many authors. One of the difficulties is laid on the large density gradient in the
undisturbed atmosphere. Unno et al. (1960) proposed a variation method for computing the condition of the onset of thermal convection. With this method they
derived a nondimensional parameter which determines the condition of marginal
stability for polytropic atmospheres.
2. Radial and nonradial oscillations
In the 1960s, Unno mostly worked on the stabilities of stars for radial and
nonradial pulsation under collaboration with Kamijo Fumio and Kato Shoji. These
works became the basis of later development in this field, as seen in Sect. 7.3.
3. Supermassive objects.
In the late 1960s, Unno, along with Osaki Yoji and Kato Shoji, paid attention to
the super-massive objects as a possible origin of extremely energetic objects. In
1963, Fred Hoyle and William A. Fowler suggested the possibility of super-massive
stars as the origin of strong radio sources. Since then, several investigations have
been made on this subject. Among them, Osaki (1966) studied the evolution and
pulsation of super-massive stars and found that the masses larger than 10
5.5 solar
mass eventually continue to collapse towards the Schwarzschild limit, whereas
masses smaller than this level underwent pulsation of large amplitude. In 1971,
Unno considered the physical state and stability of super-massive objects which
were defined as a homogeneous mixture of gas and stars. He supposed that the
evolution of a super-massive object takes three steps: (1) contraction of gas, (2) dissipation of stellar kinetic energy, and (3) formation of dense stellar system. The
structure and stability at each step were analyzed in detail. Unno suggested that when
mass is higher than 10
6 solar mass, this object possibly evolves to a quasar (Osaki
1966; Unno 1971).
7.10.3 Development of Theoretical Astrophysics
7.10.3.1 Accretion Disks and Jets
Disk phenomena are ubiquitous in stars and galaxies. Disks are formed mostly by
accretion flows from the accompanying objects. The theoretical study of accretion
disks was first conducted by Nikolai Shakura and Rashid Sunyaev in USSR in 1973.
They showed that, although the nature of viscosity was not well understood, basic
properties of thin accretion disks were explained independent of the specific form of
the viscosity, and their model was called the standard model, and this model was
successfully applied to many active objects. A general relativistic version was also
described in 1973 by I. D. Novikov and K. S. Thorne.
In Japan, theoretical studies on accretion disks began soon after. Osaki Yoji
proposed an accretion model for the outbursts of dwarf novae (Osaki 1974), Kato
Shoji and Fukue Jun examined trapped radial oscillations of accretion disks around a
236
7 Postwar Development of Astrophysics, 1946–2000 (Part II: Astrophysics)
undisturbed atmosphere. Unno et al. (1960) proposed a variation method for computing the condition of the onset of thermal convection. With this method they
derived a nondimensional parameter which determines the condition of marginal
stability for polytropic atmospheres.
2. Radial and nonradial oscillations
In the 1960s, Unno mostly worked on the stabilities of stars for radial and
nonradial pulsation under collaboration with Kamijo Fumio and Kato Shoji. These
works became the basis of later development in this field, as seen in Sect. 7.3.
3. Supermassive objects.
In the late 1960s, Unno, along with Osaki Yoji and Kato Shoji, paid attention to
the super-massive objects as a possible origin of extremely energetic objects. In
1963, Fred Hoyle and William A. Fowler suggested the possibility of super-massive
stars as the origin of strong radio sources. Since then, several investigations have
been made on this subject. Among them, Osaki (1966) studied the evolution and
pulsation of super-massive stars and found that the masses larger than 10
5.5 solar
mass eventually continue to collapse towards the Schwarzschild limit, whereas
masses smaller than this level underwent pulsation of large amplitude. In 1971,
Unno considered the physical state and stability of super-massive objects which
were defined as a homogeneous mixture of gas and stars. He supposed that the
evolution of a super-massive object takes three steps: (1) contraction of gas, (2) dissipation of stellar kinetic energy, and (3) formation of dense stellar system. The
structure and stability at each step were analyzed in detail. Unno suggested that when
mass is higher than 10
6 solar mass, this object possibly evolves to a quasar (Osaki
1966; Unno 1971).
7.10.3 Development of Theoretical Astrophysics
7.10.3.1 Accretion Disks and Jets
Disk phenomena are ubiquitous in stars and galaxies. Disks are formed mostly by
accretion flows from the accompanying objects. The theoretical study of accretion
disks was first conducted by Nikolai Shakura and Rashid Sunyaev in USSR in 1973.
They showed that, although the nature of viscosity was not well understood, basic
properties of thin accretion disks were explained independent of the specific form of
the viscosity, and their model was called the standard model, and this model was
successfully applied to many active objects. A general relativistic version was also
described in 1973 by I. D. Novikov and K. S. Thorne.
In Japan, theoretical studies on accretion disks began soon after. Osaki Yoji
proposed an accretion model for the outbursts of dwarf novae (Osaki 1974), Kato
Shoji and Fukue Jun examined trapped radial oscillations of accretion disks around a
236
7 Postwar Development of Astrophysics, 1946–2000 (Part II: Astrophysics)