Astronomy is the study of the universe and the objects in it. Astronomers observe the sky with telescopes of different kinds that gather not only visible light but also invisible forms of energy, such as radio waves. They investigate nearby bodies, such as the sun, planets, and comets, as well as distant galaxies and other faraway objects. They also study the structure of space and the past and future of the universe.
Astronomers seek answers to such questions as: How did the universe begin? What processes release energy deep inside stars? How does one star “steal” matter from another? How do storms as big as Earth arise on Jupiter and last for hundreds of years?
To answer such questions, astronomers must study several subjects besides astronomy. Almost all astronomers are also astrophysicists because the use of physics is essential to most branches of astronomy. For example, some parts of cosmology, the study of the structure of the universe, require an understanding of the physics of elementary particles, such as the bits of matter called quarks that make up protons and neutrons. Astronomers use chemistry to analyze the dusty, gaseous matter between the stars. Specialists in the structure of planets use geology.
Astronomy is not only a modern but also an ancient science. Like today’s researchers, ancient scholars based their ideas of the universe on what they observed and measured and on their understanding of why objects move as they do. However, the ancients developed some incorrect ideas about the relationships between Earth and the objects they saw in the heavens. One reason for their errors was that they did not understand the laws of motion. For example, they did not know that a force—which we know as gravitation—controls the movements of the planets. Another reason was that their measurements, made without telescopes, revealed little detail about the movements of the planets.
The ancients noted that the positions of the sun, moon, and planets change from night to night. We now know that these movements are a result of the revolution of the moon about Earth and the revolution of Earth and the other planets about the sun. The ancients, however, concluded that the sun, moon, and planets orbit a motionless Earth. In many places, religious teachings seemed to support this conclusion until the 1600’s.
Although ancient people misinterpreted much of what they saw in the heavens, they put their knowledge of astronomy to practical use. Farmers in Egypt planted their crops each year when certain stars first became visible before dawn. Many civilizations used the stars as navigational aids. For example, the Polynesians used the positions of the stars to guide them as they sailed from island to island over much of the Pacific Ocean.
Observing the sky
If you look at the night sky without a telescope or a pair of binoculars, you will see what the ancients saw. If the night is clear and you are far from city lights, you will see about 3,000 stars. Stretching across the sky will be a splotchy band of bright and dark areas called the Milky Way. In addition, a few fuzzy spots will be visible.
Ancient people in the Western world noticed that certain stars are arranged in patterns shaped somewhat like human beings, animals, or common objects. Many ancient civilizations associated such patterns, called constellations, with mythology. Many names of constellations have come to us from Greek myths. In one myth, Artemis, goddess of hunting, was greatly saddened by the death of a human hunter named Orion. In her sorrow, she placed Orion in the sky as a constellation.
How the stars move.
If you map or photograph the location of several stars for a few hours, you will observe a regular motion. The stars move relative to Earth because of Earth’s rotation about its own axis. All stars move in circles around a point in the sky known as a celestial pole. Stars rotate counterclockwise around the celestial north pole and clockwise around the celestial south pole. Stars that are far from the pole rise from below the horizon in the east, move upward, and then set in the west.
One faint star, Polaris, is so close to the celestial north pole that it moves little. Because of its location, Polaris is also known as the North Star. It is only the 49th brightest star in the sky, but it is in an important location. There is no “South Star,” but the constellation Octans (Octant) includes the celestial south pole.
The sun, like the other stars, rises in the east and sets in the west. But the sun also moves eastward relative to the other stars about 1° each day. By noting which stars are visible above the horizon just before sunrise and just after sunset, people have mapped the path of the sun among the stars for thousands of years. This path is known as the ecliptic. The band of constellations near this path is called the zodiac.
How the planets move.
Without a telescope, you can see the planets Mercury, Venus, Mars, Jupiter, and Saturn. They move every night within the constellations of the zodiac. That is, when viewed at the same time on successive nights, the planets move relative to the stars. Most of the year, they move from west to east.
The planets also have a motion that differs from that of any other celestial object. In their night-to-night movement relative to the stars, they occasionally appear to slow down and then move westward in what is known as retrograde motion. They then seem to slow down again, stop, and resume their eastward motion in what are called retrograde loops.
How the moon moves.
The moon is the brightest and most easily seen object in the night sky. As a result, the most familiar observation is of the moon’s phases, such as the full moon, quarter moon, and crescent. The moon moves from east to west as it rises and sets. From night to night, the moon moves eastward about 13° relative to the stars, rising about 50 minutes later each night.
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Earth-centered theories.
Ancient scholars produced elaborate schemes to account for the observed movements of the stars, sun, moon, and planets. In the 300’s B.C., the Greek philosopher Aristotle developed a system of 56 spheres, all with the same center. The innermost sphere, which did not move, was Earth. Around Earth were 55 transparent, rotating spherical shells. The outermost shell carried the stars, believed to be merely points of light. Other shells carried the sun, moon, and planets. These shells rotated inside other shells that rotated within still other shells in ways that accounted for almost all the observed movements.
During the A.D. 100’s, Ptolemy, a Greek astronomer who lived in Alexandria, Egypt, offered an explanation that better accounted for retrograde motion. Ptolemy said that the planets moved in small circles called epicycles. The epicycles moved on large circles called deferents. Earth was near the center of all the deferents.
Sun-centered theories.
By the early 1500’s, the Polish astronomer Nicolaus Copernicus had developed a theory in which the sun was at the center of the universe. This theory correctly explained retrograde motion as the changing view of the planets seen from a moving Earth. The theory also correctly explained the east-to-west motion of the sun and stars across the sky. This movement is due to the west-to-east rotation of Earth about its own axis, rather than an actual motion of the sun and stars.
Several decades later, the Danish astronomer Tycho Brahe built gigantic instruments that he used to make precise measurements of planetary movements. The German mathematician Johannes Kepler analyzed Tycho’s measurements of the movement of Mars. He discovered that Mars moves in an ellipse (oval) with the sun at a key point inside the ellipse. Kepler also found a relationship between how far Mars is from the sun and how rapidly it moves. He concluded that all planets have elliptical orbits and that the relationship between distance from the sun and speed applies to all the planets. These findings, which became known as Kepler’s first two laws of planetary motion, were published in 1609. In 1619, Kepler announced his third law, which shows the relation between the sizes of the planets’ orbits and the time they take to orbit the sun.
Thus, by the early 1600’s, astronomers had used observations made with the unaided eye to map the movement of the planets. However, in 1609, the Italian astronomer Galileo ushered in the era of modern astronomy by using the newly invented optical telescope to observe the heavens.
Modern astronomy
Today’s astronomers gather information in many ways. Perhaps most importantly, astronomers use telescopes and other instruments to detect visible light and other forms of radiation that are emitted (sent out) by celestial objects. Instruments also detect particles called neutrinos and cosmic rays from outer space. In addition, astronomers study chunks of matter that originated in outer space. They send spacecraft to land on other objects or to study them close-up. They also search for a type of radiation called gravitational waves. Gravitational waves arise in giant explosions or from the merging of extremely dense and compact objects, such as black holes. A black hole is an object whose gravitation pull is so strong that nothing—not even light—can escape.
Using modern techniques, astronomers have made discoveries beyond the imagination of the ancients. They have discovered two planets outside the orbit of Saturn—Uranus and Neptune—and many other distant bodies, including smaller round objects known as dwarf planets. They have found that millions of smaller bodies called asteroids revolve about the sun, most of them between the orbits of Mars and Jupiter. Astronomers have learned that the sun is merely one of hundreds of billions of stars in a vast, disk-shaped galaxy, the Milky Way. They now understand that many fuzzy spots visible with telescopes in the night sky are other galaxies.
In addition, astronomers have discovered exotic objects called pulsars, rapidly spinning collapsed stars from which regular bursts of radiation are received, and quasars that emit vast amounts of energy. They have found evidence of black holes and studied exploding stars called supernovae.
Units of distance.
Many distances involved in astronomy are so huge that they are measured in special units. One such unit is the light-year, the distance that light travels in a vacuum in a year. This distance equals about 5.88 trillion miles (9.46 trillion kilometers). The star nearest the sun, Proxima Centauri, is about 4 light-years from Earth. The Milky Way is about 100,000 light-years across. The sun is roughly 25,000 light-years from its center. The nearest large galaxy is the Andromeda Galaxy. It is about 2 million light-years away. Some galaxies are more than 10 billion light-years distant.
Astronomers measure distances in the solar system in astronomical units (AU). One AU is the average distance from Earth to the sun—about 93,000,000 miles (150,000,000 kilometers). This distance equals about 8 light-minutes. The average distance from the sun to Neptune, the farthest planet, is about 30 AU.
In their technical work with extremely long distances, astronomers use a unit called a parsec, rather than the light-year. The parsec is based on parallax, an angular measurement. One parsec equals about 3.26 light-years.
Locating objects in space.
Astronomers still use two concepts developed by the ancients to specify locations of celestial objects: (1) an imaginary celestial sphere and (2) the constellations.
The celestial coordinate system.
Astronomers specify locations in terms of the celestial coordinate system, a set of imaginary lines drawn on the celestial sphere. The celestial sphere is similar to the outermost shell in Aristotle’s system—the shell that was thought to carry the stars. The lines are similar to the lines of longitude and latitude used by geographers. The poles of the celestial sphere are the celestial north pole and the celestial south pole, which lie over Earth’s north and south geographic poles. The sphere also has a celestial equator over the earthly equator.
Longitude in the sky, marked by half-circles going from the north celestial pole to the south celestial pole, is called right ascension. Latitude in the sky, marked by circles parallel to the celestial equator, is known as declination. Declination north of the celestial equator is positive, while declination south of the equator is negative.
Using the constellations.
To locate and assign names to stars, astronomers have divided the sky into 88 parts, each associated with a constellation. Astronomers still use a system developed in the early 1600’s to identify the brightest stars in the constellations. The brightest star of all in a constellation is usually designated by alpha, the first letter of the Greek alphabet; the second brightest by beta, the second letter; and so forth. The brightest star in the constellation Lyra (the Harp) is thus Alpha Lyrae. Lyrae is Latin for of Lyra.
Because the Greek alphabet has only 24 letters, this system is limited to 24 stars per constellation. Later astronomers developed naming systems in which numbers are assigned to fainter stars and Roman letters to variable stars (stars that vary in brightness).
Electromagnetic radiation
is the most plentiful source of information about heavenly bodies. Its name comes from the fact that it consists of waves of electric and magnetic energy. Visible light is electromagnetic radiation, and objects in space also emit many kinds of invisible electromagnetic radiation. Scientists can identify the various forms of this radiation by their wavelength, frequency, or energy.
Wavelength
is the distance between successive crests of a wave. From the shortest wavelength to the longest, the forms of electromagnetic radiation are gamma rays, X rays, ultraviolet rays, visible light, infrared rays, microwaves, and radio waves. These forms together make up the electromagnetic spectrum.
Frequency.
All electromagnetic radiation travels at the same speed in a vacuum. Therefore, a relatively short wave passes a given point more quickly than does a relatively long wave. Thus, more of the shorter waves pass the point each second. Scientists say that a shorter wave has a higher frequency. The unit used to measure frequency is the hertz (symbol Hz). One hertz represents the passing of one wave past a point in one second.
Energy.
According to quantum theory, a cornerstone of modern physics, electromagnetic radiation can also be thought of as particles of energy called photons. The amount of energy of a given photon depends on the wavelength—or frequency—of the corresponding wave. Radiation that has a short wavelength and therefore a high frequency also has high energy. Radiation with a long wavelength has a low frequency and low energy.
Optical astronomy
is the study of the heavens by detecting and analyzing visible light. Visible light of different wavelengths has different colors. The wavelengths range from about 400 nanometers for deep violet to 700 nanometers for deep red. One nanometer equals a billionth of a meter, or 1/25,400,000 inch.
Modern astronomy began with observations of the sky through optical telescopes. Today, astronomers often use not only observations made with visible light but also observations made in other parts of the electromagnetic spectrum.
An optical telescope gathers and focuses light with a lens or mirror. For the faintest objects, a large light-collecting area is needed. The largest all-purpose telescopes now in general use include the twin Keck Telescopes on Mauna Kea, an extinct volcano on the island of Hawaii. Each telescope has a mirror 33 feet (10 meters) in diameter. Telescopes are now being constructed with much larger compound mirrors, up to 75 feet (22 meters) and 130 feet (42 meters) in diameter.
Major optical telescopes are installed on mountains so that starlight does not have to travel far through the atmosphere or so that the telescope can take advantage of smoothly flowing air. These locations minimize blurring due to the atmosphere. The atmosphere bends light due to a phenomenon known as refraction, and the atmosphere is constantly moving. As a result, starlight jiggles and changes in brightness as it passes through the air. Thus, stars appear to twinkle. Twinkling blurs images.
Only a telescope operating in space can avoid blurring entirely. The best-known orbiting telescope is NASA’s Hubble Space Telescope, launched in 1990 to observe infrared, visible, and ultraviolet radiation. The Hubble provided images of astronomical objects in detail never before observed. These images included pictures of galaxies on the edge of the universe and what appear to be stars in the process of forming planets.
Professional astronomers rarely look through telescopes. Instead, they study recorded images. Astronomers began to photograph images through telescopes in the 1850’s. The use of long exposure times revealed faint objects that the eye could not see through a telescope. Film has been replaced by electronic devices that can detect and record even fainter light. The charge-coupled device (CCD), for example, is about 50 times or more sensitive to light than film is.
Optical astronomers have developed three special techniques that have also been used in other kinds of astronomy. They are (1) spectroscopy, (2) interferometry, and (3) adaptive optics.
Spectroscopy
is the breaking down of incoming radiation into a spectrum of its parts. The spectrum of visible light, for example, is a rainbowlike band of colors. At one end is red, which has the longest wavelength. At the other end is violet, with the shortest wavelength. Spectroscopy is based on a discovery made in 1814 by the German optician Joseph von Fraunhofer. He found that the spectrum of sunlight contains dark lines where specific colors are absent. Later, scientists discovered that light from other stars also has such dark lines. When the object emitting the light is a hot gas without a central star or a star behind it, the spectrum has bright lines. Other kinds of electromagnetic radiation from celestial objects also have spectral lines.
Studies of spectral lines reveal the temperature, density, and chemical composition of the object emitting the radiation. The spectral lines arise in energy processes in atoms. An electron orbiting an atomic nucleus can have only certain definite levels of energy. These levels can be thought of as stairsteps. When light energy passes through a group of atoms, the electrons can absorb just the right amount of energy to jump from a lower step to a higher one. Because the energy is absorbed from the passing light, a kind of spectral line called an absorption line appears in the spectrum. The dark lines discovered by Fraunhofer were absorption lines. Each kind of atom has its own pattern of absorption lines for a given range of temperature.
Bright spectral lines called emission lines occur when electrons in atoms of hot objects “jump down the stairs” by emitting energy. In the early 1940’s, an analysis of emission lines seen when gas on the edge of the sun was silhouetted against a dark sky confirmed an earlier discovery about the temperature of the sun’s corona. The corona, the outer edge of the sun’s atmosphere, has a temperature of millions of degrees. The entire band of radiation emitted by the surface of a star also contains information about the star’s temperature. Cooler stars are red-hot and can appear slightly reddish to the eye. Hotter stars can become blue-white.
Spectroscopy can even reveal the speed and direction of motion of a star. In the spectrum of any moving object, the spectral lines shift from where they would appear in the spectrum of a stationary object. An object’s motion can produce a Doppler effect. One familiar example of this effect is a change in the pitch of sound waves emitted by a vehicle. For example, when a car approaches you, the pitch of the sound made by the engine is higher than it is when the car is going away. The shift of spectral lines also depends on whether the object is becoming farther away or closer. If the lines shift toward the blue (shorter-wavelength) end of the spectrum, the object is becoming closer. If they shift toward the red (longer-wavelength) end, the object is drawing farther away. This effect is called the Doppler redshift.
In 1929, the American astronomer Edwin Hubble discovered that distant galaxies show redshifts in their spectra. Other scientists interpreted the redshifts as showing that the galaxies are receding (becoming farther away) from our own galaxy. Hubble also found that farther galaxies have higher redshifts, indicating that they are receding at higher rates. Unlike the Doppler redshift, this cosmological redshift—which applies only to faraway objects—occurs because the universe is expanding. That is, every point in the universe is receding from every other point. Thus, astronomers can determine the distance of a galaxy by measuring its redshift.
Interferometry
uses a phenomenon called interference in which rays of light combine. Astronomers use the resulting patterns, with the aid of computers, to produce extremely detailed images. In the simplest example in optical astronomy, a ray of light emitted by a star strikes the mirror of a telescope, and another ray emitted by the same star strikes the mirror of a nearby telescope. Optical and mechanical devices combine the rays so that a series of bright and dark bands of light called an interference pattern appears. The pattern reveals any difference in the routes taken by the rays as they travel from the star to the telescopes. For example, if the rays were emitted at opposite edges of the star, the pattern could reveal the diameter of the star. A computer helps astronomers analyze the interference pattern and use the results to produce an image.
Adaptive optics
can make up for atmospheric blurring in ground-based telescopes. In this technique, light reflects from a telescope’s main mirror to a special deformable mirror, then to a CCD. Pistons mounted on the underside of the deformable mirror can change its shape several hundred times a second to make up for atmospheric blurring. A special control system senses the amount of blurring and operates the pistons.
In one arrangement, the deformable mirror is in the main telescope and the control signals come from a smaller auxiliary telescope. A laser beam emitted by the auxiliary telescope reflects off atoms in the atmosphere and returns to the auxiliary telescope. As the beam travels, the atmosphere distorts it slightly. A computer analyzes the reflected beam, then operates the pistons in a way that would remove the distortion from the image of the beam. This operation also removes much of the distortion in the image viewed by the main telescope.
Infrared astronomy
deals with invisible electromagnetic waves whose wavelengths are longer than those of visible light. Objects that are bright in infrared wavelengths include relatively cool stars and stars in the process of forming. Planets and other objects that glow by reflecting sunlight or starlight are also best studied in the infrared spectrum.
The infrared spectrum covers a range from about 700 nanometers to 1 millimeter. Infrared astronomers commonly express wavelengths in micrometers (thousandths of a millimeter), specifying the overall range as about 0.7 to 1,000 micrometers.
A photon of infrared radiation has less energy than a photon of visible light. Most infrared photons do not have enough energy to cause the chemical reaction that produces images on film. Infrared astronomy therefore did not develop fully until about the 1960’s, when better electronic sensors could produce infrared images.
One problem with infrared astronomy is that Earth’s atmosphere absorbs rays of most wavelengths in the infrared spectrum. However, some of the shorter waves can be detected at mountaintop observatories. Notable infrared telescopes include the Infrared Telescope Facility of the National Aeronautics and Space Administration (NASA) of the United States and the United Kingdom Infrared Telescope. Both are on Mauna Kea. The telescopes are above most of the water vapor in Earth’s atmosphere, one of the main absorbers of infrared rays.
An orbiting infrared telescope has a limited useful life because it must be cooled artificially. Its lifetime is limited by the amount of coolant it carries. Cooling is necessary to prevent the telescope’s own infrared radiation from overwhelming the faint rays coming from outer space. The telescope must be cooled to the temperature of liquid helium—about 4 Celsius degrees above absolute zero (–459.67 °F or –273.15 °C). Absolute zero is the theoretical temperature at which atoms and molecules would have the least possible energy.
The largest telescope in space was an infrared telescope, the Herschel Space Observatory, launched in 2009. The Herschel telescope collected light from the far-infrared part of the spectrum using a mirror 11 1/2 feet (3.5 meters) in diameter. It ran out of coolant and was shut down in 2013.
Radio and microwave astronomy.
Astronomers use radio waves emitted by celestial objects to produce images of the objects and to study the objects spectroscopically. These are the same kinds of waves that radio and television broadcasters create to transmit programs—astronomers just use them differently.
The radio spectrum includes all electromagnetic waves longer than about 1 millimeter. Microwaves are often included as part of the radio spectrum. Waves longer than about 1 millimeter and shorter than about 10 meters pass readily through Earth’s atmosphere. Astronomers receive radio signals from a wide variety of objects, including pulsars, distant galaxies, quasars, particles swirling in Jupiter’s magnetic field, and gas clouds orbiting the center of the Milky Way.
Karl G. Jansky, an American engineer, discovered radio waves from outer space in the early 1930’s. However, the science of radio astronomy was not established until after World War II (1939-1945).
A radio telescope is essentially a large dish antenna. Unlike the reflecting surfaces of other kinds of telescopes, the dish surface does not have to be extremely smooth. The smoothness required of a reflecting surface depends on the length of the waves to be reflected. The shorter the wavelength is, the smoother the surface must be. Because radio waves are long, some dish surfaces are even made of metal mesh.
The largest radio telescopes that can be steered to point anywhere in the sky are 328 feet (100 meters) in diameter. One is in Effelsberg, Germany, near Bonn. The other is in Green Bank, West Virginia. The most powerful radio telescope of all is a nonsteerable telescope in Guizhou Province, China. Its reflecting dish follows a natural bowl formation some 1,600 feet (500 meters) in diameter.
Radio astronomy is the field in which interferometry is most useful. To use separate telescopes as an interferometer, the distance between them must be controlled to a fraction of the wavelength of the radiation to be detected. This requirement severely limits the use of interferometry in the other branches of observational astronomy, where wavelengths are shorter. Because radio waves are so long, however, the dishes of radio interferometers can be tens, hundreds, or even thousands of miles or kilometers apart.
The Very Large Array (VLA) near Socorro, New Mexico, links 27 movable dishes, each 82 feet (25 meters) in diameter. The telescopes move along railroad tracks built in the shape of a Y. Individual dishes can be moved as far as 22 miles (35 kilometers) from the center of the Y. In 2014, the VLA was upgraded with a system to detect short burst radio emissions from astronomical objects. The VLA’s full name is the Karl G. Jansky Very Large Array. The Very Long Baseline Array (VLBA) consists of 10 telescopes, each 82 feet across, spread across one side of Earth. Their locations range from the Virgin Islands north to New Hampshire, northwest to Washington, and west to Hawaii. As an interferometer, the VLBA is equivalent to a single telescope with a diameter roughly that of Earth.
Spacecraft orbiting between Earth and the sun have mapped the sky at microwave wavelengths. These spacecraft included the Wilkinson Microwave Anisotropy Probe (WMAP), launched by NASA in 2001, and the Planck spacecraft, launched by the European Space Agency (ESA) in 2009. Both measured tiny variations in the cosmic microwave background radiation, a type of energy left over from the early universe. By studying the maps, astronomers can “see” the conditions in the early universe.
Radio astronomers have learned much by studying redshifts and blueshifts of spectral lines. Because the radio spectrum is not visible, astronomers see these “lines” as low and high points in a graph of radio brightness versus wavelength. The low points represent wavelengths of radiation absorbed by celestial objects. The high points represent wavelengths of radiation emitted by strong radio sources.
Redshift and blueshift work with radio waves exactly as they work with waves of visible light: If the lines in the spectrum of radiation emitted by an object shift toward the shorter-wavelength end, the object is approaching and is said to be blueshifted. If the lines shift toward the longer-wavelength end, the object is receding and is said to be redshifted. The amounts of the shifts are usually so small that no actual change in color occurs.
Astronomers have analyzed redshifts and blueshifts in radio radiation emitted by gas clouds in the Milky Way. Their analysis showed how rapidly the galaxy is rotating and how the speed of the revolution of its stars changes with the stars’ distance from the galactic center. The astronomers then applied a formula relating the speed of the stars’ revolution to the stars’ masses. They discovered that the amount of mass in the galaxy is about 1 trillion times the mass of the sun.
Both radio astronomers and optical astronomers have studied a phenomenon known as gravitational lensing. This phenomenon occurs, for example, where radiation emitted by a small, distant galaxy passes near a massive galaxy that is between the object and Earth. The gravitational force of the galaxy apparently bends the radiation much as an ordinary optical lens bends light rays that pass through it. Gravitational lensing can produce an image of the small galaxy in the shape of an arc or even a ring. Astronomers can study the radiation in the arc or ring to learn about the small galaxy.
Ultraviolet astronomy.
The ultraviolet part of the electromagnetic spectrum has wavelengths shorter than those of visible light. Wavelengths of ultraviolet light range from near the limit of the shortest waves that the eye can see, about 400 nanometers, down to about 10 nanometers. Radiation from 400 to 300 nanometers, called near ultraviolet, passes through Earth’s atmosphere and therefore can be detected on the ground. But ultraviolet astronomers get much of their information from shorter wavelengths—in the far ultraviolet, from 300 to about 100 nanometers; and in the part of the extreme ultraviolet from 100 to 10 nanometers.
Studies in the far and extreme ranges must be carried out by satellites. From 1992 to 2001, the Extreme Ultraviolet Explorer, launched by NASA, studied wavelengths from 76 down to 7 nanometers. In 1999, NASA launched the Far Ultraviolet Spectroscopic Explorer to observe wavelengths of 120 to 90 nanometers.
Another major ultraviolet telescope is the Solar and Heliospheric Observatory (SOHO), which orbits between Earth and the sun. The ESA launched SOHO in 1995 to monitor the sun with visible-light and ultraviolet cameras and spectrographs. NASA provided some of this equipment. Highly detailed images from SOHO show the sun in, for example, the ultraviolet light of helium gas at 60,000 °C or iron gas at 1,500,000 °C. NASA’s pair of Solar Terrestrial Relations Observatory (STEREO) spacecraft are sending ultraviolet images that are even more detailed than SOHO’s. The Solar Dynamics Observatory (SDO), launched in 2010, is giving the most detailed ultraviolet images yet.
X-ray astronomy.
X rays have wavelengths from about 10 nanometers down to about 0.1 nanometer. The hottest regions in space produce X rays. These regions include the sun’s corona and disks of material around black holes. The material in these disks heats up due to friction as it spirals into the black holes. As the material heats up, it emits X rays. The hot gas at the center of clusters of galaxies also emits X rays. Quasars are another source of X rays.
Celestial X rays do not penetrate Earth’s atmosphere and therefore can be studied only from spacecraft. X rays would pass through ordinary telescope mirrors and lenses, so one kind of X-ray telescope has specially designed mirrors. X rays strike these mirrors at low angles, then skip away like stones skipping off water. Rays from all the mirrors meet at a single focal point.
Other X-ray telescopes do not have mirrors. The rays enter the telescope through openings between lead or iron slats, then strike special detectors.
An American X-ray telescope on the Japanese Hinode spacecraft, launched in 2006, produced detailed images of the sun. These images show the corona and eruptions on the sun’s surface called solar flares.
In 1999, NASA launched the Chandra X-ray Observatory. Chandra produces much more detailed images than any other X-ray telescope. It has observed a wide range of astronomical objects, including especially hot or violent ones. Also in 1999, the ESA launched an X-ray observatory called XMM-Newton. This satellite’s telescopes can detect much fainter X rays than Chandra can, though with lower resolution. The main mission of XMM-Newton is to investigate the spectra of X-ray sources.
Gamma-ray astronomy.
Electromagnetic waves that have the shortest wavelengths—about 0.1 nanometer and shorter—are known as gamma rays. Gamma-ray photons have the highest energy in the electromagnetic spectrum. Thus, they form in the regions of the highest energy in the universe.
Gamma-ray sources include places where matter and antimatter are annihilating each other. Antimatter is matter composed of particles called antiparticles. Each antiparticle has the same mass as a corresponding particle of ordinary matter but with electric charge or certain other properties reversed. If a particle and its antiparticle collide, they annihilate each other, releasing gamma rays and other energy.
Astronomers have observed that matter-antimatter annihilation occasionally occurs near the center of the Milky Way. Other gamma-ray sources include the Crab Nebula, in the constellation Taurus, and a nearby collapsed star known as Geminga. The Crab Nebula consists of matter that was thrown out into space during a supernova observed in A.D. 1054.
In 2000, an international team of research centers launched the High Energy Transient Explorer-2 (HETE-2) satellite to detect and locate gamma-ray bursts. HETE-2 carried gamma-ray and X-ray detectors. When the satellite detected a gamma-ray burst, it located the source and relayed this information to a control center at the Massachusetts Institute of Technology in Cambridge. The center quickly sent the data to ground-based observers so that they could study the source with optical telescopes.
In March 2003, HETE-2 sensed a bright gamma-ray burst in the direction of the constellation Leo that lasted 25 seconds. Telescopes on the ground spotted leftover light from the burst, which showed evidence that the burst had come from an extremely powerful supernova 2 billion light-years from Earth. Astronomers believe that many gamma-ray bursts may come from supernovae.
The ESA’s INTEGRAL (International Gamma-Ray Astrophysics Laboratory) satellite went into orbit in October 2002. Researchers have used INTEGRAL to study black holes, neutron stars (collapsed stars without enough mass to become black holes), the highly energetic centers of certain galaxies, and supernovae.
INTEGRAL has an unusual orbit—its distance from Earth varies from 5,600 to 96,000 miles (9,000 to 155,000 kilometers). Scientists selected that orbit so that INTEGRAL would spend most of its time far above the Van Allen belts, bands of charged particles that surround Earth. INTEGRAL needs to be above the belts because their particles emit radiation that could interfere with the satellite’s sensitive detectors.
In 2004, an international team led by scientists from the United States, the United Kingdom, and Italy launched a satellite called Swift. It was designed to help astronomers determine the origins of gamma-ray bursts. It uses a wide-angle telescope to detect gamma-ray bursts, then quickly adjusts its position to point more precise instruments directly at the source of the burst.
In 2005, four teams of astronomers led by scientists from the United States and Denmark reported that they had discovered the origin of short gamma-ray bursts lasting less than 2 seconds. The scientists used the HETE-2 and Swift satellites to determine that the bursts were caused by the collisions of two collapsed stars, such as black holes or neutron stars.
NASA launched the Fermi Gamma-ray Space Telescope in 2008. This orbiting observatory was designed to be more sensitive and to gather data faster than previous gamma-ray instruments. Fermi has discovered a number of pulsars emitting gamma rays along with many gamma-ray bursts. In 2009, scientists announced that Fermi had detected the presence of antimatter particles coming from lightning strikes on Earth.
Neutrino astronomy.
A type of particle that arrives from outer space is the neutrino. Neutrinos rarely interact with particles on Earth. Neutrino detectors therefore use large amounts of matter as targets for the neutrinos. One detector, known as Super-Kamiokande, is deep underground in a mine in Japan. Its main part is a cylindrical tank of water 131 feet (40 meters) deep and 131 feet in diameter. Electronic devices in the detector can sense flashes of light produced when a neutrino collides with an atomic nucleus or an electron in the water. Super-Kamiokande began operating in 1996, but all its phototubes that detect the bursts of light had to be replaced after an accident in 2001 destroyed all the detectors.
The most sensitive neutrino detector is the Sudbury Neutrino Observatory (SNO), in a mine near Greater Sudbury, Ontario. The facility uses 1,000 metric tons (1,100 tons) of heavy water. In heavy water, the nucleus of each hydrogen atom consists of a proton and a neutron, instead of only a proton. SNO began taking measurements in 1999. Other neutrino detectors operate in Italy and in Russia.
In the 1960’s, scientists discovered that certain neutrinos were “missing.” These neutrinos were electron-neutrinos created by nuclear reactions in the sun. Electron-neutrinos are one of three known neutrinos. The others are muon-neutrinos and tau-neutrinos. Scientists measured only about one-half to one-third of the expected number of electron-neutrinos from the sun.
Physicists suspected that the missing electron-neutrinos turn into muon-neutrinos and tau-neutrinos on their way to Earth. In 2001, scientists at Sudbury claimed that a comparison of measurements made there and at Super-Kamiokande shows that the transformation does occur. For such a transformation to occur, neutrinos, previously thought to be without mass, must have mass. Thus, this result led to a major change in physics.
Cosmic-ray astronomy.
Cosmic rays are electrically charged, high-energy particles. There are two kinds of cosmic rays: (1) primary cosmic rays, often called primaries, which originate in outer space; and (2) secondary cosmic rays, or secondaries, which form in Earth’s atmosphere. Secondaries originate when primaries collide with atoms at the top of the atmosphere.
Most primaries are protons or other nuclei of atoms. They do not usually penetrate the atmosphere. Astronomers therefore use instruments aboard high-flying airplanes or satellites to detect them. Secondaries can reach low altitudes. A small fraction of them even strike Earth’s surface, where special sensors can detect them.
Some primary cosmic rays come from the sun, but most of them are galactic cosmic rays, which originate outside the solar system. Many galactic cosmic rays have tremendous energy. Astronomers do not know how they acquire this energy—though it may come from supernova explosions.
Gravitational-wave astronomy.
Astronomers have built equipment to detect another type of radiation, gravitational waves. These waves are predicted by the general theory of relativity announced in 1915 by German-born American physicist Albert Einstein. Researchers found indirect evidence for them in certain variations in the orbits of two dense stars that revolve about each other. In 2016, gravitational waves were detected for the first time by the Laser Interferometer Gravitational-Wave Observatory (LIGO), a pair of detectors located in Louisiana and in Washington state.
Direct sampling
is the examination of pieces of material from celestial objects. In their examinations, scientists often use techniques of geology, including chemical analysis. The most common samples are meteorites, rocks that fell through the atmosphere from farther out in our solar system. Thousands of meteorites have been found. Most come from asteroids. A handful come from the moon or Mars. The best place to find meteorites is Antarctica. They show up on the polar ice much more clearly than they do among ordinary rocks elsewhere.
Scientists have also studied hundreds of kilograms of moon rocks brought to Earth by astronauts from 1969 to 1972. In 1970, the Soviet Union’s Luna 16 spacecraft, a remotely controlled vehicle, returned small samples of soil from the moon. In addition, researchers have analyzed bits of space dust collected by devices on high-altitude aircraft. They have determined that some of the dust came from beyond our solar system.
From 2001 to 2004, NASA’s Genesis spacecraft gathered samples of the solar wind, a continuous flow of particles from the sun. Genesis returned to Earth in September 2004. Also in 2004, NASA’s Stardust spacecraft passed near Comet Wild 2 and captured particles from its coma, a cloud of gas and dust surrounding the comet’s core. Stardust dropped a capsule containing the samples to Earth in 2006.
Computer modeling.
Astronomers use computers to build scientific models (sets of mathematical equations) that represent certain processes, such as the formation of a star. After entering the equations into a computer, an astronomer inserts numbers into the equations. The computer then simulates (represents) how the process would develop. In some cases, the computer produces a moving picture on a computer screen. The picture can run much faster than the actual process. This kind of model can help astronomers because many important processes occur much too slowly for astronomers to observe. Other important processes that can be simulated occur in inaccessible places, such as the interiors of stars.
Space probes
launched from Earth have explored the sun, the moon, and all the other planets in the solar system. They have also visited asteroids, comets, and the other planets’ satellites. The probes carry scientific instruments that gather information and transmit data back to Earth. Probes have landed on the surface of the moon, Venus, Mars, and Saturn’s moon Titan. See Space exploration (Space probes).
Learning about astronomy
It is easy for students and the general public to take part in astronomy. Amateur astronomers range from individuals who make casual nighttime or solar observations to people for whom astronomy is a serious pursuit. Amateur astronomers have their own associations and local and regional clubs. Some of the larger clubs hold “star parties” at which members set up their own telescopes and observe the skies.
Amateur astronomers make major contributions to various branches of astronomy. For example, the American Association of Variable Star Observers, whose headquarters are in Cambridge, Massachusetts, collects observations from amateurs throughout the world and puts them together. The association shares the data with professional observers. Other amateur astronomers search for new comets or supernovae.
Much technical information is available to amateur astronomers. Books on observing the heavens contain tables that indicate where stars and planets can be found each month. Many cities have planetariums, which can present shows demonstrating the movements of celestial objects. Some planetariums sponsor lectures by professional astronomers. Computer programs available on DVD or over the Internet simulate sky conditions for any location, date, and time. Maps and images of the sky are also available on the Internet.
History
The roots of astronomy extend back to the dawn of civilization. More than 4,000 years ago, in what is now England, several generations of people built Stonehenge, an “observatory” consisting of huge, cut stones arranged in circles. Certain stones and alignments of stones appear to mark locations of astronomical importance, such as the point at which the sun rose on the longest day of the year. Stonehenge was apparently also a place of worship.
Some of today’s constellations have their roots in patterns in the sky noted by the Sumerians in perhaps 2000 B.C. Chinese constellation patterns, which are largely different from those used in Europe, may also date from that time. Babylonian tablets show that astronomers there were noting the positions of the moon and planets by 700 B.C. The Babylonians also noted eclipses.
The ancient Greeks carried forward ideas from the Babylonians and invented some of their own. Aristotle’s system of physics and astronomy, developed in the 300’s B.C., survived for almost 2,000 years. Aristotle formalized a geocentric system of astronomy, in which Earth was the center of the universe. During the A.D. 100’s, Ptolemy modified Aristotle’s system to account for the retrograde motion of the planets. Ptolemy also maintained that Earth was the center of the universe, however.
Developing the modern view.
By the early 1500’s, Nicolaus Copernicus had developed a theory in which Earth and the other planets revolved about the sun—the heliocentric model of the solar system. In the early 1600’s, Johannes Kepler analyzed precise measurements of planetary positions that had been made by Tycho Brahe. Kepler then developed three laws that correctly describe the shapes of the orbits of planets, indicate how rapidly a planet moves at various times of its year, and account for the length of the planet’s year.
Kepler based his laws of planetary motion on observations, but the English astronomer and mathematician Isaac Newton proved them mathematically. Newton’s work, usually called Principia mathematica (1687), set forth not only laws of motion but also the law of gravity that is still in general use.
By Newton’s time, the field of optical astronomy had already begun. In 1609, Galileo heard that an optical device had been built that made distant objects appear closer. He soon built his own telescope. The discoveries Galileo made with this instrument backed the Copernican theory over the theories of Aristotle and Ptolemy. In 1616, however, the Roman Catholic Church warned Galileo not to teach that Earth revolves about the sun. A book of Galileo’s published in 1632 was interpreted as a violation of the ban, and Galileo was put under house arrest. Only in 1992 did the Catholic Church confirm that Galileo should not have been tried or convicted, though he was not pardoned.
Finding new objects.
The British astronomer William Herschel discovered a new object in the sky in 1781. At first, he thought the object was a comet. It turned out to be a planet—later named Uranus. It was the first planet discovered since ancient times.
In 1845, astronomers John C. Adams of the United Kingdom and Urbain Le Verrier of France declared that the gravitational influence of an unknown planet was affecting the orbit of Uranus. Using one of their predictions the next year, the German astronomer Johann G. Galle found the planet, Neptune.
The American astronomer Clyde W. Tombaugh discovered Pluto in the 1930’s. Tombaugh found Pluto on photos he had taken using a wide-angle telescope at Lowell Observatory in Flagstaff, Arizona. Astronomers considered Pluto a planet until 2006. Then, the discovery of Eris, an object about the same size as Pluto in the outer solar system, led to a new class of nearly planet-sized objects called dwarf planets. The first bodies to be classified as dwarf planets include Pluto, Eris, Haumea, and Makemake. A team led by the American astronomer Michael E. Brown announced the discovery of Eris, Makemake, and Haumea in 2005. Ceres, the largest asteroid, is also called a dwarf planet.
In the early 2000’s, Brown and other astronomers began to discover an increasing number of large objects whose orbits lay beyond the orbit of Neptune. Some of these bodies are similar in size to Pluto. They appear to be part of a region of objects called the Kuiper belt.
Discovering other galaxies.
In the early days of optical astronomy, the fuzzy regions of the sky became known as nebulae—Latin for clouds. When viewed through the telescopes then available, the nebulae resembled comets. Someone who was trying to discover comets could easily mistake a nebula for a comet. To prevent such errors, the French astronomer Charles Messier made a list from the 1750’s to 1784 of the most prominent nebulae. This Messier catalog now contains 110 objects, known by their Messier numbers.
In Messier’s time, no one knew what the nebulae were. But in the mid-1800’s, Lord Rosse of Ireland built a telescope whose superior light-gathering power enabled him to discover that many nebulae have spiral shapes. The telescope’s mirror measured about 6 feet (1.8 meters) across—gigantic for that time.
It took decades to discover what the spiral nebulae were. The answer came only in 1924 with the discovery by Edwin Hubble that the nebulae are so far away that they must be beyond the Milky Way. Astronomers concluded that they are independent galaxies. The “nebula” with Messier number M31, for example, is actually the Andromeda Galaxy. Astronomers now use nebula to mean a cloud of dust and gas. The remaining objects in the Messier catalog are star clusters, groups of closely placed stars.
Advances in astrophysics.
By the end of the 1930’s, the German-born physicist Hans Bethe, working at Cornell University in Ithaca, New York, had suggested how nuclear fusion powers the stars. For example, a process known as the proton-proton chain powers the sun. In this process, six protons come together in several steps to produce a helium nucleus and two protons. The final products contain slightly less mass than the original ingredients. The missing matter is converted to energy according to Albert Einstein’s formula E = mc 2. In the formula, E is energy, m is the mass that is no longer present, and c 2 is the speed of light multiplied by itself.
The American physicist William A. Fowler and the British astronomers Geoffrey Burbidge, Margaret Burbidge, and Fred Hoyle showed in the 1950’s how nuclear reactions could have built up all but the lightest chemical elements. Astronomers now know that the lighter elements formed minutes after the big bang, the event that began the universe. Moderately heavy elements formed inside stars. The heaviest elements formed in supernova explosions.
In 1965, the American physicists Arno Penzias and Robert Wilson discovered faint radio radiation coming equally from all directions in space. Scientists showed that the radiation was emitted about 300,000 years after the big bang and has been cooling ever since. Its temperature is now about 3 Kelvin (3 Celsius degrees above absolute zero).
For four years starting in 1989, instruments aboard the Cosmic Background Explorer (COBE) satellite measured more precisely the temperature of the radiation detected by Penzias and Wilson. Another instrument aboard the satellite found small variations in the temperature from one location in the sky to another. These so-called ripples in space may be the “seeds” from which the galaxies and clusters of galaxies grew long ago. These ripples were mapped in more detail by the Wilkinson Microwave Anisotropy Probe starting in 2001 and the Planck spacecraft starting in 2009.
Finding quasars.
In 1963, the Dutch-born American astronomer Maarten Schmidt identified the starlike objects now known as quasars. He showed that the spectra of quasars have huge redshifts, indicating that the spectra are produced by powerful energy sources in distant galaxies. Most of them are billions of light-years away. Astronomers have since identified quasars as extremely bright regions that appear to get their energy from supermassive black holes in the centers of distant galaxies.
Finding pulsars.
On a much smaller scale, in 1967, the British astronomer Jocelyn Bell Burnell identified a new type of object in radio observations she was making as part of her Ph.D. thesis. These objects emit radio waves that arrive at Earth in regular pulses about 1 second apart. The objects came to be known as pulsars. Later work showed that pulsars are rapidly spinning neutron stars. With every spin, a narrow beam of radio waves sweeps over Earth, producing a pulse. Astronomers have found pulsars that pulse as often as 600 times per second.
Advances in cosmology.
Perhaps the biggest discovery in cosmology since Edwin Hubble’s expanding universe was the discovery in the late 1990’s by two teams that the expansion of the universe is speeding up. Many scientists had thought that the expansion was slowing down due to the gravity of all the objects in the universe. But studies of distant supernovae have showed them to be fainter than expected, a sign that they are farther away than predicted. An unknown force that is transmitted in an unknown way makes the universe expand more rapidly. Scientists have given this force the name dark energy and are trying to determine its nature. At least two-thirds of the energy in the universe consists of dark energy.
Careers
What astronomers do.
Most professional astronomers work at observatories or research institutes or teach and conduct research at colleges and universities. Planetariums employ astronomers to lecture and conduct classes for the public. A few astronomers work for companies that build equipment for scientific satellites and space probes. Others work for firms that do such work as monitoring the environment from space.
Becoming an astronomer.
The most important characteristic for a person who wishes to become an astronomer is a powerful spirit of inquiry. The person should also have a strong ability to learn mathematics.
High-school students who are interested in becoming astronomers should take as many math courses as they can to prepare for college mathematics and physics. A high-school physics course is also useful. Some branches of astronomy deal more with chemistry or geology than physics, so courses in those subjects can also help. Visits to planetariums and science museums as well as participation in an amateur astronomy club can help prepare a student for a career in astronomy.
To conduct research and teach astronomy at the college level requires a Ph.D. degree. Students usually take about six years to obtain this degree after receiving their bachelor’s degree. During most of this time, Ph.D. students perform research. After obtaining their degree, most astronomers take postdoctoral positions for two or more years before searching for permanent jobs.
Astronomy associations.
Astronomers from throughout the world gather every three years at the General Assembly of the International Astronomical Union. Professionals in the United States and Canada belong to the American Astronomical Society. Both professional and amateur astronomers may join the Astronomical Society of the Pacific, which has a major function in education. Many countries also have organizations devoted to astronomy, such as the Astronomical Society of India, the Royal Astronomical Society of Canada, and the United Kingdom’s Royal Astronomical Society.
