Facts of the Matter
Complex processes shape life of stars
OUR SUN is a rather ordinary star, at the cool, dim end of the range of temperatures and luminosities as stars go.
Far from the static object that shines steadily with its constant yellow light, it is a dynamic object that is the complex product of myriad physical processes.
It is a huge, exploding ball of gas, large enough to hold nearly 3 million Earth-size planets and held together by gravity against the tremendous force of the nuclear fusion taking place within its core.
All stars function on the same principles, but the crucial difference is the relationship between a star's mass and its core temperature.
Nuclear fusion begins when protons get squeezed close enough together that a stronger, short-range force, the strong nuclear force, takes over and binds them into an atomic nucleus.
A star is born when gravity draws hydrogen and other chemical elements inward to reach a critical density.
Nuclear fusion begins with the fusion of two protons (the nuclei of hydrogen atoms). This proton-proton fusion, or hydrogen "burning," is a five-step sequence of events that has a final result of four protons fusing to form a helium nucleus consisting of two protons and two neutrons.
The mass of the heavier helium nucleus is seven-tenths of 1 percent less than the combined masses of the four reacting protons, and each proton-proton fusion cycle releases only a small amount of energy.
Because atoms are so small, 1 gram of hydrogen "burns" to produce 0.9929 grams of helium and 2 billion kilowatt-hours of energy.
EACH SECOND, the sun converts 660 million tons of hydrogen into helium. In its core the sun has enough hydrogen to keep it burning for about 10 billion years. In 4.5 billion years, it has used a little less than one-half of the available hydrogen in its core but has lost only about four-hundredths of 1 percent of its total mass.
Stars come in a wide range of masses. The more massive the star, the more intense the pressure and the higher the temperature at its center. Above 16 million degrees or so, more energetic nuclear fusion cycles begin.
A star 10 times as massive as the sun will generate energy 1,000 times faster but will only last about 20 million years. A star one-tenth of the sun's mass might only be one-thousandth of its brightness but might last about a trillion years.
The sun, along with the bulk of the stars visible to the naked eye, are main sequence stars. A star arrives on the main sequence after it starts hydrogen burning in its core, and remains there throughout its core hydrogen-burning phase.
As a star ages and uses up its supply of hydrogen fuel, it will suffer different fates depending on its mass.
When the sun's core hydrogen begins to run out in another 5 billion years, gravity will overwhelm the outward explosive pressure of nuclear fusion. As it collapses, its internal temperature will increase, causing fusion to occur faster and farther out into the lower temperature zone that surrounds the core.
Faster fusion will create an outward expansion that will overwhelm gravity temporarily as the surface cools and reddens. It will expand to become a red giant and eventually engulf the inner planets, including Earth and perhaps even Mars.
As it expands, it will lose some mass because of weak gravity at the outer boundary. Meanwhile, the core will continue to heat until it reaches a temperature of 100 million degrees, at which time helium nuclei, the ashes of hydrogen burning, will begin to fuse into carbon atoms.
This "helium flash" lasts only a short time. In a star of solar mass, there is not enough mass to reach temperatures high enough for carbon atoms to fuse, and the star will collapse to become a white dwarf.
At this time a star of less than 1.5 solar masses will be only about the size of Earth, but the matter would be so dense that one teaspoon would weigh about 5 tons.
AT THAT POINT, the collapse will stop due to electron degeneracy pressure by electrons that refuse to be forced into the same quantum state.
Above 4 solar masses, a carbon fusion cycle will begin, and the future of the star will take a different course as carbon undergoes fusion to produce nitrogen and oxygen.
In very massive stars, the core temperature is high enough to fuse elements beyond carbon and oxygen. This process, called nucleosynthesis, can produce atomic nuclei up to iron. Beyond that, the energy that binds the nuclei is too low, and there can be no further equilibrium fusion.
In massive stars there is an "onion skin" of fusion shells, with the outer layers dropping fuel to lower layers and heavier and heavier nuclei being cooked up nearer the center of the star.
Iron is the most stable of all the nuclei. If the temperature of the universe was high enough, this is where everything would be headed.
Elements heavier than iron are created in the spectacular explosions of a supernova as a supermassive star collapses and the core reaches such high temperatures that it emits gamma radiation. This has disastrous effects. The intense pressure crushes electrons below their quantum limits, and they merge with protons to create neutrons and neutrinos.
THE WEAKLY interacting neutrinos escape from the core, further escalating the collapse as the outer layers detach from the core in a matter of milliseconds. Some of the neutrinos are absorbed by the outer layers, which begins the explosion.
The collapse of the core could stop when the entire star is at the density of an atomic nucleus and consists entirely of neutrons.
An even more massive star will crush the neutron "out of existence" to form a black hole.
These nuclear reactions are complex, and it is no wonder that the world's brightest physicists, astronomers and applied mathematicians spend countless hours in honing the theories and equations to match observations.
As complex as it is, the atoms that make Earth, and those in our own bodies, have been made from the all-pervasive hydrogen that dominates the conventional matter of the universe.
Understanding it is all part of searching for our own human origins, which has been a quest of the curious human mind from the beginning. It led the late, great astrophysicist Carl Sagan to say that we are all made of "star stuff."
picks up where your high school science teacher left off. He is a professor of science at Honolulu Community College, where he teaches earth and physical science and investigates life and the universe. He can be reached by e-mail at firstname.lastname@example.org