Red Giant Star
Red Giant Star
The Hertzsprung-Russell diagram, which shows relationships between stars in astronomy, states that a red giant star is any large non-main sequence star that has a stellar classification of K or M. A red giant star is a type of star that has exhausted the primary supply of hydrogen fuel at its core and is now using another element such as helium as the fuel for its energy-producing thermonuclear fusion reactions. The star Aldebaran (the bull’s eye) in the constellation Taurus is an example of a red giant star. Hydrogen fusion continues outside the core and causes the star to expand dramatically, making it a giant. Expansion also cools the star’s surface, which makes it appear red. Red giant stars are near the end of their lives, and die either in a supernova explosion, or more quietly as a planetary nebula. Both fates involve the expulsion of the star’s outer layers, which leave behind the small, exposed core.
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 people and materials objects 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 in Figure 1. Stars much smaller than the sun are unable to ignite their helium. 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 Kelvin), the helium ignites, beginning to fuse into carbon and oxygen.
Helium-fusing stars have found a way to maintain themselves against their own gravity, but there is a catch. The amount of energy a star gets out of a particular fusion reaction depends on the binding energy of the elements involved (Figure 2).
When the helium is exhausted, the cycle just described begins anew. The core contracts and heats, and if the temperature rises to 600 million Kelvin, the carbon will begin reacting, producing even more energy than the helium-burning phase. This, however, will not happen in the sun. Its core will not get hot enough, and at the end of its red giant phase, the sun will shed its outer layers, which will expand into space as a planetary nebula. Some of these nebulae look like giant smoke rings. All that will be left is the tiny core, made of carbon and oxygen, the ashes of the final fusion processes.
Whether destined to become a planetary nebula or a supernova, a red giant loses matter by ejecting a strong stellar wind. Many red giants are surrounded by clouds of gas and dust created by this ejected
material. The loss of mass created by these winds can affect the evolution and final state of the star, and the ejected material has profound importance for the evolution of the galaxy, providing raw interstellar material for the formation of the future generations of stars.
Binding energy— The amount of energy required to break an atomic nucleus apart.
Fusion— The conversion of nuclei of two or more lighter elements into one nucleus of a heavier element. Fusion is the process stars use to produce energy to support themselves against their own gravity.
Planetary nebula— A cloud of gas that is the expelled outer layers of a medium-mass giant star (about 0.5 to 3 solar masses).
Shell burning— The fusion of lighter elements into heavier ones in a roughly spherical “shell” outside the star’s core. Shell burning occurs after the fusing element has been exhausted in the core. The fusion reactions involving that element creep away from the core like a ring of flame creeping away from a campfire.
Supernova— The final collapse stage of a super-giant star.
Massive stars, however, can heat their cores enough to find several new sources of energy, such as carbon, oxygen, neon, and silicon. These stars may have several fusion shells (Figure 3). One can think of the whole red giant stage as an act of self-preservation. The star, in a continued effort to prevent its own gravity from crushing it, finds new sources of fuel to prolong its life for as long as it is able. The rapidly changing situation in its core may cause it to become unstable, and many red giants show marked variability. An interesting field of modern research involves creation of computer models of giant stars that accurately reproduce the observed levels and variation of the giants’ energy output.
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Freedman, Roger A. Universe, Stars, and Galaxies. New York: W.H. Freeman, 2005.
Jeffrey C. Hall