The Size, Structure, and Variability of Late-Type Stars Measured ...

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such as Mira variables, become unstable to pulsation. Dredge-up occurs once more in this
stage and carbon and oxygen, products of helium fusion, become abundant in the stellar
atmosphere. Polyatomic molecules of carbon and oxygen form in the stellar envelope, and
solid dust particles form at cooler temperatures further out. The greatly extended envelope
implies very low surface gravities allowing radiation pressure to cause some of the stellar
mass to become gravitationally unbound from the star. The high opacity of dust and
molecules helps drive this process. Mass-loss is observed to be as high as 10 −4 M ⊙ /year in
some AGB stars. It is this assortment of characteristics for AGB stars that sets the stage
for the observations described in this thesis.
The evolution of stars becomes less well known after this stage. Such high rates
of mass loss cause the AGB star to break up long before its fuel would have run out.
Eventually, the shell is completely removed and only the core remains. The star is now a
white dwarf, surrounded by a planetary nebula lit up by hot photons. After ∼ 10 4 years,
the planetary nebula is gone and the white dwarf slowly cools for the rest of its life. For
high mass stars, elements higher than helium can fuse, and the star can end up a neutron
star or black hole, but most of the stages of its life are analogous.