light curve of VW Hydri. According to this model, a super-outburst occurs in the
outer edge of an accretion disk where the disk is cool enough for hydrogen to
recombine, whereas normal outbursts appear in the inner hot regions of the disk.
Other behaviors in the variations of optical and X-ray spectra have also been
interpreted in favor of the disk-instability model (Mineshige 1986).
In dwarf novae there is a rich variety in the light curves of outbursts, such as SU
UMa type, U Gem type, and Z Cam type. Osaki presented a unified theory of
outbursts within a single frame of the disk-instability model (Osaki 1997).
7.3.5 Supernovae
Supernovae are an order of magnitude brighter than ordinary novae at their maximum light in absolute magnitude. The appearance of supernovae is quite rare in our
Galaxy. Only a few cases have been recorded, for example, SN 1054 (Crab Nebula)
and SN 1572 (Tycho Brahe’s star).
In the latter half of the twentieth century, the discovery of supernovae in other
galaxies drastically increased due to the development and popularization of telescopic observations globally, including among Japanese amateur astronomers. This
enabled detailed studies of the types and physical properties of supernovae.
Supernovae are basically separated into two types, I and II, depending on whether
hydrogen lines are present (Type II) or lacking (Type I) in their spectra. Both types,
however, can be classified into several subtypes by their spectral features and
different patterns of light curves.
A Type Ia supernova manifests Si absorption at λ6153 Å, whereas a Type Ib
supernova shows no Si line, but it does show He lines in emissions. Type Ia
supernovae are ordinarily attributed to the runaway explosions of white dwarfs in
old binary systems composed of a red giant and a white dwarf. When the mass of the
white dwarf exceeds the Chandrasekhar limit by the accreting flow from the red
giant, a runaway burst occurs in the white dwarf as a supernova explosion. Since
both component stars are already exhausted hydrogen, Type Ia supernovae exhibit
no hydrogen spectrum. Type Ib is lacking both hydrogen and Si lines. This type of
supernova has similarity with Type II as their core collapse type as seen in what
follows.
Type II supernovae occur in massive stars when the nuclear energy source has
been exhausted and the iron core has collapsed. The collapse causes a violent
expulsion of the outer layers of the star as a supernova, while the central part of
the star collapses into a neutron star or a black hole according to the original mass of
the star.
On February 23, 1987, a supernova appeared in the Large Magellanic Cloud
(SN 1987A) (Fig. 7.12). SN 1987A is a Type II supernova characterized by the
existence of a hydrogen-line spectrum. The progenitor of this SN was also found to
be a blue supergiant with a luminosity of around 10
5 L ☉ , effective temperature of
15,000 K, and a size of 40 solar radii. Shigeyama Toshikazu, Nomoto Kenichi, and
190
7 Postwar Development of Astrophysics, 1946–2000 (Part II: Astrophysics)
outer edge of an accretion disk where the disk is cool enough for hydrogen to
recombine, whereas normal outbursts appear in the inner hot regions of the disk.
Other behaviors in the variations of optical and X-ray spectra have also been
interpreted in favor of the disk-instability model (Mineshige 1986).
In dwarf novae there is a rich variety in the light curves of outbursts, such as SU
UMa type, U Gem type, and Z Cam type. Osaki presented a unified theory of
outbursts within a single frame of the disk-instability model (Osaki 1997).
7.3.5 Supernovae
Supernovae are an order of magnitude brighter than ordinary novae at their maximum light in absolute magnitude. The appearance of supernovae is quite rare in our
Galaxy. Only a few cases have been recorded, for example, SN 1054 (Crab Nebula)
and SN 1572 (Tycho Brahe’s star).
In the latter half of the twentieth century, the discovery of supernovae in other
galaxies drastically increased due to the development and popularization of telescopic observations globally, including among Japanese amateur astronomers. This
enabled detailed studies of the types and physical properties of supernovae.
Supernovae are basically separated into two types, I and II, depending on whether
hydrogen lines are present (Type II) or lacking (Type I) in their spectra. Both types,
however, can be classified into several subtypes by their spectral features and
different patterns of light curves.
A Type Ia supernova manifests Si absorption at λ6153 Å, whereas a Type Ib
supernova shows no Si line, but it does show He lines in emissions. Type Ia
supernovae are ordinarily attributed to the runaway explosions of white dwarfs in
old binary systems composed of a red giant and a white dwarf. When the mass of the
white dwarf exceeds the Chandrasekhar limit by the accreting flow from the red
giant, a runaway burst occurs in the white dwarf as a supernova explosion. Since
both component stars are already exhausted hydrogen, Type Ia supernovae exhibit
no hydrogen spectrum. Type Ib is lacking both hydrogen and Si lines. This type of
supernova has similarity with Type II as their core collapse type as seen in what
follows.
Type II supernovae occur in massive stars when the nuclear energy source has
been exhausted and the iron core has collapsed. The collapse causes a violent
expulsion of the outer layers of the star as a supernova, while the central part of
the star collapses into a neutron star or a black hole according to the original mass of
the star.
On February 23, 1987, a supernova appeared in the Large Magellanic Cloud
(SN 1987A) (Fig. 7.12). SN 1987A is a Type II supernova characterized by the
existence of a hydrogen-line spectrum. The progenitor of this SN was also found to
be a blue supergiant with a luminosity of around 10
5 L ☉ , effective temperature of
15,000 K, and a size of 40 solar radii. Shigeyama Toshikazu, Nomoto Kenichi, and
190
7 Postwar Development of Astrophysics, 1946–2000 (Part II: Astrophysics)