What's the difference between a meteoroid, a meteor and a meteorite?

A meteoroid is a piece of stone, metal, or dust that travels through space and sometimes enters the Earth's atmosphere.

Most meteoroids burn when they enter the atmosphere, and some reach the Earth's surface. When a meteoroid is large enough to be seen when it burns in the Earth's atmosphere, it is called a meteor.

Every day, about 200,000 meteors (or shooting stars) are visible on Earth, and 100 tons of meteoroid material are added to our planet's atmosphere every day. But only about 150 meteoroids reach the Earth's surface in an average year. When they do, they are called meteorites.

Very large meteorites have crashed into the Earth, creating huge craters. However, the largest meteorite ever discovered in one piece was found in Namibia. This block of outer space material was 2.70 metres wide and weighed about 59 tonnes!

A meteoroid in outer space that measures more than 10 metres is called an asteroid.

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Is the Sun yellow or white?

 Throughout the day we observe that our star changes tonalities: yellow at dawn, white at noon and red at dusk. Why?

This happens when the light of the Sun penetrates the atmosphere, the blue color tends to be dissipated, but not the red and yellow, so we see our star with those tonalities: from the characteristic yellow at dawn, to the strong red at dusk.

The color of a star, on the other hand, depends on its age and size: there are red giants or white dwarfs, for example our Sun, is actually white. Moreover, human sight is not able to distinguish colors in the dark, for this reason we see all white stars at night, except Betelgeuse in the constellation of Orion and Antares in Scorpio, which because they are at the end of their lives, look indistinguishably red if compared to other stars. If the Sun were one of those distant stars, we would see it as a white rather than a yellow star.

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What are impact craters?

Impact craters are geological structures formed when an asteroid, comet or meteorite hits a planet or satellite. The Earth has suffered more impacts than the Moon, but most craters have been erased by erosion, tectonic activity and volcanism.

The first of these geological structures was catalogued in 1920. It was the Meteor Crater, in Arizona, discovered by a group of workers who found meteorite fragments inside.

For years, the only way to recognize impact craters was to find these cosmic fragments. Most of the time, however, the impact is so brutal that any trace of the body that produced it is erased.

Today the chemical and magnetic footprints of meteorites and comets can be recorded, so the number of classified impact craters has increased to more than 150.

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What are gamma rays?

With the help of a satellite and a global network of telescopes, astronomers can see and locate stellar explosions at a very long distance. A gamma-ray burst is an event caused by a star that died in one of the early stages of the universe.

The record of light that generates explosions of this type serves researchers to collect data on the evolution of the cosmos, ''its intensity exceeds that of galaxies in brightness; it releases an enormous amount of gamma rays and particles that travel at the speed of light,'' says Donald Lamb, an astrophysics expert at the University of Chicago.

The challenge faced by astronomers in the face of this phenomenon is that the gamma ray shots that produce light last only seconds before dissipating. To locate them, NASA launched the Swift satellite, which has a gamma-ray telescope that locates them in fractions of a second and sends their coordinates by e-mail to almost a thousand astronomers around the world.

The light from the bursts comes from stars that exploded when the universe was just a few hundred million years old. A gamma-ray burst offers a rare glimpse through space and time," says Nial Tanvir, an astronomer at the University of Hertfordshire, England.

George Ricker, an astronomer at the Massachusetts Institute of Technology, says: "If we can locate such ancient explosions, we may finally go back in time to know the origin and constitution of the first stars before galaxies existed.

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Deneb star

    Deneb, also called Alpha Cygni, is the brightest star in the constellation Cygnus. Deneb is the 19th brightest star seen from the Earth and has an apparent magnitude of 1.26. Its absolute brightness is about 60,000 times greater than that of the Sun. Deneb is a bluish-white giant star with a surface temperature of about 20,000° F., or twice that of the Sun.
    Deneb reaches its highest point in the sky in mid-September. Its distance from the Earth is approximately 1,600 light-years.

Comets

   The Sun has a whole family of heavenly bodies traveling around it. Some of the most interesting are the comets. "Comet" comes from a Latin word that means "hair." A comet looks like a star that has shining hair streaming out from it.
   There are hundreds of comets in our solar system, but many people have never seen one. Most comets do not come close enough to the Earth to be seen without a telescope.
   People of long ago were very much frightened when there was a comet bright enough to be seen. They thought it was a sign that something terrible was about to happen. Today people have given up that old idea.
    Comets have paths around the Sun just as the Earth has. But their paths differ in shape. They are like a long oval. The paths of many comets cross the Earth's path. Some day the Earth may have a head-on collision with a comet. It has already gone through the tail of one. This happened on May 18, 1910. Newspapers carried the news in advance. Many people feared the world would end. But nothing happened at all. If the news had not been in the papers, few people would have known that the Earth had had a strange adventure.
   No one knows what would happen if the Earth ran into the head of a comet. The head of a comet is very big, but it is not a solid ball. Scientists believe it is made up of millions of chunks of rock and iron surrounded by gas. Possibly the Earth would be badly damaged if it did hit the head of a comet.
   When a comet is far away from the Sun, it does not have a tail. But when a comet comes close to the Sun, the light of the Sun drives out some of the gas in the head. This forms a tail which shines in the sunlight. A comet's tail always streams out away from the Sun.
   Halley's comet is the most famous comet. It was named for Edmund Halley, a famous English astronomer. Halley's comet comes close to the Earth every 76 years. It was through the tail of this comet that the Earth passed in 1910. This same comet was seen during the Crusades when the Christians were fighting the Turks. One of the Christians' prayers was "Lord, save us from the Devil, the Turk, and the Comet."
   Sometimes a comet goes too close to the Sun or a large planet and is pulled to pieces. Nothing is left of it but bits of rock and iron. All the comets may finally be destroyed.

The Crow constellation

   Corvus, also called the Crow, a small kite-shaped constellation just south of the celestial equator.
   It lies between Virgo and Hydra and is visible in midnorthern latitudes when it reaches its highest point in the sky in early May. The four principal stars of the constellation are of the third magnitude, the brightest being Gamma Corvi, also called Gienah, a star of magnitude 2.59. This highly luminous star is about 450 light-years from the earth. Al Chiba, unaccountably called Alpha Corvi, is a dim star of less than fourth magnitude in the southern part of the constellation.

Orion constellation

   Orion is a large, bright CONSTELLATION which may be seen in winter. It is named after Orion, who was a great hunter. A row of three bright stars marks Orion's belt. Three fainter stars in a row represent a sword or a dagger hanging from his belt. Four more stars form a rectangle around the belt and sword. These mark his shoulders and knees.
   Legends tell that Orion boasted he was the greatest hunter and no animal could kill him. A scorpion finally bit Orion and did kill him. The goddess Diana, a huntress, persuaded Jupiter to place Orion in the sky. Orion seems to be stalking the constellation, TAURUS, the bull. He is followed in his journey across the sky by Canis Major and Minor,"his dogs. The scorpion is in the sky, too, but SCORPIUS is a summer constellation. This enemy of Orion is not visible when Orion is in the sky. Near the middle star in Orion's belt is a hazy cloud. This is the great Orion nebula, a gaseous cloud that reflects light from nearby stars.
   BETELGEUSE, the bright red star on Orion's right shoulder, was supposed to be a ruby pin which held up his lion skin. Betelgeuse was the first star to have its diameter measured. Rigel, the bluish-white star diagonal to Betelgeuse, is pictured as the buckle on Orion's left shoe.

Leo constellation

   Leo, or the Lion, is a group of stars that seem to outline the shape of a lion in the sky. Leo can be seen in the night sky in the summer time. It is one of the signs of the ZODIAC.
   The stars at one end of this CONSTELLATION form a pattern like a sickle. The sickle is the front of the lion. There is a triangle of stars at the other end that marks the hind quarters of the lion.
   This constellation was called a lion because the Sun was in Leo in the summer time. Lions are associated with the tropical regions and hot climates. Lions are supposed to be very powerful. In the summertime the Sun seems hottest and most powerful. It was imagined that Leo had some effect on the heat and power of the Sun.
   Another legend, from Greek mythology, tells that Leo was once a ferocious lion that stalked in the forest of Nemea. No hunter would dare to go after him. Hercules finally went and killed the lion. Jupiter placed the lion in the sky.

Facts about meteors and meteorites

   Every few months newspapers tell their readers to watch the sky for a big shower of meteors. Meteors by the billions travel around the Sun. They are chunks of rock or iron which, as a rule, are no bigger than peas. The Earth on its travels comes close to many meteors. The Earth's gravity pulls them in. They glow white hot as they fall through the air. Most of them are changed to vapor or dust before they touch the ground. People call these meteors that flash through the air and are destroyed shooting stars, or falling stars.
   Meteors are not scattered evenly in the space around the sun. Instead there are great swarms of meteors. The big showers of shooting stars come when the earth runs into one of these swarms.
   Shooting stars puzzled the people of long ago. An old Egyptian record tells about a night when the stars all jumped about like grasshoppers. Of course, the true stars were not jumping about. There were simply a great many shooting stars that night. The Romans believed that a shower of shooting stars meant that their gods were angry.
   There are millions of shooting stars every year. Although there are more of them at some times of the year than at others, one is likely to see a shooting star or two on any clear night. The dust from shooting stars is making the earth a tiny, tiny bit bigger every year.
   It is fortunate that most shooting stars are destroyed as they plunge through the air. They are traveling very fast. Being hit by a shooting star would be like being hit by a machine-gun bullet.
Some meteors that fall from the sky are not completely destroyed during their journey through the air. They are too big. On their journey through the air they are called fireballs. After they hit the earth they are called meteorites.
   Fortunately there are not many fireballs. Fortunately, too, many of them fall in the sea. On land, if they were common, they could do a great deal of damage. In Ari-zona there is a huge hollow, or crater, which scientists think was made by a group of fireballs. In a forest in northern Russia a group of these big meteors fell not very long ago and knocked down all the trees for miles around. But in modern times no one has been hit by a fireball. There is, however, an old Chinese record telling that one killed ten men.
   There are many meteorites in museums. The biggest one now in a museum was found lying on the ice in the Far North. It was found by Peary, the explorer who first reached the North Pole. This big meteorite weighs 36½ tons.
   Some meteorites are made of stone. Others are made of iron. Probably the first iron people used came from meteorites.
   Scientists think that the big swarms of meteors come from broken-up comets. They follow paths just like the paths of comets. At least once when a comet did not show up on schedule the earth had a shower of meteors instead. Where the meteors that are not in the swarms come from is still a puzzle.

Lunisolar precession

   Lunisolar precession is the apparent change in the location of the stars in relation to the observers on Earth. It is caused by the pull of gravity of the Sun and the Moon on the Earth
   Actually, the Earth's axis is almost fixed at an angle of 23½ degrees to the plane of its equator. But the gravitational effect of the Sun and Moon (sometimes supplementing, sometimes counteracting each other) causes the axis of the spinning Earth to wobble slightly, as in the motion of a spinning top. The result is that the stars used as reference points (such as the North star or the constellations) seem to move slowly westward, completing a cycle in about 26,000 years.

Cygnus (constellation)

   Also called the Swan, Cygnus, is a prominent constellation in the northern sky. Cygnus is one of the most easily identified constellations and includes the Northern Cross. Its brightest object is Deneb, also called Alpha Cygni. The upper portion of Cygnus is in the densest part of the Milky Way. This area is a powerful source of radio signals that are thought to come from a collision of galaxies beyond the Milky Way. The star called 61 Cygni was the first to have its distance from earth measured. Cygnus reaches its highest point in the sky in mid-September, when it is directly overhead at the latitude of New York city.

What are cosmic rays?

   About 100 years ago scientists noticed that odd things were happening to some of their experiments. Rays of a strange kind were reaching them. They were much like X rays, but they were not X rays. The scientists set out to track down the strange rays. They found that these rays came from far outside the earth. The rays were named cosmic rays.
   Cosmic rays do not seem important to most of us. We do not see them or feel them. But scientists, of course, wanted to find out much more about them as soon as they found out there were such things. They went here, there, and everywhere to study them. They went into coal and iron and copper mines. They went into caves and into the great cracks in glaciers. They went deep down under the sea and up to the tops of high mountains. In balloons they went higher in the air than anyone had ever gone before. They even went into such out-of-the-way places as the mouth of a big cannon and under great piles of salt. Many turned diver or miner or mountain climber or airman. Some of these scientists even lost their lives exploring.
   Cosmic rays are still mysterious. Scientists are not yet sure exactly where they come from. But what they have found out helps them to understand better the great universe around us.

Why do molecules need clouds to survive in space?

   Molecules in space exist primarily in molecular clouds (also called "dark nebulae") because there they are protected by cosmic dust. Without this protection, the molecules would be broken down by the ultraviolet light from stars. Less than 1 percent of the mass of a dark nebula is cosmic dust, yet this amount is enough to reflect away or absorb much of the starlight.

Perseus constellation

   Perseus is a beautiful group of stars in the Milky Way. The constellation was named for a mythical Greek hero, the son of Zeus, who beheaded the wicked Medusa. A glance from Medusa would have turned him to stone, but he was aided by Mercury and Minerva. The ancient Greeks believed the constellation to be Perseus holding the severed head of Medusa.
   Though not easy to find, Perseus is visible with a small telescope. It is seen in the northern sky in December and January, and reaches from Cassiopeia to Taurus. The star Algol, visible to the naked eye, is actually a double star. The light shed by Algol is variable, caused by the revolving of the dim star around the bright star every three days, producing a partial eclipse.
   A telescope will reveal many variously colored star clusters in the constellation. About August 10th the famous meteor shower, the Perseids, can be seen.

Pegasus (constellation)

   Pegasus is a group of stars that seemed to ancient people to outline the shape of a horse. This constellation covers a large area of the sky. It can be found by locating the Square of Pegasus. Four bright stars mark the corners of this large square. The square represents the body of the horse. A line of stars ending in a triangle composes his neck and head. The horse is usually upside down. The stars that represent its forefeet usually point upward in the sky. Pegasus does not have any hind legs marked by stars.
   Pegasus can be found most easily in autumn and winter. It is near the royal family of constellations—Cassiopeia, Cepheus, Andromeda, and Perseus. In fact, one of the stars of the square is part of the constellation of Andromeda.

MYTHOLOGY

   According to legend, Pegasus was the winged horse which sprang from the head of Medusa when Perseus killed Medusa. Either Minerva or Neptune tamed Pegasus and gave him to Bellerophon. Pegasus carried his master to Lycia, where Bellerophon slew Chimera, a monster. Jupiter was displeased and sent a gadfly to sting Pegasus. The horse threw Bellerophon and flew up into the sky.

Christian Huyghens (1629-1695)

   Christian Huyghens was a Dutch mathematician and natural philosopher. He was born and died at The Hague, where the intervening time was also spent, except for a few years at school and in travel, and from 1665 to 1681 when he resided in Paris. His investigations of the mathematical theory of the compound pendulum, its application to clocks, and its use in determining the acceleration due to gravity, is alone sufficient to insure his fame. Furthermore his improvement of the telescope and discovery of the rings of Saturn, is a work in itself. Then to him, more than any other, is due the formulation of the wave theory of light, not generally adopted, however, till long after his time. His varied and practical labors have led to his being sometimes called the "Dutch Archimedes."

What are molecular clouds in the interstellar medium?

   Interstellar space contains more than sixty types of polyatomic (containing more than one atom) molecules. The substance formed in the greatest abundance is molecular hydrogen (H₂); others include water, carbon monoxide, and ammonia. Since these molecules are broken down by starlight, they are found primarily in dense, dark nebu­lae, where they are protected from the light by cosmic dust. These nebulae—known as molecular clouds—are enormous. They stretch across several light years and are one thousand to one million times as massive as the Sun.

Caroline Herschel

   German astronomer Caroline Lucretia Herschel (1750-1848) left home in 1772 and joined her brother, organist, choirmaster, and amateur astronomer William Herschel, in England. Herschel trained to become a professional singer while learning mathematics from her brother. She soon began to assist William in his astronomical studies by polishing and grinding mirrors for his telescope and copying his notes. The siblings found themselves dedicating more and more of their time to astronomy and less to music. In 1781, following William's discovery of the planet Uranus, the brother-and-sister team began receiving a yearly salary from King George III. This salary allowed them to become full-time astronomers.
   Eventually Herschel began making her own contributions to astronomy. In 1783 she discovered three new nebulae (clouds of gas and dust), and over the next decade she discovered eight comets. In 1787 King George decided to pay Herschel her own salary, something very rare for a woman then. Herschel went on to make a complete index of the star catalogue created by John Flamsteed, the first Astronomer Royal, England's honorary chief-astronomer. When her brother William died in 1822, Herschel returned to Hanover, Germany, where she lived to the age of ninety-seven. She continued working with William's son, astronomer John Herschel, for whom she put together a new catalogue of nebulae. Herschel was one of the first two women granted membership in the Royal Society, England's elite science organization. She won the Gold Medal of the Royal Astronomical Society at age seventy-eight; was elected to the Royal Irish Academy at eighty-six; and won the King of Prussia's Gold Medal for Science at age ninety-six.

What is an "emission nebula"?

   While interstellar gas is generally cold, the gas near very hot stars becomes heated and ionized (electrically charged) by ultraviolet radiation given off by those stars. The glowing areas of ionized gas in the interstellar medium are called "emission nebulae." Two well-known examples of emission nebulae are the Lagoon nebula in the constellation Sagittarius, and the Orion nebula, visible through binoculars just south of the hunter's belt in the constellation of the same name. The Orion nebula is punctuated by dark patches of cosmic dust.