Astronomy

Since the beginnings of humankind people have gazed at the heavens. Before the dawn of history someone noticed that certain celestial bodies moved in orderly and predictable paths, and astronomy—an ancient science—was born. Yet some of science's newest discoveries have been made in this same field. From simple observations of the motions of the sun and the stars as they pass across the sky, to advanced theories of the exotic states of matter in collapsed stars, astronomy has spanned the ages.

For centuries astronomers concentrated on learning about the motions of heavenly bodies. They saw the sun rise in the east, cross the sky, and set in the west. After the sun had set, they saw tiny points of light appear as the sky was growing dark.
Most of these lights seemed to stay in the same place in relation to one another, as if they were all fastened to a huge black globe surrounding Earth. These lights were called stars. Other lights, however, seemed to travel, going from group to group of stationary stars. These moving points were called planets, or “wanderers.”
The ancient astronomers thought that the positions of celestial bodies revealed what was going to happen on Earth—wars, births, deaths, and good fortune or bad. This system of belief is called astrology. Because the ancient astrologers wanted to predict precisely what would happen on Earth, they studied the motions of the celestial bodies. Most scientists no longer believe in astrology, but they have found that some ancient astrologers were good at observing the motions and positions of stars and planets.

The Visible Sky

When you look at the sky without any telescope or binoculars or any other modern instrument, you see basically the same things the ancient astronomers saw. During the day you see the sun and sometimes even a faint moon.
During a clear night you see the stars and usually the moon. If you watch the sky often enough, you can get to recognize groups of stars called constellations. You may even notice a star that seems to be in different positions from night to night: it is really a planet, one of the “wandering stars” of the ancients.

Day and Night

The day is divided into 24 hours. On average the sun is up 12 hours and down 12 hours. The daily motion of the sun is therefore the source of the time given by our clocks. But the days are not exactly alike. In winter the sun is visible less than 12 hours per day; in summer it is visible longer. This happens because the sun's path through the daytime sky is longer in summer than in winter.

Earth in Space

The apparent westward motion of the sun, the moon, and the stars is not real. They seem to move around Earth, but it is actually Earth that moves. It is rotating eastward, completing one rotation each day. This is hard to believe at first because when we think of motion we also think of the vibrations of moving cars or trains. But Earth moves freely in space, without rubbing against anything, so it does not vibrate. It is this gentle rotation, uninhibited by significant friction, that makes the sun, the moon, and the stars appear to be rising and setting.
Earth is accompanied by the moon, which moves around the planet at a distance of about 30 Earth diameters. At the same time, Earth is moving around the sun. Every year Earth completes one revolution around the sun. This motion, along with the tilt of Earth's axis, accounts for the changes in the seasons. When the northern half of Earth is tipped toward the sun, then the Northern Hemisphere experiences summer and the Southern Hemisphere, which is tipped away from the sun, experiences winter. When Earth has moved to the other side of the sun, six months later, the seasons are reversed because the Southern Hemisphere is then tipped toward the sun and the Northern Hemisphere is tipped away from the sun.
If you watch the moon for three or four weeks, you will see that it does not always look the same. Sometimes it looks like a big disk, sometimes like a tiny curved sliver. These changes are called the phases of the moon. They occur because the moon shines only when the sun's light bounces off its surface. This means that only the side of the moon that faces the sun is bright. When the moon is between Earth and the sun, the light side of the moon faces away from Earth. This is called the new moon, which is not visible. When the moon is on the other side of Earth from the sun, its entire light side faces Earth. This is called the full moon. Halfway between the new and full moons, in locations on either side of Earth, are the first quarter and the last quarter.

The Night Sky

What else can you see on a clear night with just your eyes? Naturally you can see stars. After a few nights you might even recognize a planet by its motion. Although stars and planets look alike to the unaided eye, they are very different things. The planets all circle the sun, just as Earth does. They are visible from Earth because sunlight bounces off them.
The stars are much farther away. Most stars are like the sun—large, hot, and bright. The stars shine from their own energy, just as the sun does.
As you watch the stars you may notice a broad strip of dim light across the sky. It is a clustering of faint stars known as the Milky Way. The Milky Way is a galaxy—an enormous cluster of stars, of which the sun is only one member. The Milky Way galaxy contains more than 100 billion stars. Other galaxies exist far beyond the Milky Way.

Eclipses

In ancient times people often were terrified when the sun or the moon seemed to disappear completely during the day or night when normally it would be visible. They did not understand what caused these eclipses, and they were afraid that the sun or moon might be gone forever, leaving the world in darkness.Eclipses occur irregularly because the plane of the moon's orbit around Earth is slightly different from the plane of Earth's orbit around the sun. The two planes intersect at an angle of 5° 8′. This means that the moon is usually slightly above or below the line between Earth and the sun, so neither Earth nor the moon throws a shadow on the other. Eclipses can occur only when the moon lies at one of the two points where the planes intersect. If this were not so, we would have total lunar eclipses with every full moon and total solar eclipses with every new moon.
Sometimes the moon crosses a point of intersection of the two planes at the same time that it passes directly behind Earth. The shadow of Earth blocks off the light to the moon, and the moon seems to disappear. Some two hours later, the edge of the moon appears on the other side of Earth's shadow and the whole moon gradually emerges.
The opposite happens, too. Sometimes the moon crosses the point of intersection of the two planes at the same time that it passes between Earth and the sun. It casts a shadow on Earth, causing an eclipse of the sun. Because the moon is much smaller than Earth, only a small shadow patch on Earth's surface results. To people in the darkest part of the shadow, the umbra, the moon completely blocks out the sun. This is a total eclipse of the sun: the entire bright disk is covered up and only the outer atmosphere of the sun, the corona, is visible. Because the sun is so bright, it is very dangerous to look directly at it, even during an eclipse.
Normally light from the sun's bright disk blots out the faint corona. During total solar eclipses astronomers can study the sun's atmosphere. Unfortunately, such eclipses are always brief; the longest possible total eclipse is about seven minutes.
A larger area of Earth is covered by the shadowy area around the true shadow (this blurry area is the penumbra). To people in the penumbra, the moon blocks off part of the sun, but a light silver or yellow crescent shows along the edge of the moon. This is a partial eclipse of the sun.
When the moon is farthest from Earth during an eclipse, its disk appears a bit smaller than the sun's disk, so that a ring of the sun's disk is seen around the black mass of the moon. This is what astronomers call an annular eclipse.

Rocks from Outer Space

Sometimes when observing the night sky, a flash of light may streak through the atmosphere and disappear. Although this is commonly called a shooting star, real stars do not shoot through the sky any more than the sun does. But small, solid chunks of stone or metal are in orbit around the sun. Sometimes these pieces of stone or metal enter Earth's atmosphere, and the friction generated by their great speed causes them to burn up. The fragments may either vaporize before traveling far or actually hit the ground.
Shooting stars have different names depending on where they are. According to the International Astronomical Union, a rock or metal fragment existing beyond Earth's atmosphere is called a meteoroid. A meteoroid that enters Earth's atmosphere is called a meteor. And a meteor that actually lands on Earth's surface is called a meteorite.
Meteorites, which are sturdy enough to reach the ground, apparently are pieces of asteroids. (Asteroids are huge rocks, up to 500 miles [800 kilometers] across, that orbit the sun.) Most meteors that burn up in the atmosphere are tiny dustlike particles, the remains of disintegrated comets. (Comets are flimsy objects made up primarily of frozen water and frozen gases and some gritty material. They also orbit the sun.)
Sometimes a swarm of meteoroids will enter Earth's atmosphere at one time, causing a meteor shower, with tens or hundreds of shooting stars flashing across the sky at once. But all these meteors burn up in the upper atmosphere. They are too small and fragile to reach Earth's surface, though a significant amount of dust and ash from meteors settles on Earth each day. The Leonid meteors caused the greatest meteor showers on record in 1833 and 1966. These meteors appear every November, with especially dazzling displays about every 33 years.

The Northern and Southern Lights

People who are relatively near the North or South Pole can see one of nature's most lavish and glorious displays—the aurora borealis (northern lights) or the aurora australis (southern lights). High in the skies over Earth's magnetic poles, electrically charged particles from the sun swarm down into Earth's atmosphere. As these particles collide with air molecules, brilliant sheets, streamers, or beams of colored lights are given off at heights ranging from about 50 to 200 miles (80 to 320 kilometers) up in Earth's atmosphere.
The streams of charged particles are known as the solar wind. The sun continually sends a flow of these particles out into space. During periods when the sun is unusually active—that is, when it has large sunspots on its surface—the solar wind is particularly heavy, and huge swarms of the particles reach Earth's atmosphere, causing large and brilliant auroras.

The Solar System

Earth is not the only body to circle the sun. Many chunks of matter, some much larger than Earth and some so small you would need a microscope to see them, are caught in the sun's gravitational field. The nine largest of these chunks are called planets. Earth is the third planet from the sun. The smaller chunks of matter are natural satellites, asteroids, comets, meteors, and the molecules of interplanetary gases.

Kepler's Laws of Planetary Motion

In the early 1600s, astronomers were beginning to accept the idea that Earth and the planets revolve around the sun, rather than that the sun and the planets revolve around Earth. Astronomers were still unable, however, to describe the motions of the planets with any accuracy. The German astronomer Johannes Kepler was finally able to describe planetary motions using three mathematical expressions, which came to be known as Kepler's laws of planetary motion.
If the average distance from Earth to the sun is arbitrarily called one astronomical unit (A.U.), then Kepler's third law, which describes the mathematical relation between a planet's period of revolution and its distance from the sun, can be used to find the relative distances of the other planets to the sun merely by measuring how long it takes those planets to orbit the sun. Before the actual distance from Earth to the sun was known in miles or kilometers, many distances within the solar system were known in astronomical units. The unit is still a useful one. Parallax measurements and, more recently and accurately, radar observations have allowed astronomers to determine that one astronomical unit is equal to 92,955,808 miles (149,597,870 kilometers). Another method of measurement called laser ranging—bouncing laser signals off a mirror placed on the moon's surface—has been used to verify the astronomical unit.

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, then 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 English scientist Isaac Newton calculated the acceleration of the moon toward Earth. It was much less than the acceleration of an apple falling from a tree to the ground. Newton concluded that the same force that caused the apple to fall to the ground also caused the moon to fall toward Earth. But the force grew weaker at points farther away from the planet. Following this train of thought, Newton worked out an equation that described this force as it occurred anywhere in the universe

The Planets

Up to the 18th century people knew of seven bodies, besides Earth, that moved against the background of the fixed stars. These were the sun, the moon, and the five planets that are visible to the unaided eye: Mercury, Venus, Mars, Jupiter, and Saturn. Then, in 1781, William Herschel, a German-born English organist and amateur astronomer, discovered a new planet, which became known as Uranus.
Uranus' motion did not follow the exact path predicted by Newton's theory of gravitation. This problem was happily resolved by the discovery of yet another planet, which was named Neptune. Two mathematicians, John Couch Adams and Urbain Leverrier, had calculated Neptune's probable location, but it was the German astronomer Johann Gottfried Galle who located the planet.
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 Clyde W. Tombaugh discovered a new planet, which became known as Pluto.
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 the Voyager 2 spacecraft in 1989, the deviations “disappeared.”
All the planets travel around the sun in elliptical orbits that are close to being circles. Mercury and Pluto have the most eccentric orbits. All the planets travel in one direction around the sun, the same direction in which the sun rotates. Furthermore, all the planetary orbits lie in very nearly the same plane. Again, Mercury and Pluto have the most tilted orbits: Mercury's is tilted 7° to the plane of Earth's orbit (the ecliptic plane); Pluto's is tilted about 17°.Most of the planets rotate on their axes in the same west-to-east motion (the exceptions are Venus, Uranus, and Pluto). Most of the axes are nearly at right angles to the plane of the planets' orbits. Uranus and Pluto, however, are tilted so that the spin axis lies almost in the plane of orbit.
The planets can be divided into two groups. The inner planets—Mercury, Venus, Earth, and Mars—lie within the asteroid belt, near the sun. They are dense, rocky, and small. Since Earth is a typical inner planet, this group is sometimes called the terrestrial planets.
The outer planets lie beyond the asteroid belt. With the exception of Pluto, they are much larger and more massive than the inner planets, and they are much less dense. Since Jupiter is the main representative of the outer planets, they are sometimes called the Jovian, or Jupiter-like, planets. Pluto is an outer planet, but it is not usually regarded as a Jovian planet.

Natural Satellites of the Planets

Seven of the planets—Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto—are known to have satellites. Because the moon is large in comparison with Earth, the Earth-moon system is sometimes called a double planet. The same applies to Pluto and its satellite, Charon, because Charon is estimated to be about half the size of Pluto. Although other satellites are much larger than either the moon or Charon, they are much tinier, by comparison, than the planets they circle. No other double planets are known to occur in the solar system.

Asteroids

On Jan. 1, 1801, the Italian astronomer Giuseppi Piazzi found a small planet in the large gap between Mars and Jupiter. This planet, later named Ceres, was the first and largest of thousands of asteroids, or minor planets, that have been discovered.

Comets

Comets are the most unusual and unpredictable objects in the solar system. They vary in appearance from small stellar images, like small asteroids, to huge tailed objects so bright that they can be seen in daytime near the sun. Comets are small bodies composed mostly of ices of various substances—principally water and gases—with some silicate grit mixed in. This composition and the nature of the comets' orbits suggest that comets were formed before or about the same time as was the rest of the solar system.Hundreds of millions of comets may exist in a large cloud, called the Oort cloud, that is believed to surround the solar system. Occasionally comets may leave the Oort cloud when the cloud is perturbed—perhaps by the gravitational force of a passing star. These comets enter the inner solar system and orbit the sun in long elliptical paths. Occasionally one of these intruders may be gravitationally influenced by the larger planets and pulled into a closer, shorter orbit, with a period of about seven years. Most comets, however, have much longer periods. Halley's comet takes about 76 years to complete an orbit, and many comets may take thousands or even millions of years.
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 gives the comet a long glowing tail that always points away from the sun.

The Sun

The spectrum, brightness, mass, dimension, and age of the sun and of nearby stars indicate that the sun is a normal, typical star. Like most stars, the sun produces energy by thermonuclear processes that take place at its core. These processes maintain the conditions needed for life on Earth.

Origin and Future of the Solar System

The most widely accepted model for the origin of the solar system combines theories elaborated by Gerard P. Kuiper and Thomas Chrowder Chamberlin. Astronomers believe that about 4.5 billion years ago, one of the many dense globules of gas and dust clouds that exist in the galaxy contracted into a disk. The hot, dense center of the disk became our sun. The remaining outer material formed a spinning disk, called the solar nebula, which 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.
The future of the solar system cannot be known, because accidents can happen. A star might pass right through and destroy the system, though such events are rare. If no accident occurs, the future depends on the behavior of the sun. 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. In turn, the planets will get too hot for life to endure on Earth.
Much 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 in the Solar System?

Life as we know it, and most particularly in its higher forms, can exist only under certain chemical and physical conditions. The major requirements of life are believed to be the availability of a stable liquid or very dense gas, an atmosphere, and some protection from solar radiation. A number of environments within the solar system may meet these criteria. For example, organisms may exist in the clouds of Venus or Jupiter or beneath the surface of the moon. Comets and asteroids may contain organic matter. So far there is no strong evidence either for or against life in such environments.
Analysis of samples of Martian soil obtained by the Viking 1 and 2 space probes in 1976 found no traces of living organisms. Exploratory spacecraft have since photographed surface features on Mars that suggest that liquid water once existed on the planet's surface and might still exist in the subsurface. In 1996 scientists announced possible evidence of ancient bacteria-like organisms on a meteorite thought to have originated on Mars. These discoveries reopened the possibility that life exists or existed on the planet. In any case, animal life as we know it appears excluded because there is practically no free oxygen in the atmosphere and no ozone to shield against the harmful ultraviolet sunlight. Scientists look also to Europa, one of the moons of Jupiter, in the search for life because it appears to contain a global ocean of liquid water just beneath a surface of water ice.
Earth is a paradise for life as we know it. There is no other place in the solar system that would easily support human colonization. Astronauts will have to take their environment with them to any planets they visit, just as they have taken their environment with them to the moon.

The Stars

If you have a chance to look through a telescope at the night sky, you will see a complex display indeed. You must explore this yourself to really appreciate its magnitude. The only comparable experience is to examine a drop of water from a greenish pond under a good microscope and see the drop teeming with unexpected small living creatures.

Constellations

The stars seem to form groups, or constellations. The Big Dipper is actually part of a larger constellation called Ursa Major (the Big Bear). Orion is another easy-to-recognize constellation. The first step in finding your way among the stars is to learn these constellations. If you begin with a few familiar ones and keep star charts handy, you will quickly learn to recognize others.
The stars in constellations are not necessarily close to each other in space. For example, though the middle five stars of the Big Dipper are relatively close together, the first and last stars only seem to be in the same group. They are actually much farther from Earth than the other five, and they are even slowly moving in different directions. Some parts of Orion are relatively close together, but Betelgeuse (pronounced “beetle juice”), the bright red star at the top, is much nearer to Earth.

Coordinate Systems

Astronomers need to record the exact locations of stars. Within limits, it is useful to locate objects within constellations. An ancient method of recording the motions of planets was to say that a planet was entering, in, or leaving the “house” of a zodiacal constellation. But this method is not really precise.
People who need to record the exact locations of celestial objects use numerical coordinate systems. These systems are like the coordinate system of latitude and longitude used on Earth.

 

 Different celestial coordinate systems have been devised. To be useful they must take into account that Earth has two regular motions in relation to the stars. Its rotation causes the sphere of stars to appear to make a complete circle around the planet once a day. And Earth's revolution around the sun causes the star positions at a particular hour to shift from day to day, returning to the original position after an entire year.

The horizon, or azimuth, systemis based on Earth's north-south line and the observer's horizon. It uses two angles called the azimuth and the altitude. The azimuth locates the star from the north line, and the altitude locates it from the horizon plane. For this system to be useful, the time of the observation and location from which the observation was made must be accurately known.
The equator system is based on the concept of the celestial sphere. All the stars and other heavenly bodies can be imagined to be located on a huge sphere that surrounds Earth. The sphere has several imaginary lines and points. One such line is the celestial equator, which is the projection of Earth's equator onto the celestial sphere. Another is the line of the ecliptic, which is the sun's apparent yearly path along this sphere. The celestial equator and the ecliptic intersect at two points, called the vernal equinox and the autumnal equinox. (When the sun is at either point, day and night on Earth are equally long.) The north and south celestial poles are extensions of the North and South poles of Earth along Earth's axis of rotation.In the equator system, the position of a star is given by the declination and the right ascension. The declination locates the star from the celestial equator, and the right ascension locates the star from 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.

Actual Locations of 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.
For parallax measurements of stars, scientists make use of Earth's yearly motion around the sun. This motion causes us to view the stars from different positions at different times of the year. In summer, Earth is on one side of the sun. In winter, Earth is 186 million miles (300 million kilometers) away on the opposite side of the sun. And photographs of a near star, taken through a large telescope and six months apart, will show that the star appears to shift against the background of more distant stars. If this shift is large enough to be measured, astronomers can calculate the distance to the star.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. Because they were unable to measure any parallax, they concluded that Copernicus was wrong. We know now 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 2¹/₂ 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 the 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 (5.88 × 10¹²) miles (9.46 trillion kilometers). Alpha Centauri is the star closest to the 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.

Demonstrating Parallax

You can demonstrate how parallax happens. You need a wall with a design to represent the background of distant stars. A wallpaper pattern, or even a single vertical stripe, will do. You also need something you can prop in the middle of the room to represent the nearby star whose parallax is being measured. A tall vase on a table, or even a broom leaning against a chair, will do. Put the table with the vase on it (or the broom and the chair) in the middle of the room. Stand at the opposite side of the room with the vase between you and the wallpaper.
To begin with, you are at the position of the sun. Notice where the vase is seen against the wallpaper background. Take a sideways step to your right. This is the position of Earth in summer. Notice that the vase seems to move to the left along the wallpaper background. Take a step back to the sun's position and then take another sideways step to your left. This is the position of Earth in winter. Notice that the vase seems to move to the right along the wallpaper background. The apparent motion of the vase is caused by parallax.

Estimating Distances of Stars

Each star, including the sun, has its own motion in space. This motion of the sun and the other stars causes the position of any star relative to the sun (its direction) to change evenly with time: the longer the time interval, the greater the change in direction of the star. The yearly change in direction of a star due to the space motions of sun and star is known as the proper motion of the star. Other things being equal, the nearer stars have larger proper motions than the more distant ones. This provides astronomers with a way of estimating distances of stars.
It is not possible to find the distance of an individual star in this way because there is no way to tell whether a certain measured proper motion is caused by a rapidly moving star that is far away or by a slowly moving star that is near. The average individual proper motion of a group of stars, however, can tell astronomers what the average distance of the group is. In this way, approximate distances can be found for many stars that are too far away for their annual parallaxes to be measured.

What Starlight Tells Astronomers

How can astronomers learn what the stars are made of? Since stars cannot be analyzed in the laboratory, astronomers study the feeble starlight that actually does reach Earth. Fortunately, electromagnetic radiation, including light, can provide much information about the object that emits it.
Visible light is only one form of electromagnetic radiation. There are many more kinds. Gamma rays, X rays, and ultraviolet rays are more energetic than visible light; infrared rays and radio waves are less energetic than visible light.
The forms of electromagnetic radiation differ in their frequencies—that is, in how many times per second their waves crest. Waves with high frequencies have greater energy than waves with low frequencies. Gamma rays have extremely high frequencies, and radio waves have low frequencies.
Stars are found to give off a whole range of electromagnetic radiation. The kind of radiation a star gives off is related to the temperature of the star: the higher the temperature of the star, the more energy it gives off and the more this energy is concentrated in high-frequency radiation. Spectrographs can separate radiation into the different frequencies. The array of frequencies makes up the spectrum of the star.
The color of a star is also an indication of its temperature. Red light has less energy than blue light. A reddish star must have a large amount of its energy in red light. A white or bluish star has a larger amount of higher-energy blue light, so it must be hotter than the reddish star.
Stars have bright or dark lines in their spectra. These bright or dark lines are narrow regions of extra-high emission or absorption of electromagnetic radiation. The presence of a certain chemical, such as hydrogen or calcium, in the star causes a particular set of lines in the star's spectrum. Since most of the lines found in stellar spectra have been identified with specific chemicals, astronomers can learn from a star's spectrum what chemicals it contains.
Spectrum lines are useful in another way, too. When an observer sees radiation coming from a source, such as a star, the frequency of the radiation is affected by the observer's motion toward or away from the source. This is called the Doppler effect. If the observer and the star are moving away from each other, the observer detects a shift to lower frequencies. If the star and the observer are approaching each other, the shift is to higher frequencies.
Astronomers know the normal spectrum-line frequencies for many chemicals. By comparing these known frequencies with those of the same set of lines in a star's spectrum, astronomers can tell how fast the star is moving toward or away from Earth.

Size and Brightness of Stars

Both the size and the temperature of a star determine how much radiation energy it gives off each second: this is the actual brightness of a star. It is also true, however, that the closer a star is to Earth, the more of its radiation energy will actually reach Earth and the brighter it will appear.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.
Astronomers have discovered all kinds of stars—from huge, brilliant supergiants to dense, cool neutron stars. The sun lies in about the middle range of size and brightness of stars and is considered to be a typical star. The largest stars are the cool supergiants: they have low surface temperatures, but they are so bright that they must be extremely large to give off that much energy. In the white-dwarf stars a solar mass is squeezed into a sphere about the size of Earth. A teaspoonful might weigh 10 tons.
Neutron stars are even more strongly compressed than the white dwarfs: they probably have a solar mass compressed to a radius of a few miles. The objects called pulsars are thought to be rapidly spinning neutron stars. While working at the Mullard Radio Astronomy Observatory, Jocelyn Bell Burnell and Antony Hewish discovered the first pulsar in 1967.
A black hole is an object so dense that even light is unable to escape its gravitational field. Such an object can be formed by the gravitational collapse of a massive star at the end of its life. Black holes were predicted by the general theory of relativity and later confirmed by astronomers.

Interstellar Matter

The space between the stars contains gas and dust at extremely low densities. This matter tends to clump into clouds. These clouds are called nebulas when they block more distant starlight, reflect starlight, or get heated by stars so that they glow.
Interstellar dust is made of fine particles or grains. Although only a few of these grains are spread through a cubic mile of space, the distances between the stars are so great that the dust can block the light from distant stars.
Many small, dark regions are known where few or no stars can be seen. These are dark nebulas, dust clouds of higher than average density that are thick enough to obscure the light beyond them.
The dust grains block blue light more than red light, so the color of a star can be changed 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 nebulas.
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 the gas is quite cool. These cooler regions are called H I regions.
The interstellar gas, like most stars, consists mainly of the lightest element, hydrogen, with small amounts of helium and only traces of the other elements. The hydrogen readily glows in the hot H II regions. In the cool H I regions the hydrogen gives off radio-frequency radiation. Most interstellar gas can be located only by detecting these radio waves.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.
Wherever there are large numbers of young stars, there are also large quantities of interstellar gas and dust. New stars are constantly being formed out of the gas and dust in regions where the clouds have high densities. Although many stars blow off part of their material back into the interstellar regions, the gas and dust are slowly being used up. Astronomers theorize that eventually a time will be reached when no new stars can be formed, and the star system will slowly fade as the stars burn out one by one.

The Galaxies

Stars are found in huge groups called galaxies. Scientists estimate that the larger galaxies may contain as many as a trillion stars, while the smallest may have fewer than a million. Galaxies can be up to 100,000 light-years in diameter.
Galaxies may have any of four general shapes. Elliptical galaxies show little or no structure and vary from moderately flat to spherical in general shape. Spiral galaxies have a small, bright central region, or nucleus, and arms that come out of the nucleus and wind around, trailing off like a giant pinwheel. In barred spiral galaxies, the arms extend sideways in a short straight line before turning off into the spiral shape. Both kinds of spiral systems are flat. Irregular galaxies are usually rather small and have no particular shape or form.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.
The radio galaxies that have such strong radio emission are usually rather peculiar in appearance. How do they manage to give so much energy to the charged particles and magnetic fields? Many galaxies, and the radio galaxies in particular, show evidence of interstellar matter expanding away from their centers, as though gigantic explosions had taken place in their nuclei. The giant elliptical galaxy known as M87 has a jet of material nearby that it apparently ejected in the past. The jet itself is the size of an ordinary galaxy.
Another problem has bothered 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 clusters appear to be unstable. The galaxies are moving too fast to be held together by the gravity of the matter that is visible. Then why did the clusters not disintegrate long ago? There is no doubt that galaxies contain large amounts of dark matter, matter that has little or no luminosity so that it is not directly visible. However, this matter has enough mass to exert a strong force of gravity that makes the clusters stable. Dark matter constitutes perhaps 90 percent of all matter in the universe. It could be in the form of very-low-luminosity stars, of dead stars, or possibly of other forms of matter. It may be in the form of massive black holes.

Theories on Galactic Formation

Astroomers from two United States universities announced that they had used the Hubble Space Telescope to detect 18 star clusters nearly 11 million light-years away from Earth that they believe might offer clues as to how galaxies are formed. These 18 star clusters, with more than a billion stars each, are packed into a space spanning just 2 million light-years, a remarkably small expanse for so many stars; furthermore, the star clusters appeared to be converging. The observation threatened to overturn the accepted explanation of how galaxies are formed; without much evidence from observation, astronomers long believed that galactic formation was similar to star formation. Stars form when clouds of gaseous material compress and the cloud begins to shrink. Previously, scientists generally concluded that galaxies formed as significantly larger clouds of gas collapsed. The new finding suggested that at least some galaxies form not through the collapsing of larger, gaseous structures, but rather by a convergence of clusters such as the ones under investigation. Other astronomers, while conceding that the finding was very significant, assert that the observed astronomical phenomenon may be only one of several ways in which galaxies form.

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 us to obtain an overall view of the galaxy. In fact, what you can see of its structure is a faint band of stars called the Milky Way (the word galaxy comes from the Greek word for “milk”), so our galaxy has been named the Milky Way galaxy.
The visible band of the Milky Way seems to form a great circle around Earth. This indicates that the galaxy is flat rather than spherical. (If it were spherical, the stars would not be especially concentrated in a single band.) The sun is located on the inner edge of a spiral arm. The center, or nucleus, of the galaxy is about 30,000 light-years distant, in the direction of the constellation Sagittarius. All the stars visible without a telescope belong to 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 circles 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. It is possible to calculate how much matter the galaxy must have in order to hold a star in its orbit by the force of gravity. In this way the approximate number of stars in the galaxy can be estimated.

Velocities of Galaxies

According to the Doppler effect, a general relationship seems to exist throughout the universe: the greater the speed of a galaxy, the greater its distance. This relationship suggests that the system of galaxies is expanding. Suppose the galaxies were at one time in a rather small volume of space. After a time, the fast galaxies would have sped far from the original position, while the slow galaxies would still be nearby. The result would be a velocity-distance relationship exactly like the one observed.
In the early 1960s the new and puzzling quasi-stellar radio source, or quasar, was discovered. In photographs quasars usually look like ordinary stars, but they have Doppler shifts much greater than those of galaxies. This implies that the quasars have enormously large velocities away from us.
If the same relationship between velocity and distance holds for the quasars as for the galaxies, then the quasars are at tremendously large distances from us. But if they are actually so far away, they must be far more luminous than even giant galaxies. And yet, because their energy output varies irregularly over periods of months or less, astronomers have concluded that quasars are actually smaller than ordinary galaxies. The brightest quasar lies at a distance of 2 billion light-years from the Earth.

The Universe

Cosmology is the scientific inquiry into what the universe is like. 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 the universe, except for small irregularities, has an identical appearance to all observers (and the laws of physics are identical), no matter where in the universe the observers are located. 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, alternatively, 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.
Because the universe appears to be expanding, it seems that it must have been smaller in the past. This is the basis for evolutionary theories of the universe. If one could trace the galaxies back in time, one would find a time at which they were all close together. Observations of the expansion rate indicate that this was between 10 and 20 billion years ago. Thus we have a picture of an evolving universe that started in some kind of explosion—the big bang. Some models of the universe predict that the expansion will continue forever. Others say that it will stop and be followed by a contraction back to a small volume again. Another model suggests that the universe oscillates, with alternate expansions and contractions.The steady state theory of cosmology was once popular. It is now, however, discredited. The basic assumption of steady state is a perfect cosmological principle, applying to time as well as position. The steady state theory states that the universe must have the same large-scale properties at all times; it cannot evolve, but must remain uniform. But since the universe is seen to be expanding, which would spread the matter out thinner as time goes on, steady state suggests 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.
Astronomers supporting the open universe theory believe that the universe will expand forever because they believe it is infinite. Supporters of the closed universe theory believe that at some time in the future the universe will stop expanding and will begin to contract until eventually a situation termed the “big crunch” would occur.

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 Mayas of the Yucatán peninsula (in present-day Mexico) developed a complicated calendar for keeping track of days both in the past and in the future. They could use their calendar to predict astronomical events.

From Ancient Times to the Middle Ages

In China, a calendar had been developed by the 14th century 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 geometrical 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 through the fall of Rome to the end of the Middle Ages.
In medieval times Western astronomy did not progress. During those centuries Hindu and Arabian astronomers kept the science alive. The records of the Arabian astronomers and their translations of Greek astronomical treatises were the foundation of the later upsurge in Western astronomy.

Copernicus to Today

In 1543, the year of his death, came the publication of Copernicus' theory that Earth and the other planets revolved around the sun. His suggestion contradicted all the authorities of the time and caused great controversy. Galileo supported Copernicus' theory with his observations that other celestial bodies, the satellites of Jupiter, clearly did not circle Earth.
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 reinforced 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—radar, the radio telescope, and space probes and manned spaceflights—have helped answer old questions and have opened our eyes to new problems.

Archaeoastronomy

Archaeoastronomy is an interdisciplinary field that relates archaeology, anthropology, and mythology with astronomy. It is sometimes called historical astronomy. The best-known evidence that early humankind used the sky is Stonehenge, near Salisbury, England. Built between 3100 and 1550 BC, Stonehenge consists of an impressive array of megaliths and lintels arranged as stone portals that are precisely aligned in relationship to the sun on an ancient summer solstice, which occurred on June 24. Today the summer solstice falls on June 20 or 21. Over the millennia, solstices shift slightly in the calendar. Astronomers have accounted for this shift by observing that the Earth precesses slowly as it rotates on its axis, like a top that wobbles as it spins. This precession makes one complete cycle every 26,000 years. This phenomenon causes the time of the solstices and equinoxes to change and also causes, over time, changes in the apparent location of a polestar, or North Star.There are at least 900 other structures of a similar nature that exist in the British Isles alone. At Carnac on the western coast of France are more than 3,000 stone monuments for which astronomical alignments have been claimed.
Newgrange, northwest of Dublin, Ireland, is a Neolithic tomb, part of which was built as early as 3100 BC. It also indicates probable early knowledge of astronomy. The tomb has a long, narrow passage with a slitlike opening that appears to have been designed and engineered to permit the sunlight to enter the burial chamber at the far end momentarily on the morning of the winter solstice.
From architectural studies of ancient Egypt, there is considerable evidence of early, though not prehistoric, astronomical knowledge. The base of the Great Pyramid at Giza is aligned closely with the four points of the compass, and its hidden north passage is aligned with the lower culmination of the North Star at the time the pyramids were built (between 2575 and 2465 BC). The great temple of Amon, or Amen (Ra), at El Karnak was aligned with the midwinter sunrise during the epoch of Thutmose III (1479–26 BC). A nearby temple of Khonsu, the Egyptian moon god, was built to align with the distant hills of Thebes, the northernmost extreme of the setting of the new moon crescent at the time of the summer solstice.
The Dresden Codex, written by the Maya during the 1st millennium of the Christian era, contains astronomical calculations—eclipse-prediction tables, the synodic period of Venus—of exceptional accuracy. Temples and pyramids in what are now Mexico and Guatemala were often constructed and aligned with attention to astronomical phenomena.
The Plains Indians left stone patterns called medicine (magic) wheels, found along the eastern boundary of the Rocky Mountains from Colorado to Alberta and Saskatchewan. One of the best known is on top of Medicine Mountain, in the Bighorn Mountains west of Sheridan, Wyo. It is accessible only in the summer, and calculations have been made relating the arrangements of crude sandstone to the sunrise and sunset about the time of the summer solstice and to bright stars that would have been visible at the time archaeologists estimate its construction, about 200 to 400 years ago. Another medicine wheel, at Moose Mountain in southeastern Saskatchewan, has demonstrably similar relationships but has been estimated as dating from about 2,600 years ago.
A number of stone alignments have also been found in the South Pacific. Such stones would, of course, be useless at sea, but it has been suggested that the sites were used for observation and to train voyagers to identify the correct navigational stars before their departure. For example, the Micronesians and Polynesians used as navigational tools strings of bright stars that rose or set near the same point on the horizon. The Caroline Islanders had a 32-point star compass for defining this point, using Vega, the Pleiades, and other stars.