Stars as Laboratories for Fundamental Physics - MPP Theory Group

Anomalous Stellar Energy Losses Bounded by Observations 41
Fig. 2.9. Schematic Hertzsprung-Russell diagram of the main types of stars.
The hatched band is the instability strip, the locus of Cepheid-type variables.
The “Hertzsprung gap” between the main sequence and the red-giant region
is populated with stars crossing between these branches, and by red giants
and supergiants which execute blue loops.
debate about the age of globular clusters.
Toward fainter luminosities the instability strip crosses the MS, a
region where the δ Scuti stars and dwarf Cepheids (periods of 1−3 h)
are located.
Fainter still, the only remaining population of stars are white dwarfs.
At the crossover of their locus with the instability strip the ZZ Ceti
variables are found; they have periods of a few minutes. Because the
oscillation period depends on the luminosity, an observed secular change
of the period of a ZZ Ceti star can serve as a sensitive diagnostic for a
decrease of its luminosity and thus for its cooling speed.