Stars as Laboratories for Fundamental Physics - MPP Theory Group

Anomalous Stellar Energy Losses Bounded by Observations 35
remaining for an HB star after mass loss on the RGB will still depend
on its initial value, rendering this a rather natural possibility. However,
it implies that the globular clusters of our galaxy did not form
at the same time—an age spread of several Gyr is required with the
older clusters predominantly at smaller galactocentric distances (Lee,
Demarque, and Zinn 1994 and references therein). Apart from some
anomalous cases such an age spread appears to be enough to explain
the second parameter phenomenon.
Returning to the inner structure of an HB star, it evolves quietly
at an almost fixed total luminosity (Fig. 2.6). The core constitutes
essentially a “helium main-sequence star.” Because its inner core is
convective it dredges helium into the nuclear furnace at the very center,
leaving a sharp composition discontinuity at the edge of the convective
region (Fig. 2.4). After some 12 C has been built up, 16 O also forms. At
the end of the HB phase, the helium core has developed an inner core
consisting of carbon and oxygen.
2.1.5 From Asymptotic Giants to White Dwarfs
After the exhaustion of helium at the center a degenerate carbonoxygen
(CO) core forms with helium shell burning and continuing hydrogen
shell burning (Fig. 2.5). Again, the star grows progressively
brighter and inflates. Put another way, it becomes very similar to a
star which first ascended the RGB: it ascends its Hayashi line for a
second time. The track in the Hertzsprung-Russell diagram asymptotically
approaches that from the first ascent (“asymptotic giants”).
The upper RGB can be observationally difficult to distinguish from the
asymptotic giant branch (AGB) even though they are reasonably well
separated in Fig. 2.3.
In low-mass stars carbon and oxygen never ignite. The shell sources
extinguish when most of the helium and hydrogen has been consumed
so that the star has lost its entire envelope either by hydrogen burning
or by further mass loss on the AGB. The remaining degenerate CO
star continues to radiate the heat stored in its interior. At first these
stars are rather hot, but geometrically very small with a typical radius
of 10 4 km. Their small surface area restricts their luminosity in
spite of the high temperature and so they are referred to as “white
dwarfs.” Because they are supported by electron degeneracy pressure,
their remaining evolution is cooling by neutrino emission from the interior
and by photon emission from the surface until they disappear
from visibility.