This made it possible to build heavier elements up to neon. That is, the formation of
light elements became possible by successive proton capture in Hayashi’s theory,
against neutron capture in the αβγ theory (Hayashi and Nishida 1956).
7.2.3 Stellar Evolution
Hayashi and his school considered the process of stellar evolution from its birth to
the final stage of supernova or other collapsing steps.
7.2.3.1 Evolution of Protostars (Hayashi 1966)
Stars are born in dark and cold Molecular clouds that are composed of atoms,
molecules, and dust particles. By gravitational instability, some parts of the clouds
begin to collapse and form stars. The initial contraction is in a state of free fall and
isothermal cloud. The contracting cloud is transparent to interstellar radiation and
gradually becomes opaque. The density is low enough so that the cloud remains
transparent to the thermal radiation emitted mostly by dust particles.
At some point, the cloud becomes completely opaque, and Hayashi called this
stage the initial state of an opaque protostar. The contraction process changes from
free fall to adiabatic contraction. Hayashi illustrated the evolutionary process from
this point in a Hertzsprung–Russell (HR) diagram (L/L ☉ – T eff ), as shown in Fig. 7.2,
where point A denotes the initial protostar. The star evolves by passing points B,
C, D, and E and on to point F. The physical processes through these points are as
follows.
The protostar, from A through D, is in adiabatic contraction before it reaches
gravitational equilibrium. In the adiabatic state, the increasing rate of pressure is
much higher in the central core of the protostar. When the contraction reaches a
certain point, the central core enters gravitational equilibrium and gives rise to a
bounce of the core. The interaction of the bouncing core with its outer contracting
regions produces shock waves. When shock waves reach the protostellar surface, the
star flares up more than 1000 times the original luminosity.
The time scale of the contraction from A to E is about 20 years, of which the time
for the flare-up from D to E is about 100 days. After reaching E, quasi-hydrostatic
contraction of the star starts with a wholly convective structure and continues until
the star reaches the main sequence at F, where the central temperature becomes high
enough for hydrogen to burn.
Hayashi compared the flare-up process from D to E with the flaring-up phenomena of FU Orionis, a T Tauri-type stars. This star showed a remarkable flare-up in
1936 (Herbig 1965). The star increased its brightness over six magnitudes, within
about 120 days, and gradually decreased its luminosity. Hayashi argued that this
phenomenon could be explained as the last stage of the dynamical evolution of a star.
This evolutionary track has been called the Hayashi track, as opposed to the
Henyey track (Henyey et al. 1955), which is derived based on a stellar model in
7.2 Structure and Evolution of Stars
179
light elements became possible by successive proton capture in Hayashi’s theory,
against neutron capture in the αβγ theory (Hayashi and Nishida 1956).
7.2.3 Stellar Evolution
Hayashi and his school considered the process of stellar evolution from its birth to
the final stage of supernova or other collapsing steps.
7.2.3.1 Evolution of Protostars (Hayashi 1966)
Stars are born in dark and cold Molecular clouds that are composed of atoms,
molecules, and dust particles. By gravitational instability, some parts of the clouds
begin to collapse and form stars. The initial contraction is in a state of free fall and
isothermal cloud. The contracting cloud is transparent to interstellar radiation and
gradually becomes opaque. The density is low enough so that the cloud remains
transparent to the thermal radiation emitted mostly by dust particles.
At some point, the cloud becomes completely opaque, and Hayashi called this
stage the initial state of an opaque protostar. The contraction process changes from
free fall to adiabatic contraction. Hayashi illustrated the evolutionary process from
this point in a Hertzsprung–Russell (HR) diagram (L/L ☉ – T eff ), as shown in Fig. 7.2,
where point A denotes the initial protostar. The star evolves by passing points B,
C, D, and E and on to point F. The physical processes through these points are as
follows.
The protostar, from A through D, is in adiabatic contraction before it reaches
gravitational equilibrium. In the adiabatic state, the increasing rate of pressure is
much higher in the central core of the protostar. When the contraction reaches a
certain point, the central core enters gravitational equilibrium and gives rise to a
bounce of the core. The interaction of the bouncing core with its outer contracting
regions produces shock waves. When shock waves reach the protostellar surface, the
star flares up more than 1000 times the original luminosity.
The time scale of the contraction from A to E is about 20 years, of which the time
for the flare-up from D to E is about 100 days. After reaching E, quasi-hydrostatic
contraction of the star starts with a wholly convective structure and continues until
the star reaches the main sequence at F, where the central temperature becomes high
enough for hydrogen to burn.
Hayashi compared the flare-up process from D to E with the flaring-up phenomena of FU Orionis, a T Tauri-type stars. This star showed a remarkable flare-up in
1936 (Herbig 1965). The star increased its brightness over six magnitudes, within
about 120 days, and gradually decreased its luminosity. Hayashi argued that this
phenomenon could be explained as the last stage of the dynamical evolution of a star.
This evolutionary track has been called the Hayashi track, as opposed to the
Henyey track (Henyey et al. 1955), which is derived based on a stellar model in
7.2 Structure and Evolution of Stars
179