust floating through space. Such clouds come into being when “winds” created by exploding stars blow scattered molecules of gas and particles of dust around; some become even more scattered, while others become more concentrated. When such a cloud becomes concentrated enough, gravity causes it to contract still further over time. This contraction also produces heat, which makes the gases and dust grow steadily hotter. Soon, the center of the cloud becomes hot enough to cook a steak; then it reaches the temperature of a blast furnace; and finally, after a few million years, the temperature at the cloud’s core becomes hot enough to fuse hydrogen atoms and thereby ignite nuclear reactions. At that instant, the core emits a huge burst of blinding light and other energy that blows away the cooler outer layers of the cloud, leaving behind a giant ball of white-hot gases a newborn star.

The new star has enough hydrogen in its interior to keep its self-sustaining nuclear reactions going for billions of years. And throughout this longest portion of its life cycle, it continues producing light and heat. If certain other factors in the star’s solar system are favorable such as the formation of a planet at the right distance from the star and the presence of water this abundant light and heat makes the rise of life possible in that solar system. No significant danger is posed to such life as long as the star remains stable.

The reason a typical star can remain stable for so long is that two enormous forces occurring within the body of the object oppose each other, creating an equilibrium, or balance. One of these forces is gravity, which makes the massive quantities of matter in the star’s outer layers fall inward, creating great pressure. The Sun “contains a thousand times more mass than Jupiter,” Begelman and Rees point out. If the Sun were a cold body, “gravity would compress it to a million times the density of an ordinary solid. It would be . . . about the same size as the Earth, but 330,000 times more massive.”

But as everyone can easily see and feel, the Sun is not a cold body. Stars like the Sun produce enormous amounts of energy, accounting for the second major force at work within them. The nuclear reactions taking place in a star’s core release immense amounts of heat, light, and tiny particles that travel outward toward the surface. In the Sun, for example, each and every second the core produces the same amount of energy as 100 million nuclear bombs exploding simultaneously. As this terrific stream of energy moves outward from the core, it exerts a huge amount of outward pressure. And that pressure balances the force of gravity pushing inward. The Sun’s center, Begelman and Rees summarize, has a temperature of about 15 million degrees . . . thousands of times hotter even than its glowing surface. At these high temperatures, the atomic nuclei inside the sun are moving randomly at speeds of hundreds of kilometers per second. It is the pressure of this hot interior . . . that counteracts the [internal] effect of gravity in all stars like the sun.

Thanks to this balance between outward and inward pressures, stars like the Sun maintain their structures and remain stable for long periods of time. Astronomers estimate that the Sun, which has been in this stable state for several billion years, will remain in it for several billion years to come. Like an animal or a person, however, the great luminous ball cannot live forever. Eventually, a star must use up all of its fuel and enter its death throes, producing a catastrophe in which most of its matter is forced into an extremely dense state. This state can take one of three different forms, depending on the star’s initial mass; in each case, a superdense object is created. Two of these objects a white dwarf and a neutron star are in a sense immediate precursors of and steps on the road to the black hole. The third is the black hole itself.