However, if the period is longer than a few months then we must wait until the re-expansion of the outer envelope of the evolving star after the helium flash
(the AGB stage--see Part I).
When the temperature and density in the helium shell again climb high enough to convert helium to carbon and oxygen, another "helium flash" occurs.
The helium flash brings heavier elements from the stellar interior to the surface.
The final helium flash renews the star with a vigorous source of energy, which causes both a tremendous brightening and a swelling in size.
So, in a sense, this final helium flash is really just a part of normal stellar evolution.
This is a direct consequence of the final helium flash, which removed hydrogen through convective motion.
In stars that start out with less than about 2.3 solar masses, core helium burning begins abruptly, engendering a brief thermonuclear runaway, or core helium flash
. When this happens, hydrogen burning actually slows in the surrounding shell, because it expands and cools; this makes the star's luminosity drop.
Abell 30, for example, has four nearly symmetrical knots surrounding its central star; these features might belong to an expanding shell of helium-rich material ejected from the star's surface in a "helium flash
." Abell 35, a questionable planetary reputed to be some 185,000 years old, is among the largest known.
The different classes of objects discussed in the book are grouped by spectral and pulsation characteristics, by evolutionary scenarios such as late helium flashes
and double-degenerate mergers, and by links among the classes.
During the second red-giant phase it experiences several epochs of enormous energy output--"helium flashes
"--that lead to massive, roughly 10,000-year long pulsations in size.