Asymptotic Giant Branch (AGB) Stars

graph showing location of Asymptotic Giant Branch (AGB) stars relative to Main Sequence stars

Figure 1

Cutaway view of an Asymptotic Giant Branch (AGB) star

Figure 2

For stars less massive than about nine times the mass of the sun, the last major phase of life is as an Asymptotic Giant Branch (AGB) star. This final phase of hydrogen burning happens after the star has moved from the Main sequence, through the Red Giant Phase and past the Horizontal Branch (see figure 1). At this point they are characterized by an inert carbon-oxygen core, surrounded by two separate nuclear burning layers - an inner layer of Helium and an outer layer of Hydrogen. These layers are in turn surrounded by a strongly convective outer envelope (see figure 2).

As the star evolves through the AGB phase, it cools, expands, and grows in brightness, burning its nuclear fuel faster and faster. For massive AGB stars larger than a few solar masses, the star can cool to such an extent that dust begins to condense in the outer convective envelope. At the same time, the star can begin to pulsate with very large amplitudes. As the star evolves, the pulsations become larger and longer. The large pulsation and dust formation combine to drive a wind off of the surface of the star, which can quickly lift the whole outer shell of hydrogen off, ending the hydrogen burning of the star. What is left over is a dusty shell of hydrogen slowly expanding into space, and a very hot white dwarf in the center - objects which we know of as a bipolar or planetary nebula. The nebula disperses quickly, leaving an inert white dwarf which slowly cools.

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