Chapter 13: Star Birth and Death

Hertzsprung-Russell diagram

After the main sequence phase of evolution, stars become red giants — a contracting, heating core of helium, surrounded by an energetic hydrogen burning shell, and an extended, cool atmosphere. When the contracting core becomes hot enough to begin fusing helium, the layers above it, all the way out to the atmosphere, readjust again. As a red giant, the star was climbing the red giant branch on the H-R diagram, to the right (cooler, redder) and up (larger). Once the triple-alpha process starts in the core, the core stops contracting and the outer layers of the star "deflate." The core is now fusing helium, and a shell surrounding it is still fusing hydrogen. This brings the star to the left (hotter) and down (smaller) on the H-R diagram. The amount of mass that was lost in the red giant wind will determine where the star ends up on the H-R diagram. It will be located along a line (of constant luminosity) called the horizontal branch.

This diagram shows a simplified (and not to scale) cross-section of a massive, evolved star (with a mass greater than eight times the Sun.) Where the pressure and temperature permit, concentric shells of Hydrogen (H), Helium (He), Carbon (C), Neon/Magnesium (Ne), Oxygen (O) and Silicon (Si) plasma are burning inside the star. The resulting fusion by-products rain down upon the next lower layer, building up the shell below. As a result of Silicon fusion, an inert core of Iron (Fe) plasma is steadily building up at the center. Once this core reaches the Chandrasekhar mass, the iron can no longer sustain its own mass and it undergoes a collapse. This can result in a supernova explosion.

In addition to the helium, carbon, and oxygen produced in the shell fusion zone, there is an important secondary fusion process going on. Stars usually produce heavy elements by combining sets of lighter nuclei when the temperatures are hot enough to overcome the repulsive force of the charged nuclei. There is another way up the ladder of increasingly heavy nuclei, however. A few nuclear reactions in the fusion shells produce free neutrons which have no charge. This means that neutrons can be added to charged nuclei to build higher mass nuclei at temperatures that are too cool for carbon, oxygen, or heavier nuclei to combine with each other. In AGB stars, the neutrons are produced at a moderate rate and this fusion is called the slow, or s-process. We think that the s-process is responsible for producing many elements heavier than iron, albeit in very small amounts. The amount of s-process material that is created is very sensitive to temperature, density, composition, and how many neutrons are available. Astronomers who make models of how stars evolve like processes like this because they provide very strong constraints on the models. Imagine you work at a shipping company and have two boxes that have no return addresses but need to be returned to their place of origin. One box is made of cardboard that is available all over the world, and the other box is made of a rare wood that grows only in a few places on Earth. For which box would it be easier to pinpoint a return address? In a similar way, rare, hard-to-make elements provide good clues for astronomers studying the evolution of stars.

Artist's impression of the structure of a solar-like star and a red giant. The two images are not to scale - the scale is given in the lower right corner. It is common to divide the Sun's (and solar-like stars') interior into three distinct zones: The uppermost is the Convective Zone. It extends downwards from the bottom of the photosphere to a depth of about 15% of the radius of the Sun. Here the energy is mainly transported upwards by (convection) streams of gas. The Radiative Zone is below the convection zone and extends downwards to the core. Here energy is transported outwards by radiation and not by convection. From the top of this zone to the bottom, the density increases 100 times. The core occupies the central region and its diameter is about 15% of that of the entire star. Here the energy is produced by fusion processes through which hydrogen nuclei are fused together to produce helium nuclei. In the Sun, the temperature is around 14 million degrees. In red giants, the convection zone is much larger, encompassing more than 35 times more mass than in the Sun.

Mass loss is more vigorous in the asymptotic giant branch phase than while the star was a red giant. The irregular interaction between the hydrogen and helium fusion shells leads to pulsations in the atmosphere and considerable mixing of material from deep inside the star with the atmosphere. This is a productive time in the star's life — dust may be made in the regions surrounding the star, and the pulsations and mixing expel most of the outer layers of the star (enriched in s-process nuclei) into interstellar space. Some of the most beautiful objects in the night sky are formed as this stage ends.