20-2 Dredge-ups bring the products of nuclear fusion to a giant star’s surface

As we saw in Section 16-2, energy is transported outward from a star’s core by one of two processes—radiative diffusion or convection. The first is the passage of energy in the form of electromagnetic radiation, and it dominates only when a star’s gases are relatively transparent. The second involves up-and-down movement of the star’s gases. Convection plays a very important role in giant stars, and it helps supply the cosmos with the elements essential to life.

Convection, Dredge-ups, and Carbon Stars

In the Sun, convection dominates only the outer layers, from around 0.71 solar radius (measured from the center of the Sun) up to the photosphere (recall Figure 16-4). During the final stages of a star’s life, however, the convective zone can become so broad that it extends down to the star’s core. At these times, convection can “dredge up” the heavy elements produced in and around the core by nuclear fusion, transporting them all the way to the star’s surface.

The first dredge-up takes place after core hydrogen fusion stops, when the star becomes a red giant for the first time. Convection dips so deeply into the star that material processed by the CNO cycle of hydrogen fusion (see Section 16-1) is carried up to the star’s surface, changing the relative abundances of carbon, nitrogen, and oxygen. A second dredge–up occurs after core helium fusion ceases, further altering the abundances of carbon, nitrogen, and oxygen. Still later, during the AGB stage, a third dredge–up can occur if the star has a mass greater than about 2 M_⊙. This third dredge–up transports large amounts of freshly synthesized carbon to the star’s surface, and the star’s spectrum thus exhibits prominent absorption bands of carbon–rich molecules like C₂, CH, and CN. For this reason, an AGB star that has undergone a third dredge-up is called a carbon star.

All AGB stars have very strong stellar winds that cause them to lose mass at very high rates, up to 10⁻⁴ M_⊙ per year (a thousand times greater than that of a red giant, and 10¹⁰ times greater than the rate at which our present-day Sun loses mass). The surface temperature of AGB stars is relatively low, around 3000 K, so any ejected carbon-rich molecules can condense to form tiny grains of soot. Indeed, carbon stars are commonly found to be obscured in sooty cocoons of ejected matter (Figure 20-4).

Figure 20-4: R I V U X G
A Carbon Star TT Cygni is an AGB star in the constellation Cygnus that ejects some of its carbon-rich outer layers into space. Some of the ejected carbon combines with oxygen to form molecules of carbon monoxide (CO), whose emissions can be detected with a radio telescope. This radio image shows the CO emissions from a shell of material that TT Cygni ejected some 7000 years ago. Over that time, the shell has expanded to a diameter of about ½ ly.

(H. Olofsson, Stockholm Observatory, et al./NASA)

Carbon stars are important because they enrich the interstellar medium with carbon and some nitrogen and oxygen. The triple alpha process that occurs in helium fusion is the only way that carbon can be made, and carbon stars are the primary avenue by which this element is dispersed into interstellar space. Indeed, most of the carbon in your body was produced long ago inside a star by the triple alpha process (see Section 19-3). This carbon was later dredged up to the star’s surface and ejected into space. Some 4.56 billion years ago a clump of the interstellar medium that contained this carbon coalesced into the solar nebula from which our Earth—and all of the life on it—eventually formed. In this sense you can think of your body as containing “recycled” material—substances that were once in the heart of a star that formed and evolved long before our solar system existed.