Japanese Astronomy

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Japanese Astronomy ENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICS

Japanese Astronomy

Astronomy in Japan has made rapid progress in the last

two decades, placing Japan among the leaders in the ﬁeld.

Not only mm wave and x-ray astronomy since the 1980s

but also the optical and infrared, gravitational wave and

neutrino astronomy are now at the highest level in these

ﬁelds. Astronomers are preparing ambitious future plans

in both ground-based and space-borne astronomy, in spite

of the many challenges that basic science in Japan faces.

Historical view of Japanese astronomy

Historically, astronomical research in Japan contributed

little to the human understanding of the universe before

World War II. In the Meiji era (1868–1912), Japan started

introducing science and technology to catch up with the

Western world. The government established faculties

of engineering in universities as one of the earliest of

such attempts in the world. However, most of the

systematic and organized activities of basic sciences,

including astronomy, started only after World War II.

The Norikura Solar Corona Observatory, started in

1950, and the OKAYAMA ASTROPHYSICAL OBSERVATORY with a

188 cm optical telescope (1960) of Tokyo Astronomical

Observatory (TAO) and other observatories produced

only a few observational results of the highest quality

during the period of the 1950s to 1970s. However, these

observatories established a foundation for observational

astronomy in Japan. The TAO was then with

the University of Tokyo and reorganized in 1988

as an interuniversity research institute, the NATIONAL

ASTRONOMICAL OBSERVATORY, JAPAN (NAOJ). The progress in

theoretical research in Japan, on the other hand, was

remarkable in this period. The brilliant works by

Chushiro Hayashi on the early universe, stellar structure

and origin of the solar system, Yoshihide Kozai in

celestial mechanics and Satio Hayakawa in high-energy

astrophysics were among memorable contributions to the

theoretical astronomy and astrophysics of humankind. In

particular, the systematic works on the formation of stars

and the solar system by C Hayashi and his group had built

a ﬁrm foundation in the ﬁeld.

In the 1960s small but extensive attempts started in

radio, x-ray and infrared astronomy in Japan and grew

rapidly throughout in the 1970s. It was in the 1980s that

those efforts were rewarded with remarkable scientiﬁc and

technological results.

Developments in radio astronomy

Solar radio observations in Japan started in the 1950s.

The development of solar microwave interferometers by

Haruo Tanaka was among the earliest in the ﬁeld. The

millimeter wave observations in Japan started in 1970 with

a 6 m diameter telescope at Mitaka constructed by Kenji

Akabane, Masaki Morimoto and their group. Their efforts

led TAO to establish the NOBEYAMA RADIO OBSERVATORY (NRO)

in 1982 with two top-level mm wave telescopes, the 45 m

diameter telescope (the world’s largest mm wave telescope

until now) and the mm wave interferometer composed

of ﬁve (a sixth added later) 10 m antennas with 570 m

baseline. Norio Kaifu, Masato Ishiguro and their group

led the construction and work on related developments

such as high-precision antennas, acousto-optical radio

spectrometer, Fourier correlation radio spectrometer,

high-sensitivity SIS mixer receivers, closely cooperating

with engineers from Japanese industries. Since its

establishment the Nobeyama mm wave telescopes have

been providing rich scientiﬁc results: detection of many

exotic new molecules in interstellar clouds, protoplanetary

disks, evidence of supermassive black holes in galactic

nuclei, to name a few, making Japan one of the leading

countries in radio astronomy.

Recently (1998) NRO succeeded in linking the

45 m telescope with the array of six 10 m diameter

antennas for a seven-element mm wave interferometer

called RAINBOW. It will be the most sensitive mm

wave interferometer for the time before the planned

international project ALMA (mentioned below). Another

facility of NRO is a radioheliograph, completed in 1992

under the leadership of Shinzo Enome. The 84-element

array of 80 cm diameter parabolas yields high-resolution

radio images of the sun every second.

NRO and Mizusawa Astrogeodesy Observatory of

NAOJ had started the VLBI observations with other

Japanese institutes and organized a domestic VLBI

network (J-Net). In 1997 the VLBI group led by Hisashi

Hirabayashi and his group launched the world’s ﬁrst space

VLBI satellite HARUKA as a collaboration among ISAS,

NAOJ and overseas institutes. The launched 8 m diameter

antenna is orbiting around the Earth to link with radio

telescopes worldwide to form a 30 000 km aperture radio

telescope (VSOP). It was an engineering and logistical

challenge, and recently VSOP has produced clear images

of many energetic galactic nuclei with extremely high

resolution of 0.1–1 marcsec, approaching a linear size

about 100 times that of supermassive black holes.

NRO, with its large mm wave telescopes, was the ﬁrst

top-level observational facility Japan had for astronomy.

In the ﬁeld of mm wave and sub-mm wave astronomy,

university groups are now undertaking a number of small

but excellent projects: a 4 m diameter mm wave telescope

of Nagoya University (located at Las Campanas, Chile),

a 60 cm CO molecular line mapping telescope of the

University of Tokyo (La Silla, Chile), a 1.2 m remote-

operation sub-mm carbon atom line survey telescope of

the University of Tokyo (atop Mt Fuji) etc, yielding a

large amount of observational data for speciﬁc scientiﬁc

subjects.

Developments in x-ray astronomy

HAKUCHO, the ﬁrst Japanese x-ray satellite, was

launched in 1979 by the INSTITUTE OF SPACE AND ASTRONAUTICAL

SCIENCE (ISAS) under the leadership of Minoru Oda.

Subsequent series of ISAS x-ray satellites, TENMA (1983),

GINGA (1989) and ASUKA (1993) under the leadership

of Yasuo Tanaka and his group, with their rich scientiﬁc

Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998

and Institute of Physics Publishing 2001

Dirac House, Temple Back, Bristol, BS1 6BE, UK 1

Japanese Astronomy ENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICS

results made Japanese x-ray astronomy one of the most

productive in the world. Since the early identiﬁcation of

the x-ray source Sco-X1 with an evolved star using the

balloon-borne x-ray telescope and the Okayama 188 cm

telescope, efforts by ISAS and groups in universities in

parallel with developments in space engineering have

revealed the physics of dying stars and their energetic

activities. ASUKA is a relatively small satellite (420 kg)

but is actively continuing x-ray spectroscopic observations

of various objects from star formation sites to clusters of

galaxies in the very distant universe.

Solar x-ray observations have also been made by a

successful collaboration of x-ray astronomy at ISAS and

traditional solar physics at TAO/NAOJ. YOKO, launched

in 1991 following the ﬁrst Japanese solar x-ray satellite

HINOTORI (1981), has been providing a tremendous

number of x-ray images of the Sun and contributed to

understanding of the physical process of energetic solar

ﬂares (project leaders: Yoshiaki Ogawara and Yutaka

Uchida).

Recently ISAS developed the M-V rocket with a

payload capability much higher than that of previous

L-series rockets. The ﬁrst launch of M-V was celebrated

by the success of HARUKA/VSOP mentioned earlier. By

using M-V, new astronomical projects such as Astro-E, IRIS

and Solar-B are being promoted. Astro-E is a 1650 kg mass

satellite with very high x-ray spectroscopic and imaging

capabilities, to be launched in early 2000. Solar-B, a next-

generation HINOTORI, is a 50 cm aperture optical solar

telescope slated for 2004. IRIS is described below.

Developments in optical and IR astronomy

An 8.2 m aperture optical and IR telescope ‘Subaru’ atop

Mauna Kea, Hawaii, started its test observations in early

1999 and will be opened to the astronomical community

in late 2000. It is one of the new-generation telescopes

with a large active-support primary mirror housed in

a turbulence-suppressed cylindrical enclosure and has

seven observational instruments that cover ultraviolet

to mid-infrared ranges. Its high performance was

demonstrated by stellar images with a full width at half-

maximum of 0.2 arcsec and a number of spectacular

scientiﬁc results through the test observation period. The

construction of the SUBARU TELESCOPE, led by Keiichi Kodaira

and later by Norio Kaifu and their group, started in 1991

asa9yrproject of optical and IR astronomers of Japan.

Experience of Japanese optical astronomers has been

mostly limited to the Okayama 188 cm telescope built in

1960 and the Kiso 105 cm Schmidt camera built in 1974.

Infrared astronomers studied through the pioneering

construction of the Agematsu1mIRtelescope (1973) and

observations with overseas telescopes led by Haruyuki

Okuda and Shuji Sato. To build an engineering and

technical basis for optical and infrared astronomy, the

Advanced Technology Center was established at Mitaka

campus of NAOJ headquarters. Close cooperation with

university groups and industries was also organized

for preparation, basic design and construction of the

telescope and observing instruments. To overcome the

lack of middle- to small-size modern telescopes and poor

weather conditions in Japan, a 2 m IR telescope atop

Haleakala, Hawaii, and a 1.2 m IR telescope in South

Africa are being constructed by the University of Tokyo

and by Nagoya University, respectively. Japan had never

established a permanent governmental scientiﬁc facility

abroad before the Subaru. NAOJ being a government

organization, much effort was needed to overcome this

hurdle and ﬁnally in 1997 the Subaru Telescope Facility

was formally established in Big Island, Hawaii. Further

progress toward internationalization is deﬁnitely needed

for Japanese science and its future.

The Subaru Telescope, equipped with adaptive optics

and versatile observational instruments, together with

other new-generation telescopes such as VLT and GEMINI

will embark on new frontiers including direct observations

of extra-solar-system planets, detection of many small

icy planetesimals orbiting beyond Pluto, observations of

mysterious galactic nuclei, dark matter, distant clusters of

galaxies and the youngest celestial bodies in the expanding

universe.

Developments in other ﬁelds of astronomy and

astrophysics

Recent major developments of experimental research in

astrophysics in Japan include neutrino and gravitational

wave astronomy. Kamiokande’s neutrino detection from

supernova SN1987A opened a new ﬁeld of neutrino

astrophysics. The Kamiokande (Kamioka, Japan) is a

Cherenkov light detector initially built to detect proton

decay (led by Masatoshi Koshiba and Yoji Totsuka).

Because of the importance of neutrino observations,

the Cosmic Ray Institute (CRI, University of Tokyo)

constructed SUPER-KAMIOKANDE which is 10 times larger than

Kamiokande with 50 kt of pure water in a tank 39 m

in diameter and 41 m high, at the Kamioka site, 1 km

underground. It conﬁrmed the ﬁnite mass of the neutrino

recently.

TAMA-300 is a Japanese pilot project in gravitational

astronomy led by Yoshihide Kozai and joint groups of

NAOJ and other institutes. The 300 m long L-shaped

interferometer of TAMA-300 at Mitaka started operation

in 1999. Its high sensitivity would make TAMA-300 able

to detect gravitational waves for the ﬁrst time, if they have

fortunate events such as that for Kamiokande. As 3–4 km

scale gravitational telescopes, LIGO of USA and VIRGO of

France–Italy are to be commissioned within a few years;

the Japanese gravitational telescope group is working on

a further upgrade of TAMA-300.

AGASA, a large-aperture high-energy cosmic ray

detector of CRI, is in operation at Akeno, Yamanashi,

Japan. It aims to detect cosmic rays with extremely high

energy (>1020 eV) and identify their origin, which is still

unknown.

Theoretical studies in Japan have kept their high level

through the 1950s to the 1990s. The extensive studies of

atmospheres of low-temperature stars by Takashi Tsuji,

Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998

and Institute of Physics Publishing 2001

Dirac House, Temple Back, Bristol, BS1 6BE, UK 2

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