Astronomy.

ROLCAM · Posted: Sun Oct 14, 2007 1:11 am Post subject: Astronomy.

Astronomy is the study of the stars, planets, and other objects that make up the universe. Astronomers observe the locations and motions of heavenly bodies. However, almost all astronomers are interested in more than just observing these objects. They also seek answers to such questions as "What are stars made of?" and "How do they create their light?" For this reason, most astronomers are also astrophysicists--that is, they study physical and chemical processes that occur in the universe.

Some astronomers, called observational astronomers, specialize in observing astronomical bodies through telescopes. Others are theoretical astronomers, who use the principles of physics and mathematics to determine the nature of the universe. Astronomers work in many specialized areas. For example, stellar astronomers study the stars. Solar astronomers study the sun--the star nearest the earth. Planetary astronomers study conditions on the planets. Cosmologists study the structure and history of the universe as a whole.

Unlike most other sciences, astronomy is a field in which amateurs can make significant contributions. For example, amateur astronomers play an important role in the study of variable stars. Such stars vary in brightness over time. The study of these stars provides information about distances in the universe. But there are too many variable stars for professionals to keep track of them. Members of an amateur group, the American Association of Variable Star Observers, make many of the observations of these stars. Some other amateur groups work together in search of stars that brighten suddenly. Such a star is called a nova or supernova. Amateur astronomers also observe and photograph the moon, planets, and galaxies as well as eclipses and other astronomical events.

Astronomy is one of the oldest sciences. It began in ancient times with the observation that the heavenly bodies go through regular cycles of motion. Throughout history, the study of these cycles has served such practical purposes as keeping time, marking the arrival of the seasons, and navigating accurately at sea.

By 450 B.C., the Babylonians had charted the positions of the heavenly bodies to predict events on the earth. The making of such predictions is called astrology. Astrology is based on the belief that the positions of stars and planets influence what happens on the earth. The ancient Egyptians, Greeks, and Romans also practiced astrology, and many early astronomers believed in it. By the 1700's, however, most scientists had come to reject astrology. Scientists today consider it a pseudoscience (false science). They explain events on the earth and in space by the laws of physics and chemistry, which provide no basis for a belief in astrology. In addition, many scientists do not simply ignore astrology but actively oppose it as superstition that slows the advance of science.

This article describes what can be seen in the sky and discusses the types of objects that make up the universe. It also provides information on tools and techniques used by astronomers, the history of astronomy, and careers in astronomy. World Book has separate articles on such topics as MOON, PLANET, STAR, and SUN.

Observing the sky

The daytime sky. The sun is an interesting object in the daytime sky. A variety of storms and other activities can be seen on the surface of the sun from day to day. However, the sun is too bright to observe safely without using special equipment. Sunlight also makes the sky too bright to observe other stars and planets during the day. The moon, however, is sometimes visible in daylight. As sunlight passes through the earth's atmosphere, it strikes molecules of the gases that make up the atmosphere. The molecules scatter the sunlight, which is a mixture of all colors, in every direction. The sky appears blue because blue light is scattered much more than any other color.

The nighttime sky. The moon is the brightest and most easily seen object in the sky at night. As a result, the most familiar astronomical observation is of the moon's phases, such as the full moon, half moon, and crescent. The phases of the moon occur because the amount of the moon's sunlit area that can be seen from the earth changes as the moon orbits the earth. The moon goes through a complete set of phases about once a month.

On some nights, the moon shines so brightly that few stars or planets can be seen. But on a dark, moonless night, many stars and planets become visible. The planets usually appear first. Not until the sky is truly dark do the stars come out. Planets and stars look much alike in the night sky. However, the planets shift position nightly in relation to the stars. In addition, planets shine steadily, and stars appear to twinkle. Twinkling occurs because moving layers of air in the earth's atmosphere bend starlight, making the images of the stars appear to vary in brightness and dance around slightly.

Five planets--Venus, Jupiter, Mars, Saturn, and Mercury--can be seen easily without a telescope. Venus is usually the brightest planet, and Jupiter the next brightest. Mars stands out because of its reddish tinge. Although Saturn can be seen with the naked eye, a small telescope is needed to view its beautiful rings. Mercury is often too close to the sun to be visible. But at times, Mercury appears low in the western sky just after sunset or low in the eastern sky just before sunrise.

About 6,000 stars shine bright enough to be seen without a telescope. Sirius is the brightest star. Other bright stars include Canopus, Arcturus, and Vega. In ancient times, astronomers divided the stars into classes of brightness called magnitudes. They classified the brightest stars as first magnitude, slightly fainter stars as second magnitude, and so on. The faintest stars visible to the naked eye were classified as sixth magnitude. Astronomers today use a modified version of this system.

Every few years, a bright comet may be observed with the unaided eye. A comet is a ball of ice and dust that follows a regular orbit around the sun. When nearing the sun, a comet may brighten enough to be seen from the earth. A few comets develop a tail that extends across one-sixth or more of the sky. But most comets can be seen only with a telescope. Bright comets stay visible to the naked eye for only a few days or weeks.

Streaks of light called meteors are much more common in the night sky than comets. Meteors occur when a particle or chunk of stony or metallic matter called a meteoroid enter's the earth's atmosphere. Air friction heats the meteoroid so that it glows and becomes visible as a meteor. People sometimes call meteors falling stars or shooting stars. On any clear night, a person can see a few meteors an hour. Showers of meteors take place regularly at certain times of the year. Some of these showers result from the passage of the earth through the orbits of comets that have broken up.

The sky at different latitudes. People who observe the night sky at different latitudes have different views of it. A person at the North Pole can never see the stars in the southern half of the sky. Similarly, a person at the South Pole can never see the stars in the northern half. At the equator, a person can see all the stars in the sky during the course of the year.

People anywhere on the earth can see a band of light across the night sky that is called the Milky Way. The Milky Way is the collection of stars, gas, and dust that makes up the galaxy in which our sun is located. One nearby galaxy, in the constellation Andromeda, is faintly visible in the sky of the Northern Hemisphere. Observers in the Southern Hemisphere can see two other galaxies. These galaxies are called the Magellanic Clouds.

Why the stars seem to move. The positions of the stars in the sky change slightly over many years. However, the stars seem to sweep across the sky each night. Their seeming movement is due to the earth's rotation. As the earth rotates on its axis, we on the earth are always moving from west to east. But because we do not sense this motion, it appears to us that the stars revolve overhead from east to west. Only the North Star does not seem to move because it is almost directly above the North Pole. The North Star has served as a guide for navigators since ancient times.

The sky's appearance also changes nightly because of the earth's annual revolution around the sun. The sun always blocks part of the sky--that is, some stars cannot be seen because they are in the sky during daytime. But as the earth moves around the sun, the part of the sky visible at nightchanges gradually. The earth completes a revolution around the sun in about 365 days. The stars thus rise and set 1/365 of 24 hours, or about 4 minutes, earlier each night. As the year goes on, the stars in the night sky set earlier and earlier until they are lost in the twilight. Other stars rise earlier and earlier and so become part of the night sky.

Constellations are groups of stars within a particular region of the sky. When astronomers in ancient Egypt, Greece, and other lands began to study the sky, they divided it into regions that had fairly distinct groups of stars. They named these constellations after the figures the stars seemed to form and associated the figures with stories about heroes, heroines, and beasts. Most of the constellations to which we refer today are the groupings devised by the ancient Greeks.

Some star groups, such as the Big Dipper and Little Dipper, do not make up complete constellations. Such groups are called asterisms. The Big Dipper is part of the constellation Ursa Major (Big Bear), and the Little Dipper is part of Ursa Minor (Little Bear).

The stars in a constellation do not necessarily have any relationship with one another. Some stars in a constellation may be relatively near the earth, and others relatively far away. For mapping purposes, however, astronomers divide the sky into 88 constellations. Each star is associated with only one constellation.

An astronomer's view of the universe

Early astronomers thought that the earth was the center of the universe and that everything revolved around it. However, the earth is only one of nine planets that orbit the sun. The sun itself is only an average-sized star--one of the billions of stars in the Milky Way. The Milky Way, in turn, is only one of countless galaxies.

The solar system consists of one star--our sun--and all the objects that orbit it. These objects include (1) the nine planets and their moons; (2) thousands of smaller bodies called asteroids; (3) meteoroids; (4) thousands of comets and bits of rock and ice that may become comets; and (5) particles of dust and gas. Other stars have similar objects traveling around them.

Going outward from the sun, the order of the planetary orbits is Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Although Pluto's orbit is larger than Neptune's, it is flattened. Thus, every 248 years, Neptune becomes the most distant planet. Pluto moves inside the orbit of Neptune and remains there for about 20 years. Pluto entered Neptune's orbit on Jan. 23, 1979, and will stay there until March 15, 1999.

The four planets closest to the sun--Mercury, Venus, Earth, and Mars--are rocky and relatively small. Jupiter, Saturn, Uranus, and Neptune are gaseous and relatively large. These giant planets are surrounded by rings, though only Saturn's are visible from Earth with a small telescope. Pluto is comparatively small and may once have been a moon of Neptune. All the planets except Mercury and Venus have one or more moons. The larger moons have mountains, craters, volcanoes, trenches, and other interesting features.

Astronomers measure distances within the solar system in astronomical units (AU). One astronomical unit is the average distance between the earth and the sun, which is about 93 million miles (150 million kilometers). The distance between Jupiter and the sun averages about 5 AU. Pluto's average distance from the sun is about 39 AU.

Stars are glowing balls of gas that exist throughout space. Except for the sun, all stars are too far from the earth for their distances to be conveniently measured in miles or kilometers. Astronomers often measure distances to and between stars in light-years. A light-year is the distance that light travels in a year--about 5.88 trillion miles (9.46 trillion kilometers). The star nearest the sun, Proxima Centauri, is 4.3 light-years from the sun.

Stars vary in temperature, color, brightness, size, and mass. Mass is the quantity of matter in a star. An average-sized star's temperature and color depend on its mass. The hottest stars have the most mass and appear blue. The stars with the least mass are red and much cooler than other stars. In most cases, the more mass a star has, the brighter it is.

Our sun and other average-sized stars are called main-sequence stars or dwarf stars. Some stars are much brighter than main-sequence stars of the same mass. These stars are far larger and so are called giants and supergiants. Stars that are smaller and dimmer than main-sequence stars of the same mass are called white dwarfs. Main-sequence stars, giants, supergiants, and white dwarfs represent stages in the lifetime of stars.

New stars are always forming in space. A new star begins to form when a cloud of gas and dust contracts into a ball. Most of the gas is hydrogen. The gas in the core grows hotter and hotter until hydrogen atoms collide with such force that they fuse (combine) and form helium. This process, called nuclear fusion, produces a huge amount of energy, and a main-sequence star is born. The star then remains stable, and nuclear fusion continues in its core for millions or billions of years.

After the star has used up the hydrogen in its core, its outer layers swell. The star then becomes brighter and is a giant. The giant's mass determines how the star will eventually die. A giant with about the same mass as the sun throws off its outer layers. The core cools, and the star becomes a white dwarf. Some white dwarfs revolve around larger stars. If small amounts of matter from the larger star fall on the white dwarf, nuclear fusion can occur. The white dwarf then brightens temporarily, becoming a nova. When so much matter from the larger star accumulates on the white dwarf that it collapses and burns, a supernova results.

A giant with more than three times the mass of the sun swells even further and becomes a supergiant. A supergiant ends as an exploding star, which is another type of supernova. If less than three times the mass of the sun remains after the explosion, the remnant becomes a neutron star. A neutron star is a small, dense star that is composed primarily of tightly packed neutrons or perhaps of elementary particles called quarks. Some neutron stars, called pulsars, send out beams of radio waves. A pulsar also rotates such that waves from it sweep over the earth periodically. Astronomers detect these regular pulses of radio waves as the waves sweep by the earth. If more than three times the mass of the sun remains after a supernova explosion, the remnant collapses and may form an invisible object called a black hole. A black hole has such powerful gravity that not even light can escape from it.

Galaxies and quasars. The solar system is only a minor member of the giant grouping of stars, dust, and gas that makes up our galaxy, the Milky Way. The Milky Way is flat like a compact disc, but it bulges at the center. Arms curve out from the center in a spiral pattern. The sun is in one of the arms, about 25,000 light-years from the center of the Galaxy.

There are innumerable other galaxies in the universe. Many of them are spiral galaxies like the Milky Way. Even more galaxies have an elliptical (oval) shape and no spiral arms. The remaining galaxies have an irregular shape. The Milky Way is part of a group of galaxies called the Local Group. Scientists have discovered about 30 galaxies in the Local Group, including 3 spiral galaxies. The Local Group, in turn, is part of a larger grouping called the Virgo Cluster. Most galaxies, if not all, are found in such clusters. On a larger scale, galaxies are arranged in long, chainlike structures.

The most distant objects that can be detected from the earth are quasars. Quasars give off enormous amounts of radiation. Measurements show that some quasars are as far as 12 billion to 16 billion light-years away. Research suggests that a giant black hole with a mass thousands of times that of the sun may be in the core of each quasar. According to this theory, the radiation from a quasar is the energy that is released when matter falls into the black hole.

The universe consists of all space and all the matter and energy that space contains. Astronomers do not know how large the universe is. It may even extend to infinity.

Nearly all astronomers believe that the universe began between 10 billion and 20 billion years ago with an explosion called the big bang. According to the big bang theory, the universe has been expanding since the instant it began. At first, the universe consisted chiefly of radiation. But as the universe continued to expand, most of this radiation changed into matter. The remainder can be detected today as faint radio waves coming from all parts of the universe. Astronomers call this radiation the primordial background radiation.

Today, all of the clusters of galaxies in the universe continue to rush away from one another. Whether the universe will continue to expand forever or whether it will eventually contract is the subject of much astronomical research.

Astronomers at work

Locating objects in the sky requires a system like the one geographers use to locate places on the earth. In the geographer's system, circles of latitude are measured parallel to the equator, and lines of longitude run from the North Pole to the South Pole. The astronomer's system corresponds to the geographer's, but astronomers use the terms declination for latitude and right ascension for longitude. Declination is measured in degrees north or south of the celestial equator, an extension of the earth's equator into space. Circles of right ascension cross the celestial equator and pass through the celestial poles, which are directly over the earth's poles. Right ascension is measured in hours east of the point where the sun crosses the celestial equator about March 21. One hour of right ascension corresponds to 15 degrees of longitude.

The stars are essentially fixed in their positions in the sky. But the sun goes through the whole range of right ascension each year. The sun's path through the sky with respect to other stars is called the ecliptic.

Observing with telescopes. Astronomers use telescopes to observe the radiation that reaches the earth from objects in space. This radiation consists of related patterns of electric and magnetic force that move rapidly through space. These patterns, called electromagnetic waves, differ widely in wavelength. Wavelength is the distance between the crest of one wave and the crest of the next. The chief types of radiation, in order of increasing wavelength, are gamma rays, X rays, ultraviolet light, visible light, infrared rays, and radio waves. Astronomers use various types of telescopes to observe the different wavelengths or radiation.

Optical telescopes are used to observe visible light. There are two main types--reflecting and refracting. A reflecting telescope uses a mirror to form the image. A refracting telescope uses a lens to do so. Reflecting telescopes can be made much larger than refracting telescopes and thus can detect fainter objects. Most major telescopes built today are the reflecting type.

Astronomers use optical telescopes to form magnified images of the sun, the planets, and other relatively nearby objects. But stars are so far away they appear as mere points of light no matter how greatly they are magnified. For observations of stars and other distant objects, optical telescopes are used to collect enough light to make the objects detectable. The fainter the object, the larger and stronger the telescope must be.

Astronomers often use a photographic plate or other equipment to record the images formed by an optical telescope. A photograph made in this way provides a permanent record of the appearance of a certain region of the sky at a particular moment. In addition, photographs of the sky reveal many details that cannot be seen with the eye, even through a telescope. An image that appears dim to the eye remains dim no matter how long a person looks at it. But if film is exposed to a dim image for a long time, a bright picture results. To measure the intensity of a star's light, astronomers often use a telescope equipped with an electronic device called a photomultiplier (see PHOTOMULTIPLIER TUBE). Other electronic devices have replaced film for making images. Astronomers usually use a charge-coupled device (CCD). A CCD is much more sensitive to light than film is.

Radio telescopes collect and focus radio waves. Most radio telescopes have a bowl-shaped metal reflector called a dish. The dish concentrates the weak radio signals from space and focuses them onto an antenna. The antenna converts the radio waves into electric signals. A radio receiver strengthens these signals, which are then recorded by a computer. In many cases, radio telescopes are built in groups, called arrays, that work together.

Other telescopes. Astronomers use instruments similar to optical telescopes to study ultraviolet light and some wavelengths of infrared light. Water vapor in the earth's atmosphere interferes with ground-level observations of infrared light. For this reason, astronomers sometimes send balloons and aircraft carrying infrared telescopes high into the earth's atmosphere. Infrared studies enable astronomers to observe the birth of stars and to study the dust between stars.

The development of artificial satellites has enabled astronomers to obtain information from telescopes in space. Space telescopes are used to study many types of radiation. But astronomers find them to be especially vital in the study of gamma rays, X rays, and ultraviolet rays that are blocked by the earth's atmosphere.

To study gamma rays and X rays, astronomers often use instruments that count the number of photons (particles) of radiation. Astronomers can make X-ray images by using a technique that resembles skipping a stone along the surface of a lake. They bounce X rays off a telescope at a very small angle called grazing incidence. Astronomers use grazing incidence telescopes to study X rays from the sun and other celestial objects.

Astronomers have also developed techniques to detect particles from space. For example, they use 100,000-gallon (400,000-liter) tanks of cleaning fluid to capture subatomic particles called neutrinos formed deep inside the sun. Another experiment uses 66 short tons (60 metric tons) of gallium metal in order to capture neutrinos.

Using spectroscopy. Astronomers often analyze the light collected by a telescope to determine the chemical composition of stars and other objects. The most commonly used technique for analyzing visible light is spectroscopy, which involves breaking up light into its individual colors. This range of colors, called a spectrum, begins with violet and blue at one end and continues through green, yellow, orange, and red at the other. Spectroscopic techniques are also used to break up other types of radiation into individual wavelengths.

All atoms give off light when they are heated to high temperatures. The atoms of any particular element give off especially large quantities of radiation at certain wavelengths. As a result, the spectrum of that element has bright lines, called spectral lines, at those wavelengths. Each element has a pattern of spectral lines that differs from that of any other element. Spectral lines of another type are produced when light passes through an element that is in gaseous form and at a lower temperature. Under these conditions, the atoms absorb the same wavelengths of radiation that they give off when they are heated. Thus, instead of bright lines, gaps that look like dark lines appear at the same positions in the spectrum.

By analyzing the spectrum of a star, astronomers can identify the types of atoms that make up the star's gaseous outer layers. Spectral analysis also enables astronomers to identify the molecules in the atmosphere of a planet. In addition, the spectrum of a star or planet reveals the relative abundance of the various atoms and molecules that are present. If one element or compound is more abundant than the others, its characteristic spectral lines will appear especially strong.

Astronomers use instruments called spectrographs to study spectra. A spectrograph that produces a spectrum to be viewed with the eye is called a spectroscope. Other spectrographs record the image of a spectrum on a photographic plate or some other device.

Measuring distances in space. Astronomers have devised three main methods to measure distance in space. They are (1) parallax measurement; (2) brightness measurement; and (3) red shift measurement.

Parallax measurement is used to determine the distances to about 100,000 of the nearest stars. When viewed from two widely separated points, a nearby star appears to shift its position slightly against the background of more distant stars. The star's parallax is the angle it would appear to move across the sky if viewed from two points that are 1 astronomical unit (AU) apart. Astronomers determine a star's parallax by observing the star at intervals of several months, during which the earth has moved between widely separated points in its orbit around the sun. On the basis of these observations, astronomers can calculate the star's parallax and then use trigonometry to determine the distance to the star.

Astronomers use a unit of distance that relates directly to parallax. This unit is called the parsec. One parsec is the distance to a star that has a parallax of one second of arc ( 1/3,600 degree). One parsec equals 3.26 light-years, or 19.2 trillion miles (30.9 trillion kilometers). Parallax can be used to measure distance up to about 300 parsecs, which is less than 5 percent of the distance to the center of the Milky Way.

Brightness measurement. Astronomers can determine the distance to certain stars by comparing their luminosity (actual brightness) with their apparent brightness as observed with a telescope. This type of measurement is based on the fact that the greater the distance to a star of a given luminosity, the fainter the star appears from the earth.

Astronomers commonly use brightness measurement to calculate distances to some kinds of variable stars. Each of these stars goes through a cycle of variation in brightness during a specific period of time. Astronomers have discovered that the length of this period indicates the luminosity of the star. For example, studies of a type of variable star called Cepheid variables revealed that Cepheids with longer periods have greater luminosities than those with shorter periods. Thus, a simple measurement of the period gives the luminosity, which can then be used to calculate the distance to the star. Observations of Cepheids in the Magellanic Clouds showed that those glowing regions are not within the Milky Way but so distant that they are separate galaxies. Cepheid variables provide the principal means of determining distances to the nearest galaxies.

Astronomers look for objects of known brightness even beyond the region where they can detect individual stars. For example, the brightest galaxy in a cluster of galaxies has about the same luminosity as the brightest galaxy in any other cluster. Comparing this luminosity with the apparent brightness is one of the best methods for measuring the distances to clusters of galaxies.

Red shift measurement involves the study of spectral lines in the light received from an object in space. In the spectrum of any moving object, the spectral lines are shifted from where they would appear in the spectrum of a stationary object. If they shift toward the red end of the spectrum, the object is moving away from the earth. If they shift toward the blue end, the object is approaching. The greater the shift, the faster the object is moving.

All galaxies except those nearest the earth have large red shifts. In 1929, the American astronomer Edwin Hubble discovered that the farther away a galaxy is, the faster it is receding, and thus the greater is its red shift. Hubble's law states that the speed at which a galaxy is receding is directly proportional to its distance from the earth. Thus, astronomers can determine the distance of a remote galaxy simply by measuring its red shift. Hubble's law is the only way astronomers have to measure distances to the farthest objects in the universe. Astronomers think that quasars are the most distant objects because they have the largest red shifts.

Using computers is an important part of modern astronomy. Computers aid observational astronomers in many ways. For example, computers guide telescopes, and they control devices that measure the radiation gathered by telescopes. Astronomers also use computers to work out designs for new telescopes and to analyze data collected with telescopes. In addition, computers have a major role in theoretical studies. An astrono-mer might use a computer to produce a mathematical model of the history of a star from its birth to its death. See COMPUTER (Scientific visualization software).

History

People have always been fascinated by the sky. As early as the 1300's B.C., Chinese astronomers charted the positions of the stars and recorded eclipses of the sun and moon. By about 700 B.C., the Babylonians could predict when planets would appear closest to and farthest from the sun. They also predicted when various astronomical objects would be visible for the first or last time in a year. The ancient Egyptians determined the beginning of springtime by noting the position of Sirius, the brightest star in the sky. They also used their astronomical knowledge to build temples whose walls lined up with certain heavenly bodies.

The Chinese, Babylonians, and Egyptians left written records of their astronomical observations, which are of great interest to modern scholars. Scholars today also study the art and architecture of other early civilizations to determine the extent of their astronomical knowledge. These studies link archaeology and astronomy and so are called archaeoastronomy or astroarchaeology. For example, research suggests that Stonehenge, an ancient stone monument in southern England, was used to predict the positions of the sun and moon. Other studies indicate that American Indians tracked the sun and stars long before Europeans arrived. For example, researchers have discovered that stone rings laid out by early tribes have piles of stones that mark the position of the sunrise and sunset on the longest day of the year. One such ring, the Bighorn Medicine Wheel in Wyoming, dates from about A.D. 1400.

Greek astronomy. Beginning about 600 B.C., Greek philosophers and scientists developed a number of important astronomical ideas. Pythagoras, who lived during the 500's B.C., argued that the earth is round. He also attempted to explain the nature and structure of the universe as a whole and so developed an early system of cosmology.

By about 370 B.C., Eudoxus of Cnidus had worked out a mechanical system to explain the motions of the planets. Eudoxus taught that the planets, the sun and moon, and the stars revolved around the earth. In the 300's B.C., the philosopher Aristotle incorporated this geocentric (earth-centered) theory into his philosophic system.

Also during the 300's B.C., Heraclides of Pontus proposed that the seemingly westward movement of the heavenly bodies is actually due to the eastward rotation of the earth on its axis. He also argued that Mercury and Venus revolve around the sun, not around the earth. During the 200's B.C., Aristarchus of Samos went even further and suggested that all the planets, including the earth, revolve around the sun. Heraclides and Aristarchus were far ahead of their time, and their ideas failed to replace the geocentric theory.

About 125 B.C., a Greek astronomer named Hipparchus divided the stars he could see into classes of brightness. The system of magnitudes that astronomers use today is a modified version of his scale.

The Ptolemaic system. During the A.D. 100's, the theories of Aristotle and Hipparchus were further developed by Ptolemy, a Greek astronomer who lived in Alexandria, Egypt. In his major published work, the Almagest, Ptolemy presented his ideas and summarized those of earlier Greek astronomers, especially Hipparchus. The Almagest is the chief source of our knowledge of Greek astronomy.

Ptolemy's geocentric theory of the universe became extremely influential. Astronomers accepted versions of his ideas and of his tables of planetary motions for nearly 1,500 years. Throughout most of this period, Europeans gave little attention to astronomy. However, Arab astronomers in the Middle East and northern Africa continued to observe the heavens and to preserve and refine the writings of Ptolemy. Finally, during the 1100's, a Latin translation of the Almagest introduced Ptolemy's ideas into Europe.

The beginnings of modern astronomy. The first modern breakthrough in understanding the universe came in 1543 with the publication of Concerning the Revolutions of the Celestial Spheres by the Polish astronomer Nicolaus Copernicus. The ideas that Copernicus presented in this book differed so greatly from the traditional Ptolemaic theory that historians of science often speak of the "Copernican Revolution."

Copernicus proposed that the sun is the center of the universe and that the earth and the other planets travel around it. Copernicus's heliocentric (sun-centered) theory simplified the explanation of the observed motions of the planets. Ptolemy's geocentric theory required a complicated system to explain why the planets sometimes appear to drift backwards with respect to the stars. Copernicus explained that this backward drift is not an actual motion of the planets. Instead, the planets appear to move backwards because of the earth's own movement around the sun. But Copernicus's system did not accurately predict the positions of the planets.

During the late 1500's, the Danish astronomer Tycho Brahe observed the motions of the planets far more precisely than they had ever been observed before. His observations, especially of Mars, revealed the inaccuracy of the tables then used to predict the positions of the planets. Brahe died in 1601. Brahe's assistant, Johannes Kepler of Germany, then assumed the task of analyzing the results of Brahe's observations.

On the basis of Brahe's data, Kepler discovered that the planets orbit the sun in ellipses (ovals). Until that time, even supporters of the heliocentric theory assumed that the planets moved in circular orbits. Kepler also discovered two principles that govern the speed of a planet in its orbit. His discoveries greatly improved the accuracy of calculations of planetary positions and so provided support for the Copernican theory.

Galileo and Newton. During the early 1600's, an Italian named Galileo was the first person to use a telescope to study the sky. Galileo's observations helped confirm the Copernican system. For example, he found that several moons revolve around Jupiter. This discovery showed that, contrary to the theories of Aristotle and Ptolemy, not all bodies revolve around the earth.

In 1642, about a year after Galileo died, Isaac Newton was born in England. Newton became the greatest scientist of his time. He discovered the law of gravitation and showed that it explains the motions of the planets and comets and the behavior of objects on the earth. According to this law, every object in the universe attracts every other object. The strength of the attraction between any two objects depends on their masses and the distance between them. Newton also discovered that visible light can be broken down into a spectrum. This laid the basis for the development of spectral analysis.

Explaining the origin of the solar system. By the time Newton died in 1727, most scientists and philosophers agreed that the sun was the center of the universe. They then began to develop theories to explain the origin of the solar system. In 1755, a German philosopher named Immanuel Kant suggested that the planets and the sun were formed in the same way. In 1796, Pierre Simon Laplace, a French mathematician, proposed that the sun and planets formed from a spinning cloud of gas called a nebula. Laplace's nebular hypothesis lost favor for a while. However, astronomers today again tend to accept theories that stem from the ideas of Kant and Laplace. They think that the sun and planets condensed out of what is called a primeval solar nebula. According to this theory, the nebula condensed and formed the sun and many small bodies called planetesimals. The planetesimals eventually combined into nine large bodies, the planets.

The discovery of new planets. Until the 1700's, astronomers knew of only six planets--Mercury, Venus, Earth, Mars, Jupiter, and Saturn. In 1781, the British astronomer William Herschel discovered Uranus. Astronomers had occasionally seen Uranus during the preceding 172 years, but they had not noticed its motion, and so they had considered it a star.

After the discovery of Uranus, astronomers found that the planet's path through space varied from its predicted orbit. The gravitation of an unknown planet seemed to be affecting Uranus' orbit. Two astronomers, John C. Adams of Britain and Urbain Leverrier of France, predicted the location of such a planet. On the basis of these predictions, the German astronomer Johann G. Galle and his assistant, Heinrich L. d'Arrest, discovered Neptune in 1846.

The discovery of Pluto resulted from a long search for an unknown planet that was thought to cause variations in the orbits of Neptune and Uranus. Finally, in 1930, the American astronomer Clyde Tombaugh determined that a faint image on his photographic plates was Pluto. He identified it as a planet because of its slow motion against the background of the stars. Pluto's mass, however, turned out to be too small to have a measurable effect on the orbits of Neptune and Uranus.

The development of spectral analysis. During the 1800's, scientists began to study the significance of the spectrum, which Newton had discovered in the 1600's. During the early 1800's, two physicists--William Wollaston of Britain and Joseph von Fraunhofer of Germany--studied sunlight spread out into its rainbow of colors. After Wollaston had noticed a few gaps in the spectrum at certain colors, Fraunhofer discovered there are many gaps that look like dark lines across the spectrum. These gaps are called spectral lines.

During the 1850's, two Germans--the chemist Robert Bunsen and the physicist Gustav Kirchhoff--together designed the first spectroscope to observe the details in a spectrum. They discovered that atoms of each chemical element produce a certain set of spectral lines. This knowledge enabled astronomers to identify the elements that make up a star by studying the spectral lines in the star's light.

A new view of the universe emerged during the early 1900's, chiefly from the work of the famous German-born physicist Albert Einstein. In 1905, Einstein proposed his special theory of relativity. According to this theory, nothing can travel faster than the speed of light. From this theory comes the idea that mass and energy are interchangeable and are related by the equation E equals m times c-squared. In this equation, E stands for energy, m for mass, and c-squared for the speed of light multiplied by itself. During the 1930's, astronomers discovered that stars get their energy through the transformation of mass into energy as described by Einstein's equation.

In 1915, Einstein announced his theory of gravitation, called the general theory of relativity. This theory links the three dimensions of space with a fourth dimension, time. In most cases, the results obtained by using Einstein's theory do not differ significantly from those obtained by using Newton's theories. However, the general theory of relativity must be used in studies of the universe as a whole or of events that occur in extremely strong gravitational fields. For example, the general theory explains how the mass of a black hole can affect space in such a way that not even light can escape.

The general theory of relativity implies that the universe is expanding. But in 1915, Einstein had no observational evidence to support this idea. He therefore changed his equations to describe a universe of constant size. In 1929, however, the American astronomer Edwin Hubble demonstrated that the universe is expanding. As a result, Einstein restored his original equations. Modern theories of cosmology are based on solutions to these equations.

The development of radio astronomy. In 1931, Karl Jansky, an American engineer at the Bell Telephone Laboratories in New Jersey, studied static that was interfering with short-wave communication systems. He found that the static appeared four minutes earlier each day. Jansky knew that the stars rise four minutes earlier daily, and so he concluded that the static must be coming from beyond the solar system. He was actually receiving radio waves from the center of our galaxy.

Professional astronomers did not follow up on Jansky's discovery. However, an American amateur astronomer named Grote Reber designed a radio telescope and operated it in his backyard beginning in the late 1930's. Radio astronomy finally began to flourish after World War II (1939-1945). The study of radio waves from space greatly expanded astronomers' knowledge of the structure, size, and history of the universe. For example, it revealed a great deal of new information about the clouds of gas and dust between the stars in our galaxy. During the 1960's, radio astronomers played an important role in the discovery of quasars and pulsars. In 1965, astronomers testing a radio telescope and receiver system discovered the primordial background radiation, which astronomers believe was produced when the universe began with the big bang.

Space exploration began on Oct. 4, 1957, when the Soviets launched the first artificial satellite. The development of space travel has benefited astronomy in many ways. For example, American astronauts have conducted experiments on the moon and brought back rock samples for study. Unmanned space probes have explored the planets and provided a great variety of information that should help astronomers answer questions about the formation of the solar system.

Space travel has also made it possible to observe celestial objects from above the earth's atmosphere. The atmosphere blocks some wavelengths of radiation and may interfere with the detection of others. To overcome this barrier, the United States began to launch unmanned orbiting observatories in the 1960's. In 1973 and 1974, American astronauts made valuable observations with a telescope aboard the Skylab space station. Today, astronauts sometimes make astronomical observations from space shuttles.

The National Aeronautics and Space Administration(NASA) made gamma-ray and X-ray astronomy one of its top concerns in the 1970's. Each gamma-ray or X-ray photon has an extremely high level of energy. The study of the processes that produce high-energy photons and cosmic rays is called high-energy astrophysics. In the late 1970's, NASA launched three High-Energy Astronomical Observatories to study gamma rays, X rays, and cosmic rays from neutron stars, quasars, and supernovae. From 1983 to 1986, the European Space Agency (ESA) made X-ray observations from its satellite Exosat. In 1990, NASA launched Rosat, a European satellite used to study X-ray sources in outer space. In 1991, NASA launched the Compton Gamma Ray Observatory.

Satellites also aid in the study of ultraviolet and infrared radiation. From 1972 to 1982, an orbiting observatory called Copernicus studied ultraviolet light from stars and from the space between stars. A series of satellites studied ultraviolet radiation and X rays from the sun. The International Ultraviolet Explorer, a satellite launched in 1978, returns information about stars, planets, quasars, and other astronomical objects. In 1983, the Infrared Astronomical Satellite sent back observations of hundreds of thousands of infrared sources. In 1989, the Cosmic Background Explorer satellite began to map the background radiation, which had been discovered in 1965. This mapping provided information on how the universe came to have its present structure.

In 1990, NASA launched the Hubble Space Telescope (HST), a reflecting telescope with a 94-inch (240-centimeter) mirror for studies of visible and ultraviolet light. A flawed mirror blurred the HST's images. But, in 1993, astronauts aboard the space shuttle Endeavor installed devices that make up for the flaw. The HST can observe objects 50 times fainter than can telescopes on the earth and provides details about 10 times smaller than the smallest ones visible from the ground.

Astronomy today remains one of the most active and exciting sciences. New telescopes on the earth--as well as orbiting observatories--enable astronomers to study increasingly distant regions and to make increasingly accurate observations. The new telescopes include Keck I and Keck II, two identical instruments on the island of Hawaii. Each telescope's light-gathering optical equipment consists of 36 hexagonal mirrors mounted close together. Computers adjust these mirrors to form a reflecting surface 33 feet (10 meters) in diameter. Keck I was completed in 1992; Keck II, in 1996.

The largest American telescope project ever undertaken was completed in 1980 near Socorro, New Mexico. This instrument, called the Very Large Array (VLA), consists of 27 radio telescopes, each 82 feet (25 meters) in diameter. The VLA enables astronomers to make radio maps of the sky. The Very Long Baseline Array (VLBA) consists of 10 radio telescopes that are scattered across the United States. The VLBA began to operate in 1993. It provides even finer details about distant galaxies.

Advances in observational astronomy continue to raise new questions for theoretical astronomers. For example, many astronomers seek a better understanding of the processes that produce gamma rays and X rays. The physical characteristics of black holes and quasars remain subjects of study and debate. Also, cosmologists are studying the idea of the inflationary universe--that is, that the universe expanded extremely rapidly during the first fraction of a second after the big bang.

Astronomers also search for planets around distant stars and try to detect signals from intelligent beings who might live on such planets. Astronomers use radio telescopes in most of these searches.

Careers

High school students who want to become professional astronomers should take as many mathematics and physical science courses as possible. In college, most future astronomers major in astronomy or physics. In either case, they take as many physics courses as possible. For certain fields of astronomy, courses in chemistry and geology are also important. After receiving a bachelor's degree, students interested in working at a college, university, or research institution go on to earn a Ph.D. degree. Earning a Ph.D. degree usually requires four to seven years of graduate work, including the completion of a thesis based on original research.

A professional astronomer also needs certain special skills. Most astronomers must know how to program a computer. A knowledge of the techniques of microelectronics is also helpful. Typing is a useful skill, both for programming computers and for writing articles.

Most professional astronomers teach at universities or colleges. Many others work at research institutions, the national observatories, or laboratories run by the federal government. Planetariums employ astronomers to lecture and to conduct classes for the public. A few astronomers work in industry. They do such tasks as monitoring pollution in the atmosphere or applying advanced techniques in optics for industrial purposes.

The major professional organization for astronomers in the United States is the American Astronomical Society. In Canada, both professionals and amateurs belong to the Royal Astronomical Society of Canada. Astronomers from throughout the world come together every three years at the General Assembly of the International Astronomical Union. Additional information about careers in astronomy may be obtained from the University of Texas in Austin.
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Roland Camilleri

Moderator

Sydney , Australia.