Astronomy, the most ancient science, began with the study of the Sun, the Moon, and the visible planets. The modern astronomer
is still centrally concerned with recording position, brightness, motion, and other directly observable features of celestial objects and with predicting their motion according to the laws of celestial mechanics. Astrophysics, a 19th- and 20th-century outgrowth of classical astronomy, uses quantum mechanics, relativity theory, and molecular, atomic, nuclear, and elementary-particle physics to explain observed celestial phenomena as the logical result of predictable physical processes. The astrophysicist seeks to characterize the constituents of the universe in terms of temperatures, pressures, densities, and chemical compositions. Although the term astronomer is still used, virtually all
astronomers have been trained in astrophysics. The broad aim of modern astronomy is to develop encompassing theories of the origin, evolution, and possible destiny of the universe as a whole, a field of endeavor that is known as cosmology.
NEW WINDOWS ON THE UNIVERSE
In the 20th century advances in astronomy have been so rapid that the second half of the century can be considered a golden age. Traditional optical astronomy has been revolutionized by the development of new techniques of faint object detection, including more sensitive photographic emulsions and a plethora of electronic imaging devices. Using standard telescopes, the optical astronomer can now see fainter and more distant objects than ever before. In addition the astronomer is no longer limited to observing the visible light from celestial bodies. New instruments now allow the study of the heavens in entirely new regions of the electromagnetic spectrum.
Radio Astronomy
In 1931, Karl G. Jansky of the Bell Telephone Laboratories discovered extraterrestrial radiation at radio wavelengths and launched the field of radio astronomy. During the 1930s, Grote Reber, an American radio engineer, further investigated celestial radio radiation and single-handedly brought radio astronomy to the attention of professional astronomers.
As a result of theoretical investigations by astronomers in the Netherlands during World War II, an observable radio line, emitted by neutral hydrogen atoms in space, was predicted at a wavelength of 21 cm. Detection of this line caused radio astronomy to advance rapidly after the war. Today, radio telescopes and radio interferometer systems worldwide study radio emission from the stars, the planets, the interstellar medium in the Galaxy, and extragalactic sources.
Achievements in radio astronomy include the mapping of galactic structure and the discovery of quasars, pulsars, and a large number of complex organic molecules in interstellar space (see interstellar matter). radar astronomy has also been used within the solar system to determine, for example, the rotational periods of Venus and Mercury.
Infrared Astronomy
Although scientists have known since the time of William Herschel in the late 18th century that infrared radiation from celestial objects can be detected, it was not until the late 1950s and early 1960s that infrared astronomy became the subject of intensive research. Sensitive detectors were developed that allowed astronomers to explore the infrared region of the spectrum. Infrared astronomy has been helpful in studying the very young or evolved stars that are commonly associated with dense clouds of dust observed in interstellar space.
Ultraviolet, X-Ray, and Gamma-Ray Astronomy
In 1957 the USSR launched the first satellite, thus beginning the space age. Few other disciplines have benefited from artificial satellites to the extent that astronomy has. (See space exploration.) For the astronomer, the atmosphere presents a murky or opaque barrier through which observations of the far infrared, ultraviolet, X-ray, and gamma-ray spectral regions are difficult or impossible. Satellites and, to a limited extent, high-altitude balloons and rockets have become platforms from which to observe these spectral regions. Since 1962 the United States and other nations have launched a wide range of orbiting observatories that are devoted to observing the ultraviolet, infrared, and X-ray regions (see OAO; OSO; High Energy Astronomical Observatory; Uhuru). These studies have resulted in better understanding of very hot stars and have produced evidence of the existence of black holes. The impact of extraterrestrial astronomy in all parts of the spectrum is being extended in the 1990s by a continuing program of space astronomy supported by the Space Shuttle (see gamma-ray astronomy; Space Telescope; ultraviolet astronomy; X-ray astronomy).