7.10.2.1 Planetary Nebulae
Initially, V. A. Ambarzumian of Byurakan Observatory derived the radiation pressure by assuming a rectangular contour of the Lα flux in 1932. According to his
calculation, the radiation pressure is quite strong and blows up the whole nebular
matter. Unno started his work from the theoretical derivation of radiation pressure
due to hydrogen Lyman-alpha line (Lα). In 1950, Unno solved the radiative transfer
of Lα line, by taking the Zanstra effect of redistribution in frequency in the line
contour into account, and obtained significantly reduced radiation pressure, enough
to avoid the blow-up catastrophe. In highly excited planetary nebulae, hydrogen is
almost ionized, and the Lα line of ionized helium takes place the main role of nebular
motion. Unno (1952) solved the radiative transfer of this line in the same way with
hydrogen and again derived the reduced radiation pressure to give the expansion
velocity of nebulae in the order of 10 km s
À1 . Based on these results, Unno and
Takakubo Keiya examined the internal motions and stratifications of planetary
nebulae in 1952. They showed that high-excitation nebulae are stratified in the
order of HeIII, HeII, HeI and HI from inner to outer layers, and expanding velocity
increases from 5 to 15 km s
À1 toward outer layers, which well coincided with
observations (Unno 1950, 1952; Unno and Takakubo 1952).
7.10.2.2 Solar Physics
In the 1950s, Unno treated two problems in Solar physics: convection layer and
radiative transfer. On the convection layer, he derived the production rate of acoustic
waves by turbulence and showed that the acoustic energy thus produced may be
enough for the heating of the solar corona (Unno and Kawabata 1955). On the
radiative transfer, Unno (1956) considered the line formation in magnetic field such
as in sunspots, where Zeeman effects are important in the line formation. He derived
and solved the equations of radiative transfer for each Zeeman component and
calculated the strength of polarized lines (Unno 1956).
In 1974, Unno’ work shifted to the dynamic process of the solar atmosphere: the
structure and motion of spicules (1974); active regions of the photosphere (1981),
turbulent convection (1981); and nonlocal convection zones (1989). Investigations
of the stability of the solar core and the dynamics of nonradial oscillations were also
carried out in 1975.
7.10.2.3 Stellar Physics
1. Dynamical process in stellar atmospheres
Under collaboration with Kato Shoji and Makita Mitugu, Unno considered the
condition of the onset of thermal convection in polytropic atmosphere. This problem
had a long history since John W. Strutt (Lord Layleigh) (1916) and widely treated by
7.10 Theoretical Astrophysics
235
Initially, V. A. Ambarzumian of Byurakan Observatory derived the radiation pressure by assuming a rectangular contour of the Lα flux in 1932. According to his
calculation, the radiation pressure is quite strong and blows up the whole nebular
matter. Unno started his work from the theoretical derivation of radiation pressure
due to hydrogen Lyman-alpha line (Lα). In 1950, Unno solved the radiative transfer
of Lα line, by taking the Zanstra effect of redistribution in frequency in the line
contour into account, and obtained significantly reduced radiation pressure, enough
to avoid the blow-up catastrophe. In highly excited planetary nebulae, hydrogen is
almost ionized, and the Lα line of ionized helium takes place the main role of nebular
motion. Unno (1952) solved the radiative transfer of this line in the same way with
hydrogen and again derived the reduced radiation pressure to give the expansion
velocity of nebulae in the order of 10 km s
À1 . Based on these results, Unno and
Takakubo Keiya examined the internal motions and stratifications of planetary
nebulae in 1952. They showed that high-excitation nebulae are stratified in the
order of HeIII, HeII, HeI and HI from inner to outer layers, and expanding velocity
increases from 5 to 15 km s
À1 toward outer layers, which well coincided with
observations (Unno 1950, 1952; Unno and Takakubo 1952).
7.10.2.2 Solar Physics
In the 1950s, Unno treated two problems in Solar physics: convection layer and
radiative transfer. On the convection layer, he derived the production rate of acoustic
waves by turbulence and showed that the acoustic energy thus produced may be
enough for the heating of the solar corona (Unno and Kawabata 1955). On the
radiative transfer, Unno (1956) considered the line formation in magnetic field such
as in sunspots, where Zeeman effects are important in the line formation. He derived
and solved the equations of radiative transfer for each Zeeman component and
calculated the strength of polarized lines (Unno 1956).
In 1974, Unno’ work shifted to the dynamic process of the solar atmosphere: the
structure and motion of spicules (1974); active regions of the photosphere (1981),
turbulent convection (1981); and nonlocal convection zones (1989). Investigations
of the stability of the solar core and the dynamics of nonradial oscillations were also
carried out in 1975.
7.10.2.3 Stellar Physics
1. Dynamical process in stellar atmospheres
Under collaboration with Kato Shoji and Makita Mitugu, Unno considered the
condition of the onset of thermal convection in polytropic atmosphere. This problem
had a long history since John W. Strutt (Lord Layleigh) (1916) and widely treated by
7.10 Theoretical Astrophysics
235