Red Giant Star

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.

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 Figure 2. From hydrogen to helium there is a big change in binding energy, meaning the star gets a lot of energy out of each reaction. However, from helium to carbon, there is much less of a change. Each helium-to-carbon reaction produces less energy for the star than a hydrogen-to-helium reaction. At the same time, the high temperature of the core forces the reactions to occur quickly (this is part of the reason a giant star is so luminous). Less energy is produced per reaction, but the reactions are happening more frequently, and the helium-burning phase cannot last nearly as long as the hydrogen-burning phase. Illustration by Hans & Cassidy. Courtesy of Gale Group. 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.

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. You 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 Figure 3. No element heavier than iron can be fused, because the binding energy curve reaches a minimum at iron (see Fig. 2). To fuse a heavier element would require an input of energy, rather than producing energy. When a star develops an iron core, it has run out of all possible fuel sources, and soon explodes as a supernova. Illustration by Hans & Cassidy. Courtesy of Gale Group. stars that accurately reproduce the observed levels and variation of the giants' energy output.