formation of the planetary system was further extended by Sekiya Minoru, Miyama
Shoken, and others in the late 1980s (Sekiya and Miyama 1988; Sekiya et al. 1988).
7.3 Variable Stars
In the postwar period, studies on variable stars made progress from both observational and theoretical perspectives with respect to various types of variable stars.
Observations were made through an international network VSNET, or the domestic
network VWNET-J, initially organized by Nogami Daisaku and Kato Taichi of
Kyoto University and others (Nogami et al. 1997). The stars that were observed
included various types such as novae, supernovae, cataclysmic variables, symbiotic
stars, pulsating stars, and many others. Observed data are collected in VSNET or
VSNET-J. Databases were created for network members.
In this section, topics are mostly selected based on theoretical interest in variable
stars in connection with certain topics.
7.3.1 Stars of Radial Pulsation
The prototype of radially pulsating stars is δ Cephei found by J. Goodricke in 1786.
He observed the period and asymmetric light variation of this star. The binary
hypothesis on the origin of stellar variation of Cepheid variables had been adopted
for about 130 years. The theory of radial pulsation of single stars was first advocated
by H. Shapley in 1914. J. A. Edgar considered the possibility of the excitation of first
overtone modes in 1933, referring to the suggestion of Shinjo Shinzo in 1922
(Chap. 2) (Edgar 1933).
In Japan, studies of Cepheid variables had been undertaken by Takeuchi Mine,
Aikawa Toshiki, and their group at Tohoku University in the 1980s. Among
Cepheid variables some members, such as U Triangulum and TU Cassiopeia, exhibit
double periods of pulsation, like beat phenomena. The period ratios are in the range
0.70–0.71. Takeuchi explained these phenomena as a superposition of two periodic
pulsations and named these variables double-period Cepheids. He also explained
these periods as a combination of the first and second harmonics of pulsations
(Takeuchi 1992).
In double-period Cepheids, even after subtracting the harmonic components,
some irregular fluctuation remains with small amplitude. For single-period Cepheids
also similar, small, chaotic fluctuations are observable. Tanaka Yasuo (Ibaraki
University) and Takeuchi (Tohoku University) considered a model oscillator of
irregular variation and suggested that such irregular variability was caused by
chaotic motions due to nonlinear effects (Tanaka and Takeuchi 1988; Tanaka 1991).
For other Cepheid stars, Takeuchi and Aikawa conducted Fourier analyses of
light curves and found the possible formation of double-period Cepheids. Based on a
7.3 Variable Stars
183
Shoken, and others in the late 1980s (Sekiya and Miyama 1988; Sekiya et al. 1988).
7.3 Variable Stars
In the postwar period, studies on variable stars made progress from both observational and theoretical perspectives with respect to various types of variable stars.
Observations were made through an international network VSNET, or the domestic
network VWNET-J, initially organized by Nogami Daisaku and Kato Taichi of
Kyoto University and others (Nogami et al. 1997). The stars that were observed
included various types such as novae, supernovae, cataclysmic variables, symbiotic
stars, pulsating stars, and many others. Observed data are collected in VSNET or
VSNET-J. Databases were created for network members.
In this section, topics are mostly selected based on theoretical interest in variable
stars in connection with certain topics.
7.3.1 Stars of Radial Pulsation
The prototype of radially pulsating stars is δ Cephei found by J. Goodricke in 1786.
He observed the period and asymmetric light variation of this star. The binary
hypothesis on the origin of stellar variation of Cepheid variables had been adopted
for about 130 years. The theory of radial pulsation of single stars was first advocated
by H. Shapley in 1914. J. A. Edgar considered the possibility of the excitation of first
overtone modes in 1933, referring to the suggestion of Shinjo Shinzo in 1922
(Chap. 2) (Edgar 1933).
In Japan, studies of Cepheid variables had been undertaken by Takeuchi Mine,
Aikawa Toshiki, and their group at Tohoku University in the 1980s. Among
Cepheid variables some members, such as U Triangulum and TU Cassiopeia, exhibit
double periods of pulsation, like beat phenomena. The period ratios are in the range
0.70–0.71. Takeuchi explained these phenomena as a superposition of two periodic
pulsations and named these variables double-period Cepheids. He also explained
these periods as a combination of the first and second harmonics of pulsations
(Takeuchi 1992).
In double-period Cepheids, even after subtracting the harmonic components,
some irregular fluctuation remains with small amplitude. For single-period Cepheids
also similar, small, chaotic fluctuations are observable. Tanaka Yasuo (Ibaraki
University) and Takeuchi (Tohoku University) considered a model oscillator of
irregular variation and suggested that such irregular variability was caused by
chaotic motions due to nonlinear effects (Tanaka and Takeuchi 1988; Tanaka 1991).
For other Cepheid stars, Takeuchi and Aikawa conducted Fourier analyses of
light curves and found the possible formation of double-period Cepheids. Based on a
7.3 Variable Stars
183