complete radiative equilibrium. Hayashi supposed that protostars had negligible
nuclear energy production due to their low internal temperature, so energy transport
takes place predominantly through convection, that is, the stellar interior is in the
state of convective equilibrium.
It is noticed that Hertzsprung-Russel (HR) diagram usually expresses the distributions of stars on the spectral-type vs. stellar absolute magnitude diagram for the
observational purposes. Instead, Hayashi adopted the stellar-luminosity vs. stellar
effective temperature diagram for the purpose of physical understanding of stellar
evolution.
7.2.3.2 Stellar Evolution in Advanced Stage
The main-sequence stage, where hydrogen burns and produces a helium core, is the
longest period in the life of a star. The helium core develops and gradually contracts due
to its strong self-gravity, and when the central temperature exceeds 100 million degrees,
helium burning starts, forming a carbon core. The carbon core also shrinks and forms a
neon core. The cores of heavy elements are successively formed in this way depending
on the stellar mass, and finally an iron core is produced. Since iron is the most stable
element, no iron burning will take place, and stars exhaust their energy source.
In Fig. 7.3, evolutionary tracks of stars in the HR diagram are presented for
hydrogen, helium, and carbon burnings, indicated by H, He, and C, respectively, in
Fig. 7.2 Evolutionary track
of protostar of one solar
mass from initial state to
main sequence in HR
diagram (Hayashi 1966)
180
7 Postwar Development of Astrophysics, 1946–2000 (Part II: Astrophysics)
nuclear energy production due to their low internal temperature, so energy transport
takes place predominantly through convection, that is, the stellar interior is in the
state of convective equilibrium.
It is noticed that Hertzsprung-Russel (HR) diagram usually expresses the distributions of stars on the spectral-type vs. stellar absolute magnitude diagram for the
observational purposes. Instead, Hayashi adopted the stellar-luminosity vs. stellar
effective temperature diagram for the purpose of physical understanding of stellar
evolution.
7.2.3.2 Stellar Evolution in Advanced Stage
The main-sequence stage, where hydrogen burns and produces a helium core, is the
longest period in the life of a star. The helium core develops and gradually contracts due
to its strong self-gravity, and when the central temperature exceeds 100 million degrees,
helium burning starts, forming a carbon core. The carbon core also shrinks and forms a
neon core. The cores of heavy elements are successively formed in this way depending
on the stellar mass, and finally an iron core is produced. Since iron is the most stable
element, no iron burning will take place, and stars exhaust their energy source.
In Fig. 7.3, evolutionary tracks of stars in the HR diagram are presented for
hydrogen, helium, and carbon burnings, indicated by H, He, and C, respectively, in
Fig. 7.2 Evolutionary track
of protostar of one solar
mass from initial state to
main sequence in HR
diagram (Hayashi 1966)
180
7 Postwar Development of Astrophysics, 1946–2000 (Part II: Astrophysics)