Red Giant Star

Stars are self-gravitating objects, meaning that they are held together by their own gravity. A star's gravitational field tries to compress the star's matter toward its center, just as Earth's gravity pulls you toward its center. Since stars are gaseous, they would shrink dramatically if it were not for the thermonuclear fusion reactions occurring in their cores. These reactions, which in healthy stars involve the conversion of four hydrogen nuclei into one helium nucleus, produce energy that heats the star's gas and enables it to resist the force of gravity trying to compress it.

Most stars, including the sun, use hydrogen as their thermonuclear fuel for two reasons: First, stars are mostly made of hydrogen, so it is abundant; second, hydrogen is the lightest, simplest element, and it will fuse at a lower temperature than other elements. The hydrogen-to-helium reaction, which occurs in all stars, is the "easiest" one for a star to initiate.

Although stars are huge, they eventually run out of hydrogen fuel. The time required for this to happen depends on the mass of the star. Stars like the sun take about 10 billion years to exhaust the hydrogen in their cores, while the most massive stars may take only a few million years. As the star begins to run out of hydrogen, the rate of fusion reactions in its core decreases. Since not as much energy is being produced, gravity begins to overcome the pressure of the heated gas, and the core starts to shrink. When a gas is compressed, however, it gets hotter, so as the core gets smaller, it also heats up. This is a critical point in the star's life, because if the core can heat up to about 100 million kelvin, it will then be hot enough for helium fusion to begin. Helium, the "ashes" of the previous fusion reactions in the star's core, will become the new source of energy.

A star on the verge of helium ignition is shown here. Stars much smaller than the Sun cannot ignite their helium. Figure 1. The main source of energy is in the core, while hydrogen continues fusing to helium in a "shell" around the core, much as a small circle of flame might creep away from a campfire. The energy produced by this shell streams outward, pushing the star's outer layers away from its center. When the star's surface expands, it also cools, because there is less energy being emitted per unit area. This causes the star to appear red. Many of the bright, red stars in the sky at night are these red giants. Illustration by Hans & Cassidy. Courtesy of Gale Group. Not only is their gravity too weak for their cores to achieve the necessary temperature, but their interiors are more thoroughly mixed than those of more massive stars. The helium ash in low-mass stars never gets a chance to collect at the core, where it might be used as a new fuel source.

Stars like the sun, however, do develop a helium-rich core. When their cores get hot enough (about 100 million degrees kelvin), the helium ignites, beginning to fuse into carbon and oxygen.