Technology revolution

Telescopes, photography, electronics, and computers

The study of the sky continued with the development of larger and larger telescopes. Generally these were refractive instruments wherein the light passes through the lenses, as in a pair of binoculars. The glass refracts the different colors of light slightly differently (chromatic aberration) so that perfect focusing is difficult to attain. This led to reflecting telescopes that make use of curved mirrors. In this case, all wavelengths impinging at a position of the mirror from a given angle are

1 We use italic boldface characters to signify vector quantities and the hat symbol to indicate unit vectors.

reflected in the same direction. The 5-m diameter mirror of the large telescope on Palomar Mountain in California (long known as the "200 inch") was completed in 1949. It was the world's largest telescope for many years. In the 1960s and 1970s, several 4-m diameter telescopes were built as was a 6-m instrument in the Soviet Union. At this writing, the two Keck 10-m telescopes in Hawaii are the largest, but other large telescopes are not far behind. New technologies which allow telescopes to compensate for the blurring of starlight by the earth's atmosphere are now coming on line.

Photography was an epochal development for astronomy in the nineteenth century. Before this, the faintest object detectable was limited by the number of photons (the quanta of light) that could be collected in the integration time of the eye, ~30 ms (millisecond) to ~250 ms if dark adapted. If a piece of film is placed at the focus of a telescope, the photons can be collected for periods up to and exceeding 1 hour. This allowed the detection of objects many orders of magnitude fainter than could be seen by eye. A photograph could record not only an image of the sky, but also the spectrum of a celestial object. The latter shows the distribution of energy as a function of wavelength or frequency. The light from the object is dispersed into its constituent colors with a prism or grating before being imaged onto the film. Large telescopes together with photography and spectroscopy greatly enlarge the domains of quantitative measurements available to astronomers.

Since the mid-twentieth century, more sensitive electronic detection devices have come into use. Examples are the photomultiplier tube, the image intensifier, and more recently, the charge-coupled detector (CCD). Computers have come into wide use for the control of the telescope pointing and for analysis of the data during and after the observation. The greatly increased efficiencies of data collection and of analysis capability go hand in hand in increasing the effectiveness of the astronomer and his or her ability to study fainter and more distant objects, to obtain spectra of many objects simultaneously, or to measure bright sources with extremely high time resolution. In the latter case, changes of x-ray intensity on sub-millisecond time scales probe the swirling of ionized matter around neutron stars and black holes.

Non-optical astronomy

Electromagnetic radiation at radio frequencies was discovered by Heinrich Hertz in 1888. This eventually led to the discovery of radio emission from the sky by Carl Jansky in 1931. This opened up the field of radio astronomy, an entirely new domain of astronomy that has turned out to be as rich as conventional optical astronomy. Entirely new phenomena have been discovered and studied. Examples are the distant quasars (described below) and the neutral hydrogen gas that permeates interstellar space. The invention of the maser and the use of supercooled detectors have greatly increased the sensitivity and frequency resolution of radio telescopes. Multiple radio telescopes spread over large distances (1 km to 5 000 km or more) are now used in concert to mimic a single large telescope with angular resolutions down to better than 0.001'' (arcseconds).

The Very Large Array (VLA) of 27 large radio telescopes extending over about 40 km of New Mexico desert operates on this principle. With its large area it has excellent sensitivity. It has produced many beautiful images of radio objects in the sky with angular resolution comparable to that of ground-based optical astronomy en

The atmosphere of the earth is a great impediment to many kinds of astronomy. Photons over large bands of frequencies can not penetrate it. The advent of space vehicles from which observations could be made opened up the field of x-ray astronomy. Like radio astronomy, this field led to the discovery of a variety of new phenomena, such as neutron stars in orbit with ordinary nuclear-burning stars, high-temperature shock waves in supernova remnants, black holes (described below), and high-energy phenomena in distant quasars.

Space vehicles have also made possible the study of the ultraviolet radiation from nearby stars and distant galaxies and gamma-ray emission from pulsars and from the nuclei of active galaxies. Infrared astronomy can be carried out at only a few frequencies from the ground, but in space a wide band of frequencies are accessible. Infrared astronomers can peer into dust and gas clouds to detect newly formed stars. Space vehicles also carry optical/ultraviolet telescopes above the atmosphere to provide very high angular resolutions, <0.05'' compared to the ~1'' normally attained below the atmosphere. This is a major feature of the Hubble Space Telescope.

The space program also has provided a platform for in situ observations of the planets and their satellites (moons); the spacecraft carries instruments to the near vicinity of the planet. These missions carry out a diversity of studies in a number of wavebands (radio through the ultraviolet) as well as magnetic, cosmic-ray, and plasma studies. The Voyager missions visited Jupiter, Saturn, Uranus and Neptune. One of them will soon leave the solar-system heliosphere and thus be able to carry out direct measurements of the interstellar medium.

A given celestial object can often be studied in several of the frequency domains from the radio to gamma rays. Each provides complementary information about the object. For example, the x rays provide information about very hot regions (~10 million kelvin) while infrared radiation reflects temperatures of a few thousand degrees or less. The use of all this information together is a powerful way to determine the underlying nature of a class of celestial objects. This type of research has come to be known as multi-frequency astronomy. Sky maps at various frequencies (cover illustrations) illustrate the variation of the character of the sky with frequency.

Signals other than the electromagnetic waves also provide information about the cosmos. Direct studies of cosmic rays (energetic protons, helium nuclei, etc.) circulating in the vast spaces between the stars are carried out at sea level and also from space. These high-energy particles were probably accelerated to such energies, at least in part, by the shock waves of supernova explosions.

Neutrinos, neutral quanta that interact very weakly with other matter, have been detected from the nuclear reactions in the center of the sun and from the spectacular implosion of a star in the Large Magellanic Cloud, an easily visible stellar system in the southern sky. The outburst is known as supernova 1987A. Neutrino detectors are placed underground to minimize background.

The detection of gravitational waves predicted by the theory of general relativity is still a challenge. Observatories to search for them with high sensitivity are now beginning operations, such as the US Laser Interferometer Gravitational-wave Observatory (LIGO) with interferometer "antennas" in Washington State and Louisiana or the German-UK GEO-600. A likely candidate source of gravitational waves is a binary system of two neutron stars in the last stages of spiraling into each other to form a black hole.

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