The solar system consists of the sun plus all the objects that orbit it. With more than 99 percent of the solar system's total mass and a diameter more than 100 times that of Earth and 10 times that of Jupiter, the sun is quite naturally the center of the system. The spectrum, brightness, mass, size, and age of the sun and of nearby stars indicate that the sun is a typical star. Like most stars, the sun produces energy by thermonuclear processes that take place at its core. This energy maintains the conditions needed for life on Earth.
As has been mentioned above, Earth is not the only body to circle the sun. Many chunks of matter, some much larger than Earth and some microscopic, are caught in the sun's gravitational field. Eight of the largest of these chunks are called planets. Earth is the third planet from the sun. The smaller chunks of matter include dwarf planets, natural satellites (moons), asteroids, comets, meteoroids, and the molecules of interplanetary gases.
Kepler's Laws of Planetary Motion
To find these laws, Kepler had to effectively make a scale drawing of the solar system. He did this using extremely accurate observations collected by his deceased former employer Tycho Brahe. Kepler used a relative distance scale in which the average distance from Earth to the sun was called one astronomical unit.
Kepler did not have a particularly accurate value for the astronomical unit. To help find this distance, later astronomers were able to use methods such as parallax, an apparent shift in an object's position due to a difference in the observer's position (discussed below). Even more advanced methods have determined that Earth's average distance from the sun is in fact 92,955,808 miles (149,597,870 kilometers).
Newton's Law of Universal Gravitation Kepler's laws described the positions and motions of the planets with great accuracy, but they did not explain what caused the planets to follow those paths. If the planets were not acted on by some force, scientists reasoned, they would simply continue to move in a straight line past the sun and out toward the stars. Some force must be attracting them to the sun.
The Planets
Uranus' motion did not follow the exact path predicted by Newton's theory of gravitation. This problem was happily resolved by the discovery of an eighth planet, which was named Neptune. Two mathematicians, John Couch Adams and Urbain-Jean-Joseph Le Verrier, had calculated Neptune's probable location, but it was the German astronomer Johann Gottfried Galle who located the planet, in 1846.
Even then some small deviations seemed to remain in the orbits of both planets. This led to the search for yet another planet, based on calculations made by the U.S. astronomer Percival Lowell. In 1930 the U.S. astronomer Clyde W. Tombaugh discovered the object that became known as Pluto.
Pluto is an icy body that is smaller than Earth's moon. The mass of Pluto has proved so small—about 1/500 of Earth's mass—that it could not have been responsible for the deviations in the observed paths of Uranus and Neptune. The orbital deviations, however, had been predicted on the basis of the best estimates of the planets' mass available at that time. When astronomers recalculated using more accurate measurements taken by NASA's Voyager 2 spacecraft in 1989, the deviations “disappeared.”
For some 75 years astronomers considered Pluto to be the solar system's ninth planet. This tiny distant body was found to be unusual for a planet, however, in its orbit, composition, size, and other properties. In the late 20th century astronomers discovered a group of numerous small icy bodies that orbit the sun from beyond Neptune in a nearly flat ring called the Kuiper belt (discussed in full below). Many of Pluto's characteristics seem similar to those of Kuiper belt objects. Several of those objects are roughly the same size as Pluto, and one, named Eris, is known to be larger. In 2006 the International Astronomical Union, the organization that approves the names of celestial objects, removed Pluto from the list of planets. Instead, it made Pluto the prototype of a new category of objects, called dwarf planets. Pluto is also considered one of the larger members of the Kuiper belt. (See also dwarf planet; planet.)
Except for Venus and Uranus, each planet rotates on its axis in a west-to-east motion. In most cases the spin axis is nearly at a right angle to the plane of the planet's orbit. Uranus, however, is tilted so that its spin axis lies almost in its plane of orbit.
The planets can be divided into two groups. The inner planets—Mercury, Venus, Earth, and Mars—lie between the sun and the asteroid belt. They are dense, rocky, and small. Since Earth is a typical inner planet, this group is sometimes called the terrestrial, or Earth-like, planets.
The outer planets—Jupiter, Saturn, Uranus, and Neptune—lie beyond the asteroid belt. They are also called the Jovian, or Jupiter-like, planets. These are much larger and more massive than the inner planets. Jupiter has 318 times Earth's mass and in fact is more massive than all the other planets combined. Being made mostly of hydrogen and helium (mainly in liquid forms), the Jovian planets are also much less dense than the inner planets.
Natural Satellites
Many of the natural satellites are fascinating worlds in their own right. Jupiter's moon Io has numerous active volcanoes spewing sulfur compounds across its surface. Europa, Jupiter's next moon out, may well have a vast ocean of liquid water underneath its icy crust. Neptune's Triton has mysterious geysers erupting in spite of frigid surface temperatures near − 400° F (− 240° C).
Asteroids
Comets, the Oort Cloud, and the Kuiper Belt
Comets apparently originate beyond the orbit of Neptune. At such distances from the sun, they maintain very low temperatures, preserving their frozen state. They become easily visible from Earth only if they pass close to the sun. As a comet approaches the sun, some of its ices evaporate. The solar wind pushes these evaporated gases away from the head of the comet and away from the sun. This temporarily gives the comet one or more long, glowing tails that point away from the sun.
Other comets' orbits have been traced out to tens of thousands of astronomical units and have periods of millions of years. Some of these comets may in fact be making their first ever visits to the inner solar system. Such considerations led Jan Oort in 1950 to suggest the existence of a vast, spherical cloud, containing perhaps billions of comets. Disturbances such as the gravitational influence of passing stars could deflect these comets toward the sun.
Gerard P. Kuiper proposed in 1951 that another group of icy bodies, including dormant comets, might exist in a belt just outside Neptune's orbit. Discoveries starting in the 1990s have confirmed Kuiper's hypothesis, as hundreds of objects have been found at about the distance he predicted. The belt is thought to contain many millions of icy objects, most of them small. However, the largest Kuiper belt objects, Eris and Pluto, are massive enough to also be considered dwarf planets.
Current thinking suggests that many of the short-period comets, or those that complete an orbit in less than 200 years, may have originated in the Kuiper belt. They were perhaps directed into the inner solar system by collisions with each other and gravitational encounters with Neptune. Long-period comets are thought to originate in the Oort cloud (whose existence is considered highly probable, but not proven). The cloud may have been produced long ago, as icy bodies near and inside Neptune's orbit were thrown far out from the sun by gravitational encounters with the outer planets.
The Origin and Future of the Solar System The most widely accepted model for the origin of the solar system combines theories elaborated by Kuiper and Thomas Chrowder Chamberlin. Astronomers believe that about 4.6 billion years ago, one of the many dense globules of gas and dust clouds that exist in the galaxy contracted into a slowly rotating disk called the solar nebula. The hot, dense center of the disk became the sun. The remaining outer material cooled into small particles of rock and metal that collided and stuck together, gradually growing into larger bodies to become the planets and their satellites.
In the cold outer parts of the new solar system, some of these bodies collected large amounts of hydrogen and helium from the solar nebula, thus becoming the “gas giants”—Jupiter, Saturn, Uranus, and Neptune. Closer to the sun, these light elements were mostly driven off by the higher temperatures and particles streaming off the sun. Smaller, rocky planets—Mercury, Venus, Earth, and Mars—developed there. Uncollected debris became asteroids and (in the outer regions) comets.
The sun is slowly getting brighter as it consumes its reservoir of hydrogen and turns this into helium. If current computations of stellar evolution are correct, the sun will grow much brighter and larger in about 5 billion years, making Earth much too hot for life to endure. Later the sun will have exhausted its nuclear energy source and will begin to cool. In the end it will become a white dwarf star, with all its matter packed densely into a space not much bigger than Earth. Around it will orbit frozen wastelands, the planets that survived the solar upheavals.
Does Life Exist Elsewhere?
Mars is an intriguing place to look for life. Spacecraft have photographed large features that appear to be dry riverbeds. Data from NASA's Spirit and Opportunity rovers in the early 2000s strongly suggest that liquid water once existed on the planet's surface. Also, data from the European Mars Express orbiter and from Earth-based telescopes suggest that methane is being released from beneath the surface, and a possible source for this could be subsurface colonies of bacteria.
Discoveries of life existing in extreme or unusual environments on Earth—such as in hot bedrock miles beneath the surface and in colonies near volcanic vents on the deep sea floor—have widened prospects for finding life elsewhere. No place in the solar system other than Earth, however, is easily suitable for human colonization or for large land plants or animals. It is possible that other stars may be orbited by more Earth-like planets. In fact, the number of such worlds in the universe may be truly enormous. However, the only place life has been found so far is on Earth. One example is very little to go on, especially if we are part of the example. With little information regarding the likelihood of life arising in other places, even under Earth-like conditions, discussion of life elsewhere remains speculative.
The Stars
Coordinate Systems Astronomers need to record the exact locations of stars. Within limits, it is useful to locate objects within constellations. Numerical coordinate systems are used to record the locations of celestial objects more precisely. These systems are like the coordinate system of latitude and longitude used for Earth.
The horizon, or azimuth, system is based on Earth's north-south line and the observer's horizon. It uses two angles called azimuth and altitude. Azimuth locates the star relative to the north-south line, and altitude locates it relative to the plane of the horizon. For this system to be useful, the time of the observation and the location from which the observation was made must be accurately known.
In the equator system the position of a star is given by coordinates called declination and right ascension. The declination locates the star by its angular distance north or south of the celestial equator. The right ascension locates the star by its angular distance east or west of the vernal equinox. Since this system is attached to the celestial sphere, all points on Earth (except the poles) are continually changing their positions under the coordinate system.
Determining the Distance to Stars Fixing stars on an imaginary sphere is useful for finding them from Earth, but it does not reveal their actual locations. One way to measure the distances of nearby stars from Earth is the parallax method.
More than four centuries ago the phenomenon of parallax was used to counter Nicolaus Copernicus' suggestion that Earth travels around the sun. Scientists of the time pointed out that if it did, stars should show an annual change in direction due to parallax. But, using the instruments available to them, they were unable to measure any parallax, so they concluded that Copernicus was wrong. Astronomers now know that the stars are all at such tremendous distances from Earth that their parallax angles are extremely difficult to measure. Even modern instruments cannot measure the parallax of most stars.
Astronomers measure parallaxes of stars in seconds of arc. This is a tiny unit of measure; for example, a penny must be 2.5 miles (4 kilometers) away before it appears as small as one second of arc. Yet no star except the sun is close enough to have a parallax that large. Alpha Centauri, a member of the group of three stars nearest to the sun, has a parallax of about three quarters of a second of arc.
Astronomers have devised a unit of distance called the parsec—the distance at which the angle opposite the base of a triangle measures one second of arc when the base of the triangle is the radius of Earth's orbit around the sun. One parsec is equal to 19.2 trillion (19.2 × 1012) miles (30.9 trillion kilometers). Alpha Centauri is about 1.3 parsecs distant.
Another unit used to record large astronomical distances is the light-year. This is the distance that light travels within a vacuum in one year—about 5.88 trillion miles (9.46 trillion kilometers). Proxima Centauri, part of the Alpha Centauri system, is the star closest to Earth (apart from the sun), yet it is about 4.3 light-years distant. Light takes more than four years to reach Earth from that distance.
Since parallax yields distances to only relatively nearby stars, other methods must be used for more distant ones. One of these methods is statistical parallax, in which the apparent motions across the sky of groups of stars are analyzed to determine their probable distance. Another method involves observing certain stars that vary regularly in brightness (discussed below).
Size and Brightness of Stars
Astronomers express the brightness of a star in terms of its magnitude. Two values of magnitude describe a star. The apparent magnitude refers to how bright the star looks from Earth. The absolute magnitude of a star is the value its apparent magnitude would have if the star were 10 parsecs from Earth. The apparent magnitude of a star depends on its size, temperature, and distance. The temperature is found from its spectrum; if the distance is known, then astronomers can calculate the size of the star and also assign a value for its absolute magnitude. The actual brightness of stars may be compared using their absolute magnitudes.
Certain stars whose brightness varies regularly provide an important way for astronomers to estimate the distances of remote galaxies. In such stars the actual brightness (absolute magnitude) is closely related to the period of their brightness variations. Astronomers can use the observed period to determine the actual brightness and then compare this with the apparent brightness to estimate the distance.
Astronomers have discovered all kinds of stars—from huge, brilliant red supergiants more than 100 times the sun's diameter to extremely dense neutron stars only about a dozen miles across. The sun lies in about the middle range of size and brightness of stars. The largest stars are the cool, reddish supergiants: they have low surface temperatures, but they are so bright that they must be extremely large to give off that much energy. White dwarf stars, on the other hand, are very faint in spite of their high surface temperatures and thus must be very small—only about the size of Earth.
What Is a Star? Astronomers have found, using analysis of stars' spectra, that stars are made mostly of the simplest elements: hydrogen and helium. These elements are in the gaseous state. In most of the star, however, the temperature is so high (thousands to millions of degrees) that the gas is ionized (with electrons stripped away from the atomic nuclei)—a state called plasma.
The mutual gravitational attraction of a star's matter is what forces it into a roughly spherical shape. In fact, if there were nothing to counteract this inward force, the star would collapse to a very small size. The gravitational squeezing of the gas, however, heats it to very high temperatures. In the 1800s astronomers believed that this compression was actually the energy source for a star. This presented a problem. The sun could shine like this for only a few million years without shrinking so much that conditions on Earth would be greatly altered. Yet geological and biological evidence suggested that Earth has maintained the conditions for life for hundreds of millions of years.
The 20th century brought a solution to this problem. With the discovery of nuclear energy, astronomers could explain the sun's long-lasting power output as the result of nuclear fusion: hydrogen deep inside the sun was being fused together to form helium. This process is so energetic that it can counterbalance the inward force of gravity. Stars, then, are essentially battlegrounds between two forces—the inward crush of gravity and the outward pressure from the heat generated by nuclear fusion.
The Lives of Stars
This cannot last forever, though, as eventually most of the hydrogen “fuel” is converted into helium. In the largest stars, this takes only a few million years. Very-low-mass stars, with less gravitational pressure to battle, consume their fuel very slowly and may last a trillion years. The sun is intermediate, with an estimated lifetime of about 10 billion years, which it is believed to be almost halfway through.
Stars born with much more mass than the sun undergo even more dramatic events. Under tremendous pressure, such a star performs numerous additional fusion reactions in its core, producing a wide range of elements, up to and including iron. At this point, the ultradense core can collapse suddenly, leading to a colossal explosion called a supernova. Many such events have been observed from Earth, some so bright that they were visible in broad daylight. For a few weeks the exploding star can outshine an entire galaxy of a hundred billion stars. The elements thrown out into space can become part of nebulae, eventually to be incorporated into future generations of stars and planets.
Neutron Stars and Black Holes After some types of supernova explosions, an extremely dense core remains. This object, called a neutron star, is about the mass of the sun and is made mostly of neutrons. Its matter is so compact that a teaspoon of it has the mass of a small mountain. Some neutron stars spin rapidly while beaming radiation into space. If a beam intercepts Earth, astronomers may detect it as a series of pulses of radio waves or sometimes radiation at other wavelengths. Such a neutron star is referred to as a pulsar.
Often, neutron stars and black holes are detectable only because of their effects on nearby companion stars. Gas (mainly hydrogen) is drawn off the companion star and then swirls rapidly down onto (or into) the neutron star or black hole. The violent compressional heating and acceleration of the gas causes it to emit X-rays, which can be detected from Earth-based satellites. Such double star systems are called X-ray binaries.
Planets of Other Stars Astronomers have long thought that, like the sun, many or most stars should be accompanied by orbiting planets. These planets would be so distant from Earth, however, that their very faint light would be drowned out by the bright light of their “suns.” It turns out that there are indirect methods of detecting such planets. An orbiting planet would cause a star to wobble slightly, and this wobble could be detected as alternating red and blue Doppler shifts of the star's light. Furthermore, the speed and period of the wobble could enable astronomers to estimate the planet's mass and distance from the star. This technique was first successfully used in 1995 to find a planet orbiting the star 51 Pegasi. During the next 10 years, about 140 extrasolar planets were discovered in this way (plus a few by other means, such as the dip in light caused when a planet passes in front of a star).
Most of the planets found so far are at least as massive as Jupiter, yet they are closer to their stars than Mercury is to the sun. Such close-in, massive planets should be the easiest to detect, since they cause the greatest wobbles. But they are still a challenge to explain. Current theories of planet formation suggest that such large planets should form farther from the star, where temperatures are cold enough to allow collection of large amounts of gas. One possibility astronomers are considering is that these “hot Jupiters” formed farther out from their stars and migrated inward. This raises the question, though, of why our solar system has not experienced such planetary migration.
Interstellar Matter
Dust grains block blue light more than red light, so the color of a star can appear different if it is seen through much dust. To find the temperature of such a star, astronomers must estimate its color to be bluer than it appears because so much of its blue light is lost in the dust. When clouds of dust occur near bright stars they often reflect the starlight in all directions. Such clouds are known as reflection nebulae.
Interstellar gas is about 100 times denser than the dust but still has an extremely low density. The gas does not interfere with starlight passing through it, so it is usually difficult to detect. When a gas cloud occurs close to a hot star, however, the star's radiation causes the gas to glow. This forms a type of bright nebula known as an H II region. Away from hot stars interstellar gas is quite cool. Masses of this cool gas are called H I regions.
The hydrogen occurs partly as single atoms and partly as molecules (two hydrogen atoms joined together). Molecular hydrogen is even more difficult to detect than atomic hydrogen, but it must exist in abundance. Other molecules have been found in the interstellar gas because they give off low-frequency radiation. These molecules contain other atoms besides hydrogen: oxygen or carbon occurs in hydroxyl radicals (OH−) and in carbon monoxide (CO), formaldehyde (H2CO), and many others, including many organic molecules.
The Galaxies
Radio Galaxies, Quasars, and Dark Matter Galaxies were long thought to be more or less passive objects, containing stars and interstellar gas and dust and shining by the radiation that their stars give off. When astronomers became able to make accurate observations of radio frequencies coming from space, they were surprised to find that a number of galaxies emit large amounts of energy in the radio region. Ordinary stars are so hot that most of their energy is emitted in visible light, with little energy emitted at radio frequencies. Furthermore, astronomers were able to deduce that this radiation had been given off by charged particles of extremely high energy moving in magnetic fields.
Astronomers have found that, in many galaxies, stars near the center move very rapidly, apparently orbiting some very massive unseen object. The most likely explanation is that a giant black hole, with millions or even billions of times the sun's mass, lurks in the center of most large galaxies. As stars and gas spiral into these black holes, much of their mass vanishes from sight. The violent heating and compression produces a huge release of energy, including high-speed jets of matter (such as in M87).
Another problem has puzzled astronomers for years. Most, if not all, galaxies occur in clusters, presumably held together by the gravity of the cluster members. When the motions of the cluster members are measured, however, it is found in almost every case that the galaxies are moving too fast to be held together only by the gravity of the matter that is visible. Astronomers believe there must be a large amount of unseen matter in these clusters—perhaps 10 times as much as can be seen. While some of this likely consists of objects such as black holes and neutron stars, most of it is believed to be “exotic dark matter,” of unknown origin.
The Milky Way Galaxy Like most stars, the sun belongs to a galaxy. Since the sun and Earth are embedded in the galaxy, it is difficult for astronomers to obtain an overall view of this galaxy. In fact, what can be seen of its structure is a faint band of stars called the Milky Way (the word galaxy comes from the Greek word for “milk”). Because of this, the galaxy has been named the Milky Way galaxy.
Not all the galaxy's stars are confined to the galactic plane. There are a few stars that occur far above or below the disk. They are usually very old stars, and they form what is called the halo of the galaxy. Evidently the galaxy was originally a roughly spherical mass of gas. Its gravity and rotation caused it to collapse into the disklike shape it has today. The stars that had been formed before the collapse remained in their old positions, but after the collapse further star formation could occur only in the flat disk.
All the stars in the galaxy move in orbits around its center. The sun takes about 200 million years to complete an orbit. The orbits of most of these stars are nearly circular and are nearly in the same direction. This gives a sense of rotation to the galaxy as a whole, even as the entire galaxy moves through space.
Dark clouds of dust almost completely obscure astronomers' view of the center of the Milky Way galaxy. Radio waves penetrate the dust, however, so radio telescopes can provide astronomers with a view of the galactic nucleus. In that region stars travel in very fast, tight orbits—which implies the existence of a huge mass at the center. In addition, the Earth-orbiting Chandra X-ray Observatory has detected flares of X-rays, lasting only a few minutes, in the region. Most astronomers believe these findings are best explained by the existence of a black hole—3 million times the sun's mass but only about a dozen times the sun's diameter—that is violently accelerating and compressing in-falling blobs of matter.
The Universe Cosmology is the scientific inquiry into the nature, history, development, and fate of the universe. By making assumptions that are not contradicted by the behavior of the observable universe, scientists build models, or theories, that attempt to describe the universe as a whole, including its origin and its future. They use each model until something is found that contradicts it. Then the model must be modified or discarded.
Cosmologists usually assume that, except for small irregularities, the universe has a similar appearance to all observers (and the laws of physics are identical), no matter where in the universe the observers are located or in which direction they look. This unproven concept is called the cosmological principle. One consequence of the cosmological principle is that the universe cannot have an edge, for an observer near the edge would have a different view from that of someone near the center. Thus space must be infinite and evenly filled with matter, or the geometry of space must be such that all observers see themselves as at the center. Also, astronomers believe that the only motion that can occur, except for small irregularities, is a uniform expansion or contraction of the universe.
In the 1950s and 1960s there was a rival model, called the steady state theory. The basic assumption of steady state was a perfect cosmological principle, applying to time as well as position. The steady state theory stated that the universe must have the same large-scale properties at all times; it cannot evolve, but must remain uniform. Since the universe is seen to be expanding, which would spread the matter out thinner as time goes on, steady state suggested that new matter must be created to maintain the constant density. In the steady state theory galaxies are formed, they live and die, and new ones come along to take their places at a rate that keeps the average density of matter constant.
When astronomers observe an object at a great distance, they are seeing it as it looked long ago, because it takes time for light to travel. A galaxy viewed at a distance of a billion light-years is seen as it was a billion years ago. Distant galaxies do seem to be different from nearby galaxies. They seem closer together than nearby ones, contrary to steady state contentions but consistent with the view that the universe had a greater density in the past. Also, a faint glow of radiation has been discovered coming uniformly from all directions. Calculations show that this could be radiation left over from the big bang.
The History of Astronomy
In many early civilizations, astronomy was sufficiently advanced that reliable calendars had been developed. In ancient Egypt astronomer-priests were responsible for anticipating the season of the annual flooding of the Nile River. The Maya, who lived in what is now central Mexico, developed a complicated calendar system about 2,000 years ago. The Dresden Codex, a Mayan text from the 1st millennium AD, contains exceptionally accurate astronomical calculations, including tables predicting eclipses and the movements of Venus.
In China, a calendar had been developed by the 14th century BC. In about 350 BC a Chinese astronomer, Shih Shen, drew up what may be the earliest star catalog, listing about 800 stars. Chinese records mention comets, meteors, large sunspots, and novas.
The early Greek astronomers knew many of the geometric relationships of the heavenly bodies. Some, including Aristotle, thought Earth was a sphere. Eratosthenes, born in about 276 BC, demonstrated its circumference. Hipparchus, who lived around 140 BC, was a prolific and talented astronomer. Among many other accomplishments, he classified stars according to apparent brightness, estimated the size and distance of the moon, found a way to predict eclipses, and calculated the length of the year to within 61/2 minutes.
The most influential ancient astronomer historically was Ptolemy (Claudius Ptolemaeus) of Alexandria, who lived in about AD 140. His geometric scheme predicted the motions of the planets. In his view, Earth occupied the center of the universe. His theory approximating the true motions of the celestial bodies was held steadfastly until the end of the Middle Ages.
In medieval times Western astronomy did not progress. During those centuries Hindu and Arab astronomers kept the science alive. The records of the Arab astronomers and their translations of Greek astronomical treatises were the foundation of the later upsurge in Western astronomy.
The great Danish astronomer Tycho Brahe rejected Copernicus' theory. Yet his data on planetary positions were later used to support that theory. When Tycho died, his assistant, Johannes Kepler, analyzed Tycho's data and developed the laws of planetary motion. In 1687 Newton's law of gravitation and laws of motion explained Kepler's laws.
Meanwhile, the instruments available to astronomers were growing more sophisticated. Beginning with Galileo, the telescope was used to reveal many hitherto invisible phenomena, such as the revolution of satellites about other planets.
The development of the spectroscope in the early 1800s was a major step forward in the development of astronomical instruments. Later, photography became an invaluable aid to astronomers. They could study photographs at leisure and make microscopic measurements on them. Even more recent instrumental developments—including radar, telescopes that detect electromagnetic radiation other than visible light, and space probes and manned spaceflights—have helped answer old questions and have opened astronomers' eyes to new problems.
As has been mentioned above, Earth is not the only body to circle the sun. Many chunks of matter, some much larger than Earth and some microscopic, are caught in the sun's gravitational field. Eight of the largest of these chunks are called planets. Earth is the third planet from the sun. The smaller chunks of matter include dwarf planets, natural satellites (moons), asteroids, comets, meteoroids, and the molecules of interplanetary gases.
Kepler's Laws of Planetary Motion
- Kepler's theory of the solar system.
- Kepler's second law of planetary motion describes the speed of a planet traveling in an elliptical …
To find these laws, Kepler had to effectively make a scale drawing of the solar system. He did this using extremely accurate observations collected by his deceased former employer Tycho Brahe. Kepler used a relative distance scale in which the average distance from Earth to the sun was called one astronomical unit.
Kepler did not have a particularly accurate value for the astronomical unit. To help find this distance, later astronomers were able to use methods such as parallax, an apparent shift in an object's position due to a difference in the observer's position (discussed below). Even more advanced methods have determined that Earth's average distance from the sun is in fact 92,955,808 miles (149,597,870 kilometers).
Newton's Law of Universal Gravitation Kepler's laws described the positions and motions of the planets with great accuracy, but they did not explain what caused the planets to follow those paths. If the planets were not acted on by some force, scientists reasoned, they would simply continue to move in a straight line past the sun and out toward the stars. Some force must be attracting them to the sun.
- A painting shows Isaac Newton contemplating the force—gravity—that causes an apple to …
The Planets
- A montage shows the eight planets of the solar system plus Pluto, with the images placed right next …
Uranus' motion did not follow the exact path predicted by Newton's theory of gravitation. This problem was happily resolved by the discovery of an eighth planet, which was named Neptune. Two mathematicians, John Couch Adams and Urbain-Jean-Joseph Le Verrier, had calculated Neptune's probable location, but it was the German astronomer Johann Gottfried Galle who located the planet, in 1846.
Even then some small deviations seemed to remain in the orbits of both planets. This led to the search for yet another planet, based on calculations made by the U.S. astronomer Percival Lowell. In 1930 the U.S. astronomer Clyde W. Tombaugh discovered the object that became known as Pluto.
Pluto is an icy body that is smaller than Earth's moon. The mass of Pluto has proved so small—about 1/500 of Earth's mass—that it could not have been responsible for the deviations in the observed paths of Uranus and Neptune. The orbital deviations, however, had been predicted on the basis of the best estimates of the planets' mass available at that time. When astronomers recalculated using more accurate measurements taken by NASA's Voyager 2 spacecraft in 1989, the deviations “disappeared.”
For some 75 years astronomers considered Pluto to be the solar system's ninth planet. This tiny distant body was found to be unusual for a planet, however, in its orbit, composition, size, and other properties. In the late 20th century astronomers discovered a group of numerous small icy bodies that orbit the sun from beyond Neptune in a nearly flat ring called the Kuiper belt (discussed in full below). Many of Pluto's characteristics seem similar to those of Kuiper belt objects. Several of those objects are roughly the same size as Pluto, and one, named Eris, is known to be larger. In 2006 the International Astronomical Union, the organization that approves the names of celestial objects, removed Pluto from the list of planets. Instead, it made Pluto the prototype of a new category of objects, called dwarf planets. Pluto is also considered one of the larger members of the Kuiper belt. (See also dwarf planet; planet.)
- Many pieces of matter are held in the sun's enormous gravitational field. Together with the sun, …
Except for Venus and Uranus, each planet rotates on its axis in a west-to-east motion. In most cases the spin axis is nearly at a right angle to the plane of the planet's orbit. Uranus, however, is tilted so that its spin axis lies almost in its plane of orbit.
The planets can be divided into two groups. The inner planets—Mercury, Venus, Earth, and Mars—lie between the sun and the asteroid belt. They are dense, rocky, and small. Since Earth is a typical inner planet, this group is sometimes called the terrestrial, or Earth-like, planets.
The outer planets—Jupiter, Saturn, Uranus, and Neptune—lie beyond the asteroid belt. They are also called the Jovian, or Jupiter-like, planets. These are much larger and more massive than the inner planets. Jupiter has 318 times Earth's mass and in fact is more massive than all the other planets combined. Being made mostly of hydrogen and helium (mainly in liquid forms), the Jovian planets are also much less dense than the inner planets.
Natural Satellites
- Pluto, left, and its large moon Charon, right, appear in an image taken by the Hubble Space …
Many of the natural satellites are fascinating worlds in their own right. Jupiter's moon Io has numerous active volcanoes spewing sulfur compounds across its surface. Europa, Jupiter's next moon out, may well have a vast ocean of liquid water underneath its icy crust. Neptune's Triton has mysterious geysers erupting in spite of frigid surface temperatures near − 400° F (− 240° C).
- Saturn's moon Titan appears in a mosaic of nine images taken by the Cassini spacecraft and …
Asteroids
- Asteroid Ida and its tiny moon, Dactyl, are shown in an image taken by the Galileo spacecraft in …
Comets, the Oort Cloud, and the Kuiper Belt
- The Stardust spacecraft took this composite image of Comet Wild 2's nucleus during a flyby in 2004. …
Comets apparently originate beyond the orbit of Neptune. At such distances from the sun, they maintain very low temperatures, preserving their frozen state. They become easily visible from Earth only if they pass close to the sun. As a comet approaches the sun, some of its ices evaporate. The solar wind pushes these evaporated gases away from the head of the comet and away from the sun. This temporarily gives the comet one or more long, glowing tails that point away from the sun.
- Halley's comet passes near Earth about once every 76 years. The photograph was taken via telescope …
Other comets' orbits have been traced out to tens of thousands of astronomical units and have periods of millions of years. Some of these comets may in fact be making their first ever visits to the inner solar system. Such considerations led Jan Oort in 1950 to suggest the existence of a vast, spherical cloud, containing perhaps billions of comets. Disturbances such as the gravitational influence of passing stars could deflect these comets toward the sun.
Gerard P. Kuiper proposed in 1951 that another group of icy bodies, including dormant comets, might exist in a belt just outside Neptune's orbit. Discoveries starting in the 1990s have confirmed Kuiper's hypothesis, as hundreds of objects have been found at about the distance he predicted. The belt is thought to contain many millions of icy objects, most of them small. However, the largest Kuiper belt objects, Eris and Pluto, are massive enough to also be considered dwarf planets.
Current thinking suggests that many of the short-period comets, or those that complete an orbit in less than 200 years, may have originated in the Kuiper belt. They were perhaps directed into the inner solar system by collisions with each other and gravitational encounters with Neptune. Long-period comets are thought to originate in the Oort cloud (whose existence is considered highly probable, but not proven). The cloud may have been produced long ago, as icy bodies near and inside Neptune's orbit were thrown far out from the sun by gravitational encounters with the outer planets.
The Origin and Future of the Solar System The most widely accepted model for the origin of the solar system combines theories elaborated by Kuiper and Thomas Chrowder Chamberlin. Astronomers believe that about 4.6 billion years ago, one of the many dense globules of gas and dust clouds that exist in the galaxy contracted into a slowly rotating disk called the solar nebula. The hot, dense center of the disk became the sun. The remaining outer material cooled into small particles of rock and metal that collided and stuck together, gradually growing into larger bodies to become the planets and their satellites.
In the cold outer parts of the new solar system, some of these bodies collected large amounts of hydrogen and helium from the solar nebula, thus becoming the “gas giants”—Jupiter, Saturn, Uranus, and Neptune. Closer to the sun, these light elements were mostly driven off by the higher temperatures and particles streaming off the sun. Smaller, rocky planets—Mercury, Venus, Earth, and Mars—developed there. Uncollected debris became asteroids and (in the outer regions) comets.
The sun is slowly getting brighter as it consumes its reservoir of hydrogen and turns this into helium. If current computations of stellar evolution are correct, the sun will grow much brighter and larger in about 5 billion years, making Earth much too hot for life to endure. Later the sun will have exhausted its nuclear energy source and will begin to cool. In the end it will become a white dwarf star, with all its matter packed densely into a space not much bigger than Earth. Around it will orbit frozen wastelands, the planets that survived the solar upheavals.
Does Life Exist Elsewhere?
- A false-color image shows the surface of Europa, one of the satellites of Jupiter. The icy surface, …
Mars is an intriguing place to look for life. Spacecraft have photographed large features that appear to be dry riverbeds. Data from NASA's Spirit and Opportunity rovers in the early 2000s strongly suggest that liquid water once existed on the planet's surface. Also, data from the European Mars Express orbiter and from Earth-based telescopes suggest that methane is being released from beneath the surface, and a possible source for this could be subsurface colonies of bacteria.
- The twin Viking landers performed several experiments at the surface of Mars that were designed to …
Discoveries of life existing in extreme or unusual environments on Earth—such as in hot bedrock miles beneath the surface and in colonies near volcanic vents on the deep sea floor—have widened prospects for finding life elsewhere. No place in the solar system other than Earth, however, is easily suitable for human colonization or for large land plants or animals. It is possible that other stars may be orbited by more Earth-like planets. In fact, the number of such worlds in the universe may be truly enormous. However, the only place life has been found so far is on Earth. One example is very little to go on, especially if we are part of the example. With little information regarding the likelihood of life arising in other places, even under Earth-like conditions, discussion of life elsewhere remains speculative.
The Stars
- A false-color picture shows the Orion nebula (M42), a giant “stellar nursery” with over …
- Although the stars of the Big Dipper seem to belong together, they are actually widely separated. A …
Coordinate Systems Astronomers need to record the exact locations of stars. Within limits, it is useful to locate objects within constellations. Numerical coordinate systems are used to record the locations of celestial objects more precisely. These systems are like the coordinate system of latitude and longitude used for Earth.
- A person standing on Earth does not feel the motion of the planet as it spins. The sun and the …
- Earth's axis of rotation slowly wobbles in a toplike motion called precession. This means that the …
The horizon, or azimuth, system is based on Earth's north-south line and the observer's horizon. It uses two angles called azimuth and altitude. Azimuth locates the star relative to the north-south line, and altitude locates it relative to the plane of the horizon. For this system to be useful, the time of the observation and the location from which the observation was made must be accurately known.
- In the equator coordinate system the right ascension and declination of a star do not change as …
In the equator system the position of a star is given by coordinates called declination and right ascension. The declination locates the star by its angular distance north or south of the celestial equator. The right ascension locates the star by its angular distance east or west of the vernal equinox. Since this system is attached to the celestial sphere, all points on Earth (except the poles) are continually changing their positions under the coordinate system.
Determining the Distance to Stars Fixing stars on an imaginary sphere is useful for finding them from Earth, but it does not reveal their actual locations. One way to measure the distances of nearby stars from Earth is the parallax method.
- The near-Earth asteroid Eros sometimes comes within 14 million miles (22 million kilometers) of …
More than four centuries ago the phenomenon of parallax was used to counter Nicolaus Copernicus' suggestion that Earth travels around the sun. Scientists of the time pointed out that if it did, stars should show an annual change in direction due to parallax. But, using the instruments available to them, they were unable to measure any parallax, so they concluded that Copernicus was wrong. Astronomers now know that the stars are all at such tremendous distances from Earth that their parallax angles are extremely difficult to measure. Even modern instruments cannot measure the parallax of most stars.
Astronomers measure parallaxes of stars in seconds of arc. This is a tiny unit of measure; for example, a penny must be 2.5 miles (4 kilometers) away before it appears as small as one second of arc. Yet no star except the sun is close enough to have a parallax that large. Alpha Centauri, a member of the group of three stars nearest to the sun, has a parallax of about three quarters of a second of arc.
Astronomers have devised a unit of distance called the parsec—the distance at which the angle opposite the base of a triangle measures one second of arc when the base of the triangle is the radius of Earth's orbit around the sun. One parsec is equal to 19.2 trillion (19.2 × 1012) miles (30.9 trillion kilometers). Alpha Centauri is about 1.3 parsecs distant.
Another unit used to record large astronomical distances is the light-year. This is the distance that light travels within a vacuum in one year—about 5.88 trillion miles (9.46 trillion kilometers). Proxima Centauri, part of the Alpha Centauri system, is the star closest to Earth (apart from the sun), yet it is about 4.3 light-years distant. Light takes more than four years to reach Earth from that distance.
Since parallax yields distances to only relatively nearby stars, other methods must be used for more distant ones. One of these methods is statistical parallax, in which the apparent motions across the sky of groups of stars are analyzed to determine their probable distance. Another method involves observing certain stars that vary regularly in brightness (discussed below).
Size and Brightness of Stars
- The globular cluster M4 contains more than 100,000 stars. The brightest stars in this photograph …
Astronomers express the brightness of a star in terms of its magnitude. Two values of magnitude describe a star. The apparent magnitude refers to how bright the star looks from Earth. The absolute magnitude of a star is the value its apparent magnitude would have if the star were 10 parsecs from Earth. The apparent magnitude of a star depends on its size, temperature, and distance. The temperature is found from its spectrum; if the distance is known, then astronomers can calculate the size of the star and also assign a value for its absolute magnitude. The actual brightness of stars may be compared using their absolute magnitudes.
Certain stars whose brightness varies regularly provide an important way for astronomers to estimate the distances of remote galaxies. In such stars the actual brightness (absolute magnitude) is closely related to the period of their brightness variations. Astronomers can use the observed period to determine the actual brightness and then compare this with the apparent brightness to estimate the distance.
Astronomers have discovered all kinds of stars—from huge, brilliant red supergiants more than 100 times the sun's diameter to extremely dense neutron stars only about a dozen miles across. The sun lies in about the middle range of size and brightness of stars. The largest stars are the cool, reddish supergiants: they have low surface temperatures, but they are so bright that they must be extremely large to give off that much energy. White dwarf stars, on the other hand, are very faint in spite of their high surface temperatures and thus must be very small—only about the size of Earth.
What Is a Star? Astronomers have found, using analysis of stars' spectra, that stars are made mostly of the simplest elements: hydrogen and helium. These elements are in the gaseous state. In most of the star, however, the temperature is so high (thousands to millions of degrees) that the gas is ionized (with electrons stripped away from the atomic nuclei)—a state called plasma.
The mutual gravitational attraction of a star's matter is what forces it into a roughly spherical shape. In fact, if there were nothing to counteract this inward force, the star would collapse to a very small size. The gravitational squeezing of the gas, however, heats it to very high temperatures. In the 1800s astronomers believed that this compression was actually the energy source for a star. This presented a problem. The sun could shine like this for only a few million years without shrinking so much that conditions on Earth would be greatly altered. Yet geological and biological evidence suggested that Earth has maintained the conditions for life for hundreds of millions of years.
The 20th century brought a solution to this problem. With the discovery of nuclear energy, astronomers could explain the sun's long-lasting power output as the result of nuclear fusion: hydrogen deep inside the sun was being fused together to form helium. This process is so energetic that it can counterbalance the inward force of gravity. Stars, then, are essentially battlegrounds between two forces—the inward crush of gravity and the outward pressure from the heat generated by nuclear fusion.
The Lives of Stars
- The Cat's Eye nebula is a spectacular example of what happens when a star of about the sun's mass …
This cannot last forever, though, as eventually most of the hydrogen “fuel” is converted into helium. In the largest stars, this takes only a few million years. Very-low-mass stars, with less gravitational pressure to battle, consume their fuel very slowly and may last a trillion years. The sun is intermediate, with an estimated lifetime of about 10 billion years, which it is believed to be almost halfway through.
- Nebula NGC 6751 formed several thousands of years ago as a dying star in the constellation Aquila …
Stars born with much more mass than the sun undergo even more dramatic events. Under tremendous pressure, such a star performs numerous additional fusion reactions in its core, producing a wide range of elements, up to and including iron. At this point, the ultradense core can collapse suddenly, leading to a colossal explosion called a supernova. Many such events have been observed from Earth, some so bright that they were visible in broad daylight. For a few weeks the exploding star can outshine an entire galaxy of a hundred billion stars. The elements thrown out into space can become part of nebulae, eventually to be incorporated into future generations of stars and planets.
Neutron Stars and Black Holes After some types of supernova explosions, an extremely dense core remains. This object, called a neutron star, is about the mass of the sun and is made mostly of neutrons. Its matter is so compact that a teaspoon of it has the mass of a small mountain. Some neutron stars spin rapidly while beaming radiation into space. If a beam intercepts Earth, astronomers may detect it as a series of pulses of radio waves or sometimes radiation at other wavelengths. Such a neutron star is referred to as a pulsar.
- A Hubble Space Telescope image shows a disk of gas and dust that probably surrounds and feeds a …
Often, neutron stars and black holes are detectable only because of their effects on nearby companion stars. Gas (mainly hydrogen) is drawn off the companion star and then swirls rapidly down onto (or into) the neutron star or black hole. The violent compressional heating and acceleration of the gas causes it to emit X-rays, which can be detected from Earth-based satellites. Such double star systems are called X-ray binaries.
Planets of Other Stars Astronomers have long thought that, like the sun, many or most stars should be accompanied by orbiting planets. These planets would be so distant from Earth, however, that their very faint light would be drowned out by the bright light of their “suns.” It turns out that there are indirect methods of detecting such planets. An orbiting planet would cause a star to wobble slightly, and this wobble could be detected as alternating red and blue Doppler shifts of the star's light. Furthermore, the speed and period of the wobble could enable astronomers to estimate the planet's mass and distance from the star. This technique was first successfully used in 1995 to find a planet orbiting the star 51 Pegasi. During the next 10 years, about 140 extrasolar planets were discovered in this way (plus a few by other means, such as the dip in light caused when a planet passes in front of a star).
Most of the planets found so far are at least as massive as Jupiter, yet they are closer to their stars than Mercury is to the sun. Such close-in, massive planets should be the easiest to detect, since they cause the greatest wobbles. But they are still a challenge to explain. Current theories of planet formation suggest that such large planets should form farther from the star, where temperatures are cold enough to allow collection of large amounts of gas. One possibility astronomers are considering is that these “hot Jupiters” formed farther out from their stars and migrated inward. This raises the question, though, of why our solar system has not experienced such planetary migration.
Interstellar Matter
- Dark nebulae, such as the Horsehead nebula in Orion, consist of clouds of interstellar dust, which …
Dust grains block blue light more than red light, so the color of a star can appear different if it is seen through much dust. To find the temperature of such a star, astronomers must estimate its color to be bluer than it appears because so much of its blue light is lost in the dust. When clouds of dust occur near bright stars they often reflect the starlight in all directions. Such clouds are known as reflection nebulae.
Interstellar gas is about 100 times denser than the dust but still has an extremely low density. The gas does not interfere with starlight passing through it, so it is usually difficult to detect. When a gas cloud occurs close to a hot star, however, the star's radiation causes the gas to glow. This forms a type of bright nebula known as an H II region. Away from hot stars interstellar gas is quite cool. Masses of this cool gas are called H I regions.
- The Veil nebula (NGC 6992) is in the constellation Cygnus. It glows as it collides with dust and …
The hydrogen occurs partly as single atoms and partly as molecules (two hydrogen atoms joined together). Molecular hydrogen is even more difficult to detect than atomic hydrogen, but it must exist in abundance. Other molecules have been found in the interstellar gas because they give off low-frequency radiation. These molecules contain other atoms besides hydrogen: oxygen or carbon occurs in hydroxyl radicals (OH−) and in carbon monoxide (CO), formaldehyde (H2CO), and many others, including many organic molecules.
- New stars form in a “stellar nursery” of the Eagle nebula (M16, or NCG 6611). This …
The Galaxies
- In the 1920s Edwin Hubble separated galaxies into four general types according to their …
- The giant galaxy NGC 1316 is classified as an ellipitcal galaxy. It is unusual for its type in that …
- The Whirlpool galaxy (M51), at left, is a spiral galaxy. It is accompanied by a small, irregular …
- NGC 1300 is a barred spiral galaxy.
- The Cigar galaxy (M82) is classified as an irregular galaxy.
Radio Galaxies, Quasars, and Dark Matter Galaxies were long thought to be more or less passive objects, containing stars and interstellar gas and dust and shining by the radiation that their stars give off. When astronomers became able to make accurate observations of radio frequencies coming from space, they were surprised to find that a number of galaxies emit large amounts of energy in the radio region. Ordinary stars are so hot that most of their energy is emitted in visible light, with little energy emitted at radio frequencies. Furthermore, astronomers were able to deduce that this radiation had been given off by charged particles of extremely high energy moving in magnetic fields.
- A distant radio galaxy contains two gaseous jets, each emanating from one of the galaxy's twin …
Astronomers have found that, in many galaxies, stars near the center move very rapidly, apparently orbiting some very massive unseen object. The most likely explanation is that a giant black hole, with millions or even billions of times the sun's mass, lurks in the center of most large galaxies. As stars and gas spiral into these black holes, much of their mass vanishes from sight. The violent heating and compression produces a huge release of energy, including high-speed jets of matter (such as in M87).
- PKS 2349 (bright central disk) is a quasar, or quasi-stellar radio source, several billion …
Another problem has puzzled astronomers for years. Most, if not all, galaxies occur in clusters, presumably held together by the gravity of the cluster members. When the motions of the cluster members are measured, however, it is found in almost every case that the galaxies are moving too fast to be held together only by the gravity of the matter that is visible. Astronomers believe there must be a large amount of unseen matter in these clusters—perhaps 10 times as much as can be seen. While some of this likely consists of objects such as black holes and neutron stars, most of it is believed to be “exotic dark matter,” of unknown origin.
The Milky Way Galaxy Like most stars, the sun belongs to a galaxy. Since the sun and Earth are embedded in the galaxy, it is difficult for astronomers to obtain an overall view of this galaxy. In fact, what can be seen of its structure is a faint band of stars called the Milky Way (the word galaxy comes from the Greek word for “milk”). Because of this, the galaxy has been named the Milky Way galaxy.
- The name of Earth's galaxy comes from the visual phenomenon of the Milky Way, a band of stars seen …
Not all the galaxy's stars are confined to the galactic plane. There are a few stars that occur far above or below the disk. They are usually very old stars, and they form what is called the halo of the galaxy. Evidently the galaxy was originally a roughly spherical mass of gas. Its gravity and rotation caused it to collapse into the disklike shape it has today. The stars that had been formed before the collapse remained in their old positions, but after the collapse further star formation could occur only in the flat disk.
All the stars in the galaxy move in orbits around its center. The sun takes about 200 million years to complete an orbit. The orbits of most of these stars are nearly circular and are nearly in the same direction. This gives a sense of rotation to the galaxy as a whole, even as the entire galaxy moves through space.
Dark clouds of dust almost completely obscure astronomers' view of the center of the Milky Way galaxy. Radio waves penetrate the dust, however, so radio telescopes can provide astronomers with a view of the galactic nucleus. In that region stars travel in very fast, tight orbits—which implies the existence of a huge mass at the center. In addition, the Earth-orbiting Chandra X-ray Observatory has detected flares of X-rays, lasting only a few minutes, in the region. Most astronomers believe these findings are best explained by the existence of a black hole—3 million times the sun's mass but only about a dozen times the sun's diameter—that is violently accelerating and compressing in-falling blobs of matter.
The Universe Cosmology is the scientific inquiry into the nature, history, development, and fate of the universe. By making assumptions that are not contradicted by the behavior of the observable universe, scientists build models, or theories, that attempt to describe the universe as a whole, including its origin and its future. They use each model until something is found that contradicts it. Then the model must be modified or discarded.
Cosmologists usually assume that, except for small irregularities, the universe has a similar appearance to all observers (and the laws of physics are identical), no matter where in the universe the observers are located or in which direction they look. This unproven concept is called the cosmological principle. One consequence of the cosmological principle is that the universe cannot have an edge, for an observer near the edge would have a different view from that of someone near the center. Thus space must be infinite and evenly filled with matter, or the geometry of space must be such that all observers see themselves as at the center. Also, astronomers believe that the only motion that can occur, except for small irregularities, is a uniform expansion or contraction of the universe.
- According to the evolutionary, or big bang, theory of the universe, the universe is expanding while …
In the 1950s and 1960s there was a rival model, called the steady state theory. The basic assumption of steady state was a perfect cosmological principle, applying to time as well as position. The steady state theory stated that the universe must have the same large-scale properties at all times; it cannot evolve, but must remain uniform. Since the universe is seen to be expanding, which would spread the matter out thinner as time goes on, steady state suggested that new matter must be created to maintain the constant density. In the steady state theory galaxies are formed, they live and die, and new ones come along to take their places at a rate that keeps the average density of matter constant.
When astronomers observe an object at a great distance, they are seeing it as it looked long ago, because it takes time for light to travel. A galaxy viewed at a distance of a billion light-years is seen as it was a billion years ago. Distant galaxies do seem to be different from nearby galaxies. They seem closer together than nearby ones, contrary to steady state contentions but consistent with the view that the universe had a greater density in the past. Also, a faint glow of radiation has been discovered coming uniformly from all directions. Calculations show that this could be radiation left over from the big bang.
The History of Astronomy
- Stonehenge is an ancient monument in England that includes a circular setting of massive stones. …
In many early civilizations, astronomy was sufficiently advanced that reliable calendars had been developed. In ancient Egypt astronomer-priests were responsible for anticipating the season of the annual flooding of the Nile River. The Maya, who lived in what is now central Mexico, developed a complicated calendar system about 2,000 years ago. The Dresden Codex, a Mayan text from the 1st millennium AD, contains exceptionally accurate astronomical calculations, including tables predicting eclipses and the movements of Venus.
In China, a calendar had been developed by the 14th century BC. In about 350 BC a Chinese astronomer, Shih Shen, drew up what may be the earliest star catalog, listing about 800 stars. Chinese records mention comets, meteors, large sunspots, and novas.
The early Greek astronomers knew many of the geometric relationships of the heavenly bodies. Some, including Aristotle, thought Earth was a sphere. Eratosthenes, born in about 276 BC, demonstrated its circumference. Hipparchus, who lived around 140 BC, was a prolific and talented astronomer. Among many other accomplishments, he classified stars according to apparent brightness, estimated the size and distance of the moon, found a way to predict eclipses, and calculated the length of the year to within 61/2 minutes.
The most influential ancient astronomer historically was Ptolemy (Claudius Ptolemaeus) of Alexandria, who lived in about AD 140. His geometric scheme predicted the motions of the planets. In his view, Earth occupied the center of the universe. His theory approximating the true motions of the celestial bodies was held steadfastly until the end of the Middle Ages.
In medieval times Western astronomy did not progress. During those centuries Hindu and Arab astronomers kept the science alive. The records of the Arab astronomers and their translations of Greek astronomical treatises were the foundation of the later upsurge in Western astronomy.
- A celestial map of 1661 by Andreas Cellarius depicts the heavens according to the Copernican …
The great Danish astronomer Tycho Brahe rejected Copernicus' theory. Yet his data on planetary positions were later used to support that theory. When Tycho died, his assistant, Johannes Kepler, analyzed Tycho's data and developed the laws of planetary motion. In 1687 Newton's law of gravitation and laws of motion explained Kepler's laws.
Meanwhile, the instruments available to astronomers were growing more sophisticated. Beginning with Galileo, the telescope was used to reveal many hitherto invisible phenomena, such as the revolution of satellites about other planets.
The development of the spectroscope in the early 1800s was a major step forward in the development of astronomical instruments. Later, photography became an invaluable aid to astronomers. They could study photographs at leisure and make microscopic measurements on them. Even more recent instrumental developments—including radar, telescopes that detect electromagnetic radiation other than visible light, and space probes and manned spaceflights—have helped answer old questions and have opened astronomers' eyes to new problems.