SATURN

Saturn is the second largest planet in our solar system, after Jupiter. It is the sixth planet from the Sun. Saturn is surrounded by spectacular rings. Italian astronomer Galileo was the first person to see the rings around Saturn. He thought the rings looked like handles. Galileo was looking at Saturn through one of the first telescopes, in 1610. In the 1650s, a Dutch astronomer named Christiaan Huygens was the first astronomer to see that the “handles” were really huge rings.

SATURN’S RINGS

Saturn has seven main rings. The rings are made of many smaller ringlets. They are by far the biggest and brightest rings of any planet in our solar system. Jupiter, Neptune, and Uranus also have rings. Saturn’s rings go out hundreds of thousands of miles from the center of the planet. You could just fit Saturn and its rings in the space between Earth and our Moon.

Saturn’s rings are pretty thin. In places they are only 16 feet (5 meters) thick. The rings are made of dust, pieces of rock, frozen gases, and ice.

WHAT IS SATURN LIKE?

Saturn is a huge ball of gas. Like the planets Jupiter, Neptune, and Uranus, Saturn is called a gas giant. None of the gas giants have a solid surface that you could land a spacecraft on.

The gases around Saturn are poisonous to people. You need to breathe oxygen in order to live. Saturn’s atmosphere is mostly hydrogen and helium gas. An atmosphere is the layer of gases around a planet.

There are thick clouds in Saturn’s atmosphere. The clouds look like colored stripes going around the planet. Strong winds and storms make Saturn’s clouds whirl and swirl.

Saturn’s atmosphere blends into the center of the planet. The gases get thicker and heavier the farther down you go. Finally, the gas turns to liquid.

Astronomers think that the center, or core, of Saturn is very hot. They think the temperature could be about 27,000° Fahrenheit (15,000° Celsius). There are probably rocks and possibly iron in the core of Saturn.

Saturn acts like a big magnet in space. Other planets, including Earth, also act like big magnets.

DOES SATURN HAVE MOONS?

Saturn has 47 moons. Some of the moons orbit, or go around, Saturn inside the rings. Others orbit farther out, beyond the rings.

The largest moon is Titan. Titan is larger than the planets Pluto and Mercury. It has an atmosphere made of nitrogen gas. Scientists think that Titan’s atmosphere may be like Earth’s atmosphere was billions of years ago.

Saturn’s other moons are smaller and icy cold. Enceladus is covered with bright ice, and it has geysers. It is the brightest moon in the solar system. Mimas has a huge crater that formed when a meteor hit. The enormous impact must have almost shattered the moon.

HOW DO WE LEARN ABOUT SATURN?

Ancient people could see Saturn without a telescope. But they could not see Saturn’s rings. They named Saturn after the Roman god of farming.

Astronomers study Saturn through telescopes. They send spacecraft to take pictures and measurements of Saturn and its moons and rings. The first spacecraft to visit Saturn was Pioneer 11 in 1979. Voyager 1 and Voyager 2 visited Saturn in the 1980s. The Cassini spacecraft arrived at Saturn in 2004.

URANUS

There are no cars on the planet Uranus. Yet Uranus is covered with a kind of smog. Smog on Earth is pollution that comes mostly from cars that burn gasoline. Sunlight changes fumes from the cars to smog.

Smog on Uranus comes from gases called ethane and methane. Small amounts of these gases are part of the atmosphere of Uranus. An atmosphere is the layer of gases around a planet, like air on Earth. Sunlight shining on methane and ethane makes smog. The haze of smog around Uranus makes the planet look like a smooth, bluish-green beach ball.

WHO DISCOVERED URANUS?

Uranus was the first planet discovered with a telescope. William Herschel, an astronomer in England, found Uranus in 1781. Astronomers are scientists who study things in space. Astronomers had known about Uranus since 1690, but they did not know Uranus was a planet. At first, they thought it was a star or a comet. Astronomers named the planet Uranus for an ancient Greek god of the heavens.

WHAT IS IT LIKE ON URANUS?

Uranus is a huge planet. It is the third largest planet in the solar system, after Jupiter and Saturn. The diameter (width) of Uranus is more than four times bigger than the diameter of Earth.

You could not walk around on Uranus. Uranus does not have a solid surface. The planet is made up mostly of gas. It has clouds made of methane ice high in the atmosphere. The clouds get thicker and thicker the farther down you go.

The thick clouds blend into a liquid ocean. Astronomers think the ocean on Uranus is made of water and the chemicals ammonia and methane.

THE ATMOSPHERE OF URANUS

You could not breathe on Uranus. The atmosphere there is made mostly of hydrogen gas and helium gas. There are also small amounts of the gases ethane and methane.

There are thick blue clouds in the atmosphere. Uranus has some of the brightest clouds in the solar system. Strong winds blow the clouds around. The clouds make striped patterns under the smog.

URANUS LIES ON ITS SIDE

Uranus looks like it is spinning lying down. All planets spin. They spin around an imaginary line called an axis. The line goes from the north pole to the south pole. It goes from the top to the bottom of a planet.

Most planets are tilted a bit. Earth is tilted as it spins around its axis. But Uranus is tilted so much that it is lying on its side. Astronomers think that something big crashed into Uranus after it formed. The crash may have knocked Uranus onto its side.

There are weird seasons on Uranus. At times, the north pole or the south pole faces directly toward the Sun. Each season on Uranus is more than 20 years long!

WHAT ARE THE RINGS AND MOONS LIKE?

Astronomers have found 13 rings around Uranus. The rings go around the equator. An equator is an imaginary line around the middle of a planet. The rings are dark and faint. They may be made of ice and dust.

Uranus has many moons. Astronomers believe the planet has at least 27 moons. The 2 biggest and brightest moons are Oberon and Titania.

The next largest moons are Umbriel and Ariel. The surfaces of all 4 of these big moons are covered with holes called craters. Meteorites (rocks from space) probably made the craters. Astronomers think that these 4 moons are made of ice and rock.

Another smaller, icy moon named Miranda looks very strange. It has a jumble of ridges. It has three weird areas shaped like oval racetracks. There is nothing like Miranda anywhere else in the solar system. Miranda is a very mysterious little moon.

HOW DO WE LEARN ABOUT URANUS?

Astronomers use telescopes to learn how Uranus moves across the sky. Uranus is the seventh planet from the Sun. It orbits, or goes around, the Sun in an oval-shaped path. Its average distance from the Sun is a bit less than 2 billion miles (3 billion kilometers). Uranus is so far away that it takes 84 years for the planet to go around the Sun once!

Most of what we know about Uranus comes from a trip to the planet by the Voyager 2 spacecraft. Voyager 2 flew past Uranus in 1986. Voyager 2 took many pictures of the planet and its rings and moons.

Astronomers also study pictures from the Hubble Space Telescope. This telescope orbits high above Earth. They use other big telescopes on Earth to study Uranus.

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NEPTUNE

In ancient times, people didn’t even know the planet Neptune existed. Astronomers, scientists who study space, didn’t discover the big blue planet until the 1840s.

Neptune is usually the eighth planet from the Sun. So far as astronomers know, only the planet Pluto is farther from the Sun. However, Pluto’s orbit sometimes brings it closer to the Sun than Neptune is, making Neptune the planet farthest from the Sun. This happens every 248 years.

HOW DID WE FIND NEPTUNE?

Astronomers found Neptune by watching the planet Uranus. Uranus wobbles as it orbits the Sun. A British astronomer and a French astronomer used math to figure out that the gravity of another planet was making Uranus wobble. A German astronomer finally saw Neptune through a telescope in 1846. Astronomers named the newly found planet Neptune after the ancient Roman god of the sea.

STUDYING NEPTUNE

It took astronomers a long time to learn anything about Neptune because the planet is so far away. Neptune is about 2.8 billion miles (about 4.5 billion kilometers) from the Sun. That’s 30 times as far as Earth is from the Sun! It takes Neptune almost 165 years to go around the Sun once.

Looking through telescopes, astronomers can tell that Neptune is a big planet. It is about four times larger than Earth. They call Neptune a giant planet. It is the smallest of four huge planets made mostly of gas. Jupiter, Saturn, and Uranus are the other gas giants.

Astronomers learned most of what they know about Neptune from a spacecraft named Voyager 2. Voyager 2 is the only spacecraft that has visited Neptune. It flew past in 1989. It took pictures and made measurements of Neptune.

Astronomers also study pictures of Neptune made by the Hubble Space Telescope. Hubble orbits high above Earth.

WHAT IS IT LIKE ON NEPTUNE?

You could not breathe on Neptune, and the fierce winds would blow you away. Even without the winds, you could not walk on Neptune because the planet has no solid surface.

A thick layer of clouds surrounds Neptune. The clouds are part of the planet’s atmosphere. An atmosphere is made of gases. Neptune’s atmosphere is made mostly of hydrogen and helium gases. Some methane gas high in the atmosphere gives Neptune its bluish color. You could not breathe the gases in Neptune’s atmosphere. You need to breathe oxygen in order to live. There is no oxygen in Neptune’s atmosphere.

Blowing clouds make striped patterns around Neptune. Sometimes there are spots in the clouds. The spots are storms. Neptune has the fastest winds in the solar system. The winds on Neptune can blow at 1,200 miles per hour (2,000 kilometers per hour).

Scientists have made guesses about what the inside of Neptune is like. Scientists think the planet’s thick atmosphere blends into an ocean of water. The core (center) of Neptune may be made of ice and rock.

DOES NEPTUNE HAVE MOONS?

Astronomers have found 13 moons around Neptune. The biggest moon is Triton. Triton is mostly made of ice. There may be some rock in its core. Triton is one of the coldest spots in the solar system. The temperature there can drop to -390° Fahrenheit (–235° Celsius).

Triton once had volcanoes. Its volcanoes did not shoot out melted rock as volcanoes on Earth do. Triton’s volcanoes shot out slushy, half-melted ice.

Astronomers also have found four rings around Neptune. The rings are dark and hard to see, even in pictures from Voyager 2. They may be made of dust and rocks.

HOW DID NEPTUNE FORM?

Astronomers think the Sun and all the planets formed from a disk of gas and tiny particles of rock, metal, and ice. The rocky planets closest to the Sun formed mainly from rock and metal. The planets farthest from the Sun formed mostly from gas and chunks of ice.

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PLUTO

Our Sun would look like a small dot of light if you were standing on the surface of Pluto. But Pluto is so far away that no spacecraft has ever visited it. Pluto is the ninth planet in our solar system. Astronomers know less about Pluto than about any other planet.

Pluto is usually the farthest planet from the Sun. It goes around the Sun in an oval-shaped path called an orbit. It takes almost 248 Earth years for Pluto to go around the Sun once. Sometimes Pluto’s orbit brings Pluto closer to the Sun than the eighth planet, Neptune. During that time, Pluto is the second farthest planet from the Sun. Neptune is farther.

HOW WAS PLUTO DISCOVERED?

An American astronomer named Percival Lowell observed that the orbits of the distant planets Neptune and Uranus changed slightly as they went around the Sun. Lowell decided that the changes were being caused by an unseen planet. He called it Planet X.

Lowell died before he found Planet X. A young astronomer named Clyde Tombaugh found Pluto in 1930. It was not in the same place that Lowell predicted, and it turns out that Lowell made mistakes in his math. But because he called attention to it, Tombaugh worked hard to find another planet and did. He saw Pluto on pictures taken with a powerful telescope. Even with a powerful telescope, Pluto looks like just a tiny dot of light.

Astronomers in 1978 found that Pluto has a moon. They named the moon Charon. Charon is almost as big as Pluto. In 2005 astronomers discovered two small moons.

WHAT IS PLUTO MADE OF?

Scientists are not sure what Pluto is made of. They think it’s made of rock and ice. They know that Pluto is very cold because it is so far from the Sun.

Pluto has a thin atmosphere sometimes, when it comes closest to the Sun. An atmosphere is a layer of gases around a planet. The Sun warms frozen gases on Pluto’s surface. Some frozen nitrogen, carbon monoxide, and methane turn to gas and become an atmosphere. When Pluto moves farther from the Sun, the gases freeze again. For most of its orbit, Pluto has no atmosphere.

In 1994, the Hubble Space Telescope, orbiting high above Earth, took pictures of Pluto. The pictures showed that there are ice caps at Pluto’s north and south poles. They showed that there are bright and dark areas on Pluto.

Scientists are not even sure exactly how big Pluto is. They know that it is smaller than Earth’s moon. There are many things left to learn about Pluto. The New Horizons spacecraft was launched in 2006, and scientists hope it will fly past Pluto in 2015.

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SOLAR SYSTEM

Do you think our planet is the only place in the universe where there is life? Until 1995, astronomers had never found a solar system like ours. A solar system is made up of a star surrounded by planets and other objects. In 1995, astronomers found a planet orbiting (going around) a distant star like our Sun. Since then, they have found other solar systems. Astronomers now think that there are many solar systems in the universe. They do not know whether there is life in any of these other solar systems.

Our solar system is the one we know the most about. The Sun is at its center. Our solar system includes everything that orbits around the Sun. Planets, moons, asteroids, comets, gas, and dust are all part of the solar system.

Our solar system lies near the edge of the Milky Way galaxy. A galaxy is a huge collection of stars. The Milky Way is shaped like a whirlpool. All the stars in the galaxy, including our Sun, orbit around the center of the Milky Way.

THE SUN

The Sun, like other stars, is a hot ball of gas. Hydrogen and helium are the main gases in the Sun. Almost all the energy in our solar system comes from the Sun. The Sun changes hydrogen into helium to create light and heat. These changes take place deep inside the Sun.

THE NINE PLANETS

Eight of the nine planets in our solar system fall into two groups called the inner planets and the outer planets. The four planets closest to the Sun are called the inner planets. They are Mercury, Venus, Earth, and Mars. The inner planets are also called the rocky planets, because they are made mainly of rock and iron. There are four outer planets: Jupiter, Saturn, Uranus, and Neptune. The outer planets are also called the gas giants because they are huge and made mainly of gas. Pluto, the farthest planet from the Sun is a small ball of ice. Some astronomers wonder whether Pluto should be called a planet.

Mercury is the planet that is closest to the Sun. It has a large core, or center, that is made of iron. Venus, the next planet from the Sun, is the hottest of the planets. The temperature on Venus is about 890° Fahrenheit (about 477° Celsius). Earth is the only planet known to have life. It also has plenty of liquid water. There are signs that liquid water may once have flowed on Mars, but now Mars is cold and dry. What happened to the water on Mars is a great mystery that scientists are trying to solve.

Jupiter is the largest of the planets. A thick atmosphere made of hydrogen and helium surrounds it. Jupiter’s clouds look like white, brown, and orange stripes going around the planet. There is an oval-shaped red spot in the clouds. Astronomers think this spot is a big storm that has been raging for hundreds of years! Saturn is the second largest planet and the sixth planet from the Sun. It has bright rings around it. Uranus and Neptune look like smooth blue balls. Methane gas gives these planets their blue color. Pluto is the smallest planet in our solar system. It is far from the Sun and very cold. Temperatures on Pluto can drop down to -387° Fahrenheit (-223° Celsius).

MOONS AROUND THE PLANETS

Seven of the planets have natural satellites, or moons. Mercury and Venus do not have any moons. Earth has 1 moon and Pluto has 3. Mars has 2 moons. Neptune has 13 known moons; Uranus has at least 27 moons; Saturn has 47 moons; Jupiter has at least 63 moons. Jupiter’s moon Io has active volcanoes.

ASTEROIDS, COMETS, AND DUST

There are thousands of asteroids in the solar system. Asteroids are small pieces of rock and metal. Most of them orbit the Sun between Mars and Jupiter. Asteroids that crash into Earth are called meteors. Sometimes they burn up as they fall toward Earth. They make streaks of light in the night sky. Pieces that land on the ground are called meteorites.

Comets are another type of object in the solar system. Comets are like dirty snowballs. They are made of ice and dust. Comets are usually far out in the solar system. Sometimes a comet comes in close to the Sun. When it comes in close, the comet starts to melt and looks like it has a long tail streaming out behind it. In 1994 pieces of a comet called Shoemaker-Levy 9 crashed into Jupiter. The crash made huge explosions and sent up fireballs that were larger than Earth.

HOW DID THE SOLAR SYSTEM FORM?

Astronomers think that the solar system may have come from a swirling cloud of gas and dust. First a star, our Sun, formed from a clump in the cloud. It began as a spinning ball of gas at the center of the cloud. Then the planets, their moons, and the other objects in the solar system formed from the leftover gas and dust. Other stars and solar systems in the universe may have formed the same way. In fact, new solar systems are still forming from giant clouds of gas and dust.

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BIG BANG

The universe contains everything that exists: Earth, the Sun, the stars, galaxies (collections of billions of stars), and everything else in space. People have wondered how the universe got started for thousands of years. Most scientists now think they have the answer. They think the universe began about 14 billion years ago with a kind of big explosion. They call the explosion the big bang.

WHAT HAPPENED AFTER THE BIG BANG?

No one knows what caused the big bang, but scientists think they know what happened all the way back to the first seconds after the big bang.

The brand new universe was very hot and very small. It blew outwards very fast. In the first three minutes, matter started to form. Hundreds of years later, the universe looked like a big ball of fire.

You can picture the universe as something like a black balloon with white dots painted on it. The black represents space and the white dots are galaxies. Blowing air into the balloon makes it bigger. The spaces between each dot get farther apart as the balloon expands.

As it got bigger, the universe got cooler. Hydrogen gas formed. The gas broke into clumps. The clumps came together to make galaxies and stars. Other kinds of matter formed in the stars. Finally, planets like Earth formed around some stars.

IS THERE PROOF OF A BIG BANG?

The expansion of the universe is evidence for the big bang. American scientist Edwin Hubble studied light coming from galaxies far out in the universe. In 1929, he found that the galaxies were speeding away from Earth and from each other in all directions. Scientists tracked the paths of the galaxies back to their starting place. They saw that all the galaxies must have started from the same place. Packing all that matter into a small area would make a very dense, searing hot ball—the big bang.

Scientists use math to describe how the universe behaves. In the early 1900s, German American scientist Albert Einstein came up with equations that predict an expanding universe. These equations have correctly predicted the motions of stars, planets, and light.

More proof came in the 1990s from a spacecraft called the Cosmic Background Explorer (COBE). COBE saw rays coming from far off in the universe. The rays are left over from the early days of the universe. They could only have been created in a much smaller and hotter universe long ago.

WILL THE UNIVERSE KEEP EXPANDING?

Scientists are not sure what will happen to the universe. They currently think it will keep expanding forever. They even think the expansion is speeding up. But scientists are still studying this question.

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MOON

Did you ever look at the Moon and think you could see a face? Sometimes dark spots on the Moon look like eyes, a nose, and a mouth. People used to talk about “the man in the Moon.” They would joke about the Moon being made of cheese with holes in it.

The Moon is the second brightest thing in our sky, after the Sun. The Moon doesn’t make its own light. Light rays from the Sun bounce off it and make it shine. The Moon is closer to Earth than any other body in our solar system.

WHAT’S ON THE MOON?

In the 1600s, the famous Italian scientist Galileo was the first person to look at the Moon through a telescope. He saw dark spots that he thought were oceans. He called them maria, the Latin word for “seas.” Galileo thought the light areas were large landmasses called continents.

Today, we know a lot more about the Moon. We know that nothing lives on the Moon, and there are no oceans. The maria are dry, flat plains covered with rocks. The Moon is the only place in space that human beings have visited.

TOUCHING THE MOON

The first astronauts landed on the Moon in 1969. They traveled in a United States spacecraft named Apollo 11. The astronauts set up experiments on the Moon and brought some moon rocks back to Earth. Later, five more Apollo missions explored different parts of the Moon. The astronauts on these missions brought back more rocks and soil.

Scientists learned many things about the Moon from the Apollo space missions. They also learned from other spacecraft that orbited (went around) the Moon. Some of these spacecraft sent robot landers down to the surface of the Moon.

SPACE ROCKS AND CRATERS

The dry, gray Moon might seem like a boring place now. But you should have seen it several billion years ago.

Many times over the past two or three billion years, chunks of rock and ice have come whizzing toward the Moon. The space rocks and ice are asteroids and comets. They slam into the Moon’s surface. The biggest ones came just after Earth and the other planets were formed. When they hit the Moon, these large objects threw up tons of rock and dust. There are billions of big and small pits on the Moon made by the space rocks. These pits are called craters.

ANCIENT VOLCANOES

If you went to the Moon, you’d see the dark-colored maria. Scientists think the dark gray rock is lava (melted rock). They believe that billions of years ago, red-hot rock gushed up from volcanoes on the Moon. The lava flowed over the Moon’s surface. It filled in low places, including some of the big craters. Then the lava cooled to make the Moon’s gray rocks.

The lava also left round hills on the Moon called domes and carved grooves called rilles.

ROUGH HIGHLANDS

There are rough and mountainous places all over the Moon. Scientists call these places highlands.

There are highlands on the far side of the Moon but almost no maria. Only one side of the Moon faces Earth, so you can never see the far side of the Moon. Scientists learned what the far side looks like from pictures taken by orbiting spacecraft.

HOT DAYS AND COLD NIGHTS

The astronauts who walked on the Moon had to wear big space suits. The space suits provided air for the astronauts to breathe, because there is no air on the Moon. The suits also kept the astronauts cool during hot Moon days and warm during cold Moon nights.

With no atmosphere to protect it, Moon temperatures can be very high and very low. It can be 261° Fahrenheit (127° Celsius) at noon during a Moon day—hotter than boiling water! It can be as cold as -279° Fahrenheit (-173° Celsius) on a Moon night. Days and nights on the Moon each last about two weeks.

Days and nights are long because the Moon turns very slowly. It takes the Moon about 27 days to make one turn. Earth turns once every 24 hours.

ICE ON THE MOON?

There is no water on the Moon, but scientists think that there may be ice. Two spacecraft in the 1990s saw signs of the ice. If there is ice on the Moon, it could help future explorers stay there longer.

The signs of ice were found in deep craters at the north and south poles of the Moon. Because these craters are always in shadow, it stays very cold there—about -364° Fahrenheit (-220° Celsius).

THE MOON FROM EARTH

The Moon always seems to change shape. Sometimes it looks like a round ball in the sky. Sometimes it is a thin sliver. But the Moon does not really change shape. What happens to it?

The Moon reflects light from the Sun. How you see the reflected sunlight depends on where the Moon is. The Moon orbits (goes around) Earth. Sometimes it is between the Sun and Earth, and you can’t see any reflected sunlight. This is called the new moon.

Sometimes Earth is between the Moon and the Sun. You can see all of the reflected sunlight. The Moon looks round. This is called a full moon.

The rest of the time, you see only part of the reflected sunlight from the Moon. The reflected sunlight looks like slivers of Moon. It takes about 27 days to go from a new moon to a full moon and back to a new moon again.

WHERE THE MOON CAME FROM

No one knows for sure how the Moon was formed. By testing moon rocks, scientists have learned that the Moon is about 4.6 billion years old. This is the same age as the solar system.

Scientists think that at that time something as big as a planet crashed into Earth. The collision blasted huge pieces of Earth into space. Some of the pieces came together to make the Moon.

Scientists continue to study moon rocks for clues. There is still much to learn about the Moon.

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BLACK HOLES

Black holes are some of the strangest things in space. A black hole sucks in anything that gets near it. Nothing can escape from a black hole—not even light.

BLACK HOLES ARE STRONG

Nothing escapes from a black hole because its gravity is so strong. Gravity is a force that pulls one thing to another. Gravity is the force that holds you down on Earth. When you jump up, Earth’s gravity pulls you right back down. Earth’s gravity also makes the Moon orbit (go around) Earth.

The more matter (stuff) that is packed in a star, planet, moon, or other object, the stronger is its gravity. Gravity makes an object with more matter pull an object with less matter toward it. The Sun has a lot more matter than Earth. The Sun’s gravity pulls on Earth. It makes Earth orbit the Sun.

Matter is packed very tightly in some things and loosely in others. The matter that makes up an iron ball is packed much tighter than the matter that makes up a bag of feathers. A scientist would say that an iron ball is much denser than a bag of feathers.

A black hole is denser than anything you could imagine. A black hole could have a million times more stuff than our Sun. All of this stuff would be packed into an area smaller than a city. The force of gravity from so much stuff packed into such a small area is awesome.

WHERE DO BLACK HOLES COME FROM?

Astronomers and physicists think black holes come from dying stars. A dying star burns out and stops shining. All the stuff that makes up the star starts falling in on itself. The star gets denser and denser. If the star is big enough and has enough matter, it could get dense enough to become a black hole.

STUDYING BLACK HOLES

No one has really seen a black hole. You cannot see black holes because they do not give off any kind of light. Physicists used math to predict that black holes exist.

Astronomers look for signs of black holes. Astronomers study powerful rays coming from stars in deep space. The stars seem to be orbiting black holes. Astronomers think that black holes are sucking gas from the stars, and this makes the stars give off X rays.

Galaxies are enormous groups of stars. Astronomers think that most galaxies have huge black holes at their centers. The Hubble Space Telescope took pictures of a disk of hot gases at the center of our own Milky Way Galaxy. Astronomers think this disk is going around an enormous black hole right in the center of our galaxy.

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Constellations

In ancient times when people looked up at the starry night sky, they thought they saw shapes in groups of stars. We call these shapes constellations. The ancient Sumerians, Babylonians, Egyptians, Greeks, and Chinese named constellations of stars after heroes and beasts from stories and after everyday objects.

You probably know some constellations. The Big Dipper looks like a giant pot with a long handle. The constellation Orion is named after a hunter in Greek mythology. You can see his belt, marked by three bright stars, and his sword, which hangs from his belt.

The stars that form constellations are not really near each other. Some of the stars in a constellation are much farther from us than others. The stars just happen to form patterns as we view them from the Earth.

WHICH CONSTELLATION IS WHICH?

When you look up at the sky, you see shapes much like those which the ancient stargazers named after characters from magical tales. You can use a star chart, a map that shows where stars appear in the sky, to become familiar with the shapes and names of the constellations. As the night passes, these great shapes seem to move through the sky, just as the Sun appears to cross the sky during the day. But it’s actually Earth that’s moving, not the Sun and stars.

You can only see the brightest stars with the naked eye. Try looking at the sky through binoculars or a telescope. Thousands of fainter stars come into view. You can no longer see the shapes of constellations.

DO CONSTELLATIONS ALWAYS LOOK THE SAME?

Because Earth tilts as it circles the Sun, you see different constellations at different times of year. The Big Dipper, for example, is easiest to find during summer. Orion is most visible during winter.

People in Australia and elsewhere in the Southern Hemisphere see completely different constellations than people in Canada or the United States. One of the most famous southern constellations is Crux, the Southern Cross.

The shapes of constellations slowly change over very long periods of time. The familiar forms will look quite different many thousands of years from now.

HOW DO ASTRONOMERS USE CONSTELLATIONS?

Astronomers divide the sky into 88 constellations. Even though the constellations do not represent real groupings of stars, astronomers still find them useful for naming stars and mapping the sky.

Astronomers use letters of the Greek alphabet to name stars. They also use a form of the name of the constellation the star is in. The brightest star in a constellation has alpha in its name, because alpha is the first letter of the Greek alphabet. For example, the brightest star in the constellation Perseus is called Alpha Persei. And the second brightest is Beta Persei. (Beta is the second letter in the Greek alphabet.) The star closest to the Sun is Alpha Centauri, the brightest star in the southern constellation of Centaurus.

Some objects that are not stars are also named after the constellations in which they appear. Such objects include the Andromeda galaxy and the Orion nebula. At certain times of the year, the Earth passes through showers of meteors (shooting stars). Even these meteor showers, such as the Perseids and Geminids, are named after the constellations from which they seem to fall.

WHAT ARE THE CONSTELLATIONS OF THE ZODIAC?

The ancient Babylonians noticed that the Sun’s position in the sky changes through the year. They divided the stars along the Sun’s path into 12 constellations. We call these 12 the constellations of the zodiac. They consist of Aries, the Ram; Taurus, the Bull; Gemini, the Twins; Cancer, the Crab; Leo, the Lion; Virgo, the Virgin; Libra, the Balance; Scorpio, the Scorpion; Sagittarius, the Archer; Capricorn, the Goat; Aquarius, the Water Bearer; and Pisces, the Fishes.

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COMETS

People were very superstitious in ancient times. They believed that fuzzy, white streaks that sometimes appeared in the night sky meant an important event was about to happen. They believed that these comets were omens (signs) that bad things were about to happen. Astronomers now know that a comet is a ball of ice, rock, and dust. A comet is like a dirty snowball speeding though space.

WHAT ARE COMETS?

Astronomers in the 1500s and 1600s began to study comets. They found that comets are bodies in space, not part of the atmosphere.

A British astronomer named Edmond Halley discovered that comets actually orbit the Sun. They return again and again, but they go so far from the Sun that their returns take a long time. He studied a comet that showed up in 1682. He proposed that it was the same comet that had appeared in 1607 and 1531. Halley predicted that the comet would return again around 1758. The comet returned as he had said it would. The comet came to be known as Halley’s Comet. The orbit of Halley’s Comet takes it close to the Sun about every 76 years. The last time Halley’s Comet came near the Sun was in 1986.

Astronomers now study comets with telescopes and spacecraft. They are learning more about what comets are made of, where they come from, and how they move in space.

WHAT DOES A COMET LOOK LIKE?

A comet looks like a fuzzy, white ball with one or more fat, white tails trailing behind it. A comet has three main parts. It has a center made of ice and rock. A hazy cloud of gas called the coma surrounds the center. A comet can also have one or more tails that trail behind it.

Comets only have tails when they get near the Sun. When a comet comes near the Sun, heat makes some of the ice turn to gas. The comet gets very bright. This gas streams away and carries dust with it, making one or more tails. The tails always point away from the Sun.

HOW DO COMETS TRAVEL IN SPACE?

Comets orbit the Sun just as the planets do. That is why astronomers see the same comet return over and over again. But comets have huge oval-shaped orbits. They swing far out to the edge of the solar system.

It takes Earth 365 days to go around the Sun. The orbits of some comets are so big that it takes them hundreds of years to go around the Sun once.

WHERE DO COMETS COME FROM?

Comets come from places in the outer solar system called the Kuiper Belt and the Oort Cloud. The Kuiper Belt and the Oort Cloud are made up of chunks of ice and rock. Comets that orbit the Sun in less than 200 years come from the Kuiper Belt. The Kuiper Belt is just beyond the planet Neptune. Comets that take longer than 200 years to go around the Sun come from the Oort Cloud. The Oort Cloud is far out at the edge of the solar system, beyond the planet Pluto.

Astronomers think that comets formed soon after the solar system began about 5 billion years ago. The solar system formed from a big cloud of gas and dust. The Sun formed in the center of the cloud. Gas and dust farther out clumped together to make the planets. Comets may have formed from leftover gas and dust at the edge of the solar system where it is very cold.

COULD A COMET HIT EARTH?

We know that comets can hit planets. In 1994, telescopes and spacecraft watched Comet Shoemaker-Levy 9 crash into the planet Jupiter. The crash sent up fireballs that were bigger than Earth!

Some scientists think that comets hit Earth long ago. A comet or asteroid (space rock) crashing into Earth 65 million years ago may have killed off the dinosaurs. The crash would have sent up a huge cloud of dust. The dust could have blocked out the Sun’s light. Earth would have become cold and dark enough to kill off the plants that dinosaurs ate.

On a dark, clear night you can sometimes see streaks of light shooting across the sky. Some of these shooting stars are bits left behind by comets. They give off light as they burn up while falling through the atmosphere. So in a way, comets are constantly falling on Earth!

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