The Astronomical Universe

   The present conception of the universe is that of a vast complex system whose boundaries are unknown. To grasp the immensity of the universe, one must appreciate the magnitude of the units used to measure it. The most common unit is the light-year, which is the distance that light, which travels at about 186,300 miles (300,000 km) per second, travels in one year; one light-year is equal to 5.88 trillion miles (9.46 trillion km). Another common unit of astronomical measurement is the parsec, which is equal to 19.2 trillion miles (30.8 trillion km), or 3.26 light-years. The nearest star to the solar system, Proxima Centauri, is about 4.3 light-years (25 trillion miles; 41 trillion km) away. The most distant object yet observed is a quasar that has been estimated to be about 18 billion light-years (106,000 billion billion miles; 170,000 billion billion km) from the earth. However, this is only the distance that the largest optical telescopes allow man to see.

   Like the size, the age and method of formation of the universe are unknown. The age is estimated to be between 10 billion and 20 billion years. Two models of the universe vied for acceptance in the period after World War II. One, put forth by Georges Lemaître in 1931, deals with an evolutionary universe that originated in a primordial explosion called the big bang. The other, introduced by Hermann Bondi and Thomas Gold in 1948, postulates a steady-state universe in which matter is constantly being created. Observational evidence obtained since the late 1960's has provided strong support for the evolutionary, or big bang, model. (See also Cosmology, Astronomical)

   Astronomical studies of the universe can be divided into four broad categories. These are the solar system, which deals with the earth and its nearest neighbors in space, the sun, moon, and planets; stars; interstellar matter; and galaxies. (See also Interstellar Matter; Galaxies)

The Solar System

   The solar system comprises the sun and the numerous smaller bodies that revolve around it. These include the 9 principal planets and their natural satellites, or moons, of which about 50 are known; the thousands of minor planets, or asteroids; the comets; and the meteor swarms. The principal planets in order of increasing distance from the sun are Mercury, Venus, the earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. The first two are called inferior planets because they revolve inside the earth's orbit, and the last six are called superior planets because they revolve outside it. The inferior planets and the first three superior planets were called wandering stars by the ancients since they were seen to move against the background of fixed stars. (See also Solar System)

   Motions of the planets reveal to the expert in celestial mechanics the physical interrelationships in the solar system, and offer subtle evidence bearing upon basic concepts such as the theory of relativity. Skill in celestial mechanics is a key to success in earth-satellite and space-rocket projects, where the slightest miscalculations can result in expensive, discouraging failure. Astrophysical studies of the planets yield data on their dimensions, composition of their atmospheres, surface features (especially as to Mars), possible presence of life, and future use of planets and their satellites as space stations. (See also Celestial Mechanics; Orbit)

   Specialists in solar astronomy investigate physical processes occurring in and near the sun, including thermonuclear reactions and other high-temperature phenomena. Sunspots and solar radiation, with their influences on the earth's weather, climate, and inhabitants, are also studied. By intensive study of the sun, the astrophysicist learns much about points of light in the largest telescopes. (See also International Years of the Quiet Sun (IQSY); Sun)

   Lunar astronomy was revolutionized in the space age. Photographs of the moon's far side, which had never before been viewed, were returned to the earth by the Soviet lunar probe Lunik 3 in 1959, and close-up pictures with a resolution far superior to that possible in photographs taken from the earth were returned by U.S. probes of the Ranger series in 1964 and 1965. Experiments performed by the unmanned Surveyor 5, 6, and 7 spacecraft, which soft-landed on the moon in 1967 and 1968, first made possible an estimate of the moon's chemical composition. Lunar rock and dust samples brought back to earth by the U.S. manned Apollo landing missions (1969-1972) and the Soviet robotic Luna landing missions (1970-1976) made possible determinations of the moon's age, detailed chemistry, and physical characteristics. The Apollo and Luna moon rocks are still being studied by geologists and mineralogists. The moon, with its craters, maria ("seas"), and mountain chains, is a little-changed relic of the early days in the solar system, and clues obtained as to its origin may well be clues to the origin of the solar system itself. (See also Moon)

   Meteors and meteorites, too, yield clues to the origin and history of the solar system, because meteorites, uranium-dated at about 4.5 billion years, may have changed little since the system was born. Meteors and their occurrence are also of obvious interest in the study of probable hazards to space travel. Comets are related to meteors. They, too, are remnants of the early days of the solar system and are studied for clues to the past. (See also Comet; Meteor; Meteorite)

Stars

   Along with the earth, moon, and sun, the stars are the most familiar celestial objects. The ancients thought of the stars as fixed in place, since they did not appear to move in relation to one another, and mentally arranged them into groups to form outlines of mythological animals and men, dippers, crosses, and other patterns. They also believed that all the stars were at the same distance from the earth and imbedded on the inner surface of a sphere that rotated around it. (See also Celestial Sphere; Constellation)

   Modern astronomy recognizes stars as being fiery gaseous bodies like the sun, which is really a star of less than average size and temperature. Like the sun, the stars shine all the time, but they cannot be seen during the day because their light is overpowered by that from the sun. Stars vary in their distances from the earth, in their relative motions, and in color, volume, shape, mass, temperature, luminosity, density, and chemical composition. Because they are at such great distances from the earth, stars provide a valuable reference background for study of the motions of members of the solar system, and thus become a means by which to tell time. In studies of the makeup of our galaxy, distances of stars are measured. Distances of nearer stars are measured by triangulation; distances of more remote stars can be inferred from the ratio of intrinsic brightness (as indicated by the type of the star's spectrum) to apparent brightness, since the latter decreases according to the distance traveled by the starlight. Many studies center on stars (totalling perhaps half of all stars), called binary systems, that have been found to consist of two or more components. Doppler shifts in stellar spectra are observed for clues as to stellar motions (usually of the order of miles per second) and thus to past and present events in the galaxy. The movements of star clusters and of their individual members are plotted. Stars are spectroscopically analyzed and classified as to their chemical composition and probable stage of evolution. Evidence that the companion of Barnard's star, named after its discoverer, Edward Emerson Barnard, may be a planet rather than a second star points up the possible existence of numerous other "solar systems," some of which could perhaps be inhabited by intelligent life. The vicinities of certain nearby stars have been monitored by radio telescopes, thus far unsuccessfully, for possible signals from intelligent beings who may inhabit planets associated with these stars. (See also Interstellar Communication)

   Like our own sun, all stars are centers of stupendous physical happenings impossible as yet to reproduce on the earth. They represent billions of individual case histories, and each appears as it existed at some moment in the past -- as many years ago as it is light-years distant from the earth. Astronomy, from the viewpoint of an instant in time, beholds a panorama of cosmic change spanning millions and even billions of years. (See also Cosmology, Astronomical; Star)

Interstellar Matter

   Shrouding the stars and scattered through interstellar space are cosmic gas and dust. This material is here and there concentrated into the so-called diffuse nebulas, in some of which -- as astronomers believe of the glowing region in Orion's sword -- stars are now forming. These gigantic clouds, some dark, some reflecting starlight, and others luminous like giant flourescent bulbs, partly hiding the star-studded spiral arms of our Milky Way galaxy, are frequent targets for the radio telescope and the time-exposure camera. Radio noise from the interstellar clouds and from objects behind them is a clue to events never suspected by the observer at the optical telescope. It is the radio telescope that has made possible, in recent years, the mapping of the major features of the Milky Way galaxy. (See also Interstellar Matter; Milky Way; Nebula; Nova)

Galaxies

   Beyond the Milky Way, at distances ranging out to 6 billion light-years and beyond, are the uncountable billions of other galaxies. Individual stars of certain nearer galaxies, those that are not more than a few million light-years distant, can be distinguished through large telescopes. The astrophysicist analyzes the starlight and invisible radiation from these other systems, finding them remarkably similar to our own, though they occur in a vast range of sizes, shapes, and apparent stages of development. The redshift in their spectra indicates that they are racing away from the solar system at greater and greater speeds as their distances from it increase. (See also Galaxies)

   The discovery of exploding galaxies has indicated that magnetic fields exist in interstellar and even intergalactic space and that high-energy particles move through these fields. The implications of these discoveries in the studies of cosmic radiation and galactic evolution are of extreme importance. (See also Cosmic Rays)

   The use of radio telescopes has done more than extend the boundaries of the known universe. They have yielded valuable information concerning the tenuous hydrogen gas found in interstellar and intergalactic space and the spatial distribution of galaxies. Perhaps of even greater importance was the use of the radio telescope to identify the mysterious quasars. Quasars, a contraction for quasi-stellar radio sources, are far too bright to be stars and far too small to be galaxies. Their enormous energy output cannot be explained by any known mechanism. It is thought by many that the explanation of the quasar's energy mechanism may also be the key to the mechanism of the formation of the universe. (See also Quasar)