Earth was once thought to be the center of the universe.We know that Earth and its neighboring planets are merely tiny speck of agglomerated debris left behind from formation of the Sun 5billion years ago. Except for being our parent star, the sun is relatively unremarkable star. It lies in the hinterlands of the Milky Way galaxy, a vast, pancake-shaped city of 300 billion stars. Like islands scattered across an archipelago, many billions of other galaxies lie beyond the Milky Way, out to the horizon of the visible universe. On the largest scale, the structure in the universe resembles a great sponge, where long filamentary clusters of galaxies are the fabric in the sponge, and mysterious, as yet unidentified dark voids in space are the "HOLES".   

Today we know that the universe is active, dynamic, and ever changing. Stars explode; galaxies collide; black hole devour matter. The very fabric of space and time was apparently forged in an incredible explosion, called The Big Bang, some 15 billion years ago.

Our past and destiny are intertwined with the violent cosmic events that shaped the cosmos. many of the atoms in our bodies - Calcium, Nitrogen, and Iron, for instance - were forged in the hearts of ancient stars that exploded long ago, spewing material back into space. Other cosmic catastrophes - an Asteroid's collision with Earth, radiation from a nearby supernova, or changes in the Sun's brightness - may have affected the evolution of life in the past and may do so again in the future.

PLANETS - Our solar system consists of nine known planets, several dozen moons, thousands of rocky and metallic asteroids, and trillions of icy bodies called comets. All of these objects are gravitationally bound to the Sun. The solar system has two types of planets: the tiny rocky planets of Mercury, Venus, Earth, and Mars, which all lie close to the Sun; and the immense liquid and gaseous outer planets of Jupiter, Saturn, Uranus, Neptune, which lie in the colder reaches of the solar system. The farthest planet from the Sun, icy Pluto, may actually be a "double planet", because it orbited by moon nearly one-quarter its size. Thousands of icy dwarf bodies similar to Pluto may have inhabited the early solar system.

STARS - Stars are massive, self-luminous objects that from the basic building blocks of the cosmos. They coalesce under gravity to form great clusters and galaxies. Stars are also fundamental engines that generate energy by smashing lighter elements together to form a heavier elements through a process called nuclear fusion. Our Sun is the closest star to Earth, and very typical of stars in general. Every second, the Sun converts 540 million tons of hydrogen into 595 million tons of helium. In the process, 49 million tons of matter is converted to pure energy, which eventually reaches Earth as "LIGHT". 

The basic recipe for making a star is fairly simple: compress a huge cloud of interstellar dust and gas into a relatively small, dense globule of hydrogen. Then Collapse this cloud, perhaps with a shock wave from a nearby stellar explosion. The cloud continue to collapse under gravity until nuclear fusion begins, its outward force counterbalancing any further collapse. When this equilibrium is reached, a STAR is born. Even a casual glance into evening sky reveals that stars come in a wide range of brightness and colors. Some stars appear bright because they are close neighbors to the Sun. Others are intrinsically bright because they are much hotter that the Sun. Stars colors also provide clues to their intrinsic nature. Bluish-colored are hotter than the Sun; Reddish are Cooler. Extremely hot star can be 10 or even 100 times more massive than our Sun.

Galaxies - Our Milky Way was once thought to contain all stars in the universe. In the 1920's however, American astronomer Edwin Hubble discovered that the universe is filled with other island  cities of stars as well. Hubble classified galaxies according to shape. Many are spiral, or pinwheel-shaped, like our Milky Way. Others are elliptical, and still others are irregular. Today, astronomers are presented with a truly dizzying variety of galaxies-radio galaxies, infrared galaxies, X-ray galaxies, active galaxies - but there is yet no coherent scheme of galactic evolution.  

The core of some galaxies are extraordinarily bright, shining at a level equal to compressing 1 million Suns into a piece of space no larger than our solar system. This active galactic nuclei maybe powered by immense black holes that have grown perhaps from the merger of individual stellar black holes. Star dust and gas swirling down into the hole heat up to millions of degrees, allowing a prodigious amount of energy to be radiated.

Brighter than 1 million supernovas going off in unison, Quasars are probably the most energetic type of active galactic nucleus. Quasars stands for Quasi-Stellar object, because it is brilliant, point like, and virtually indistinguishable from stars in photographic sky surveys. However, Quasars are many billions of light year away, far too distant to be individual stars.     

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The Sun is the Center around which the Earth and the other planets of our solar system revolve. It is a rather ordinary star of average size. Of course, the Sun appears much bigger and brighter to us because it is much closer to the Earth than is any other star. It is about 93 million miles away. The next-nearest star, Alpha Centuari, is more than 25 trillion miles away.

Our Sun is only one of about 100 billion stars in our galaxy, the Milky Way. It is located in one of the outer, spiral arms of the Milky Way, about three-quarters of the way from the galactic center.

The Sun is a vast ball of hot, glowing gas, some 870,000 miles across - more than 100 times the diameter of the Earth. The Sun's mass, however equals that of 333,420 Earths. This tremendous weight produces a pressure at the center of the Sun of more than 1 million metric tons per square centimeter. 

The Sun's gravity is 28time stronger than that of the Earth. So a man weighing 150 pounds on Earth would weigh 28 x 150 pounds, 4,200 pounds, if he could stand on the surface of the Sun.

In spite of the great mass of the Sun, its average density -  the weigh of standards volume of its matter-is only 1.4 times the weigh of an equal volume of water. The Earth, on other hand, is 5.5 times denser than water. This low solar density is easy to explain. The center of the Sun, because of enormous pressure, is more than 100 times denser than water. but much of the Sun beyond the center is composed of gas that is often thinner than the Earth's atmosphere. When this densities averaged together, the general density of the Sun is quite low.

The Sun i like a huge furnace, fired by nuclear, or atomic, energy at its core. temperatures at the center may be 25,000,000F or more. At the surface, temperatures are much cooler-between 9,000 and 11,000F still hot enough to vaporize nearly all substances that exist as solids or liquids on the Earth.

A moon is any natural body that orbits a planet. There are at least 35 known moons in our solar system. The majority of them orbit the giant planets Jupiter and Saturn, and are little more than huge, airless ball of ice, ranging from hundreds to more than a thousand miles across. One of the largest moons, Saturn's TITAN, is so big(3,169miles) that it retains its own atmosphere of nitrogen. Mars has some of the smallest moons, a pair called Deimos and Phobos, each no bigger than an asteroid, which indeed they may have been at one time.

In the 18th century, astronomers calculated astrophysical laws that predicted they would find an as-yet-unseen planet between Mars and Jupiter. And they eagerly searched the skies for it. On the night of January 1, 1801, the Italian astronomer Giuseppe Piazzi discovered a small celestial body, which he took to be planet, in the space between the orbits of Mars and Jupiter. This body, which was later called Ceres, was found to have a diameter of only 478miles. Over the years, many more small, planet-like bodies were found in the gap between mars and Jupiter. Today more than 1,000 of these small bodies have been discovered, leading astronomers to estimate there may be more than 50,000 in all.

Astronomers once thought that asteroids were fragments of a big planet that once orbited between Mars and Jupiter and then broke apart for unknown reason. But in recent years, scientist have come to believe that asteroids are probably debris left over from the solar system's formation, debris that simply never coalesced to form a planet.

Comets are among the strangest members of the Solar System. Instead of moving as the planets do, in nearly circular orbits in the same direction, comets revolve around the Sun in every conceivable direction. Much of the time they are so far away from the Sun that they are invisible even to our largest telescopes.

It was once thought that some comets approached the Sun from far beyond the solar system, and that once they withdrew from the Sun, they would never return. Today it is generally agreed that comets are members of the Sun's family. They make up a vast shell of icy debris called the Oort Cloud. Though this region lies 50,000 times farther from the Sun than does Earth, the trillions of icy comet bodies that inhabit it are all gravitationally bound to the Sun.  

When astronomers 1st discovered a comet, it usually appears as a faint, diffused, fuzzy star, with a dense, star-like center and a veil-like region, known as its coma. As the comet approaches the Sun, its coma becomes brighter, as more and more material vaporizes off the surface of the comet's solid, icy nucleus. When they are some 100million miles from the Sun, some comets begin to show a tail streaming behind them, pointing directly away from the Sun. Comets tails appear to consist of very thin gases that fluoresce, or glow, under sunlight, as well as a fine stream of dust particles. This material is forced away from the Sun by the pressure of the solar wind.  

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As the ancients gazed at the night skies, they were occasionally startled to see strange celestial objects intruding upon the familiar pattern of stars, Moon, and planets. These mysterious apparitions looked like fuzzy stars with long trains of light, moving from one constellation to another and cutting across the paths of the planets at every conceivable angle. The trains of light suggested a woman's tresses; hence, the celestial intruders came to be known as "long-haired stars", or kometes, the Greek word for "long-haired".

A bright comet was a terrifying spectacle in antiquity. It was thought to foreshadow some dire catastrophe-plague, famine, war, or perhaps the death of a ruler. Today we realize that comets are simply another member of the solar system, and that their coming is no more portentous than the appearance of the first stars at twilight.

When a particular comet is first discovered, it usually appears as a faint, diffuse body with a dense area near its center. This dense part, which sometimes looks like a tiny star. i known as nucleus. The nebulous, or veil-like, region around it is the coma, Nucleus and coma together from the head of a comet.

In a certain number of cases, however, a spectacular transformation takes place as the comet approaches the Sun. The coma changes from a diffuse, round mass to sharply defined layers, called envelopes. Nebulous matter streams away from the comet's head in the direction opposite to the Sun and forms an immense tail. Most comets of this type have only one tail. A very few have two or more. Some comets occasionally also have forward spikes. As a comet recedes from the Sun, the tail can no longer be seen, the coma becomes diffuse again, and, in the great majority of cases, the comet itself disappears from view. 

How do comets originate? According to one theory, they represent celestial building block left over after the formation of the planets. According to another, they are remnants of shattered worlds. All this is pure conjuncture, as are the various theories that attempt to explain how comets are launched on their journey around the Sun.

1 theory, proposed by Dutch astronomer J.H. Oort in 1950, holds thate there is a vast storehouse of comets - as many as 100billion, perhaps-in the icy reaches beyond the farthermost planetary orbit. A given comet would normally remains entirely inactive in the "deep freeze" of space unless the passage of a star disturbed it. The comet then would swing into the sphere of gravitational attraction of a major planet and would revolve around the Sun a few hundred or a few thousand times until it disintegrated.

Photographs by the Hubble Telescope provide the first direct evidence that a huge belt of at least 100million comets is circling the solar system beyond orbit of Neptune. This ring, called the Kuiper belt, was first suggested in 1951 by Dutch astronomer Gerard P. Kuiper, and appears to be the origin of many comets.

The general belief is that the nucleus consists of a vast number of small, solid bodies held together by mutual attraction. The nuclei of a certain comets that have ventured close to Earth have been measured with considerable precision. The tail of a great comet of 1861, for example, stretches across two-thirds of the sky and was bright enough to create shadows on the ground. Yet it had nucleus less than 100 miles in diameter.

As the nucleus of comet approaches the Sun, the solar heat vaporize the material on the outer surface of the nucleus. Escaping gases, carrying fine dust with them, diffuse into the coma. They are the swept away by the force of the Sun's radiation to form tail. The gases and the dust they transport are illuminated partly because they absorb ultraviolet light and re-emit it in the form of visible light.

The tail which flows away from the Sun, increase in breadth as the distance from the head increases. The tail does not form an exact line between the Sun and the comet's head. The greater the distance from the head, The more the gases and dust that make up the tail lag behind. Hence the tail often has the shape of curved horn, with its tip at the comet's head.

When comet turns away from the Sun, the material that formed the tail is swept off into space. In time, comets gradually lose all their substance, unless it can be replenished by dust and by gas molecules swept up in the course of their journeys through space.

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The elegant white planet orbiting the Sun is called Venus after the Roman goddess of beauty. Although similar to the Earth in size(7,500miles) - compared to Earth's diameter(7,900miles) and mass (Venus a bit more than four-fifths the mass of Earth), the Venusian surface and atmosphere are far different from ours.

Like Mercury, Venus is at times an evening star and at other times a morning star, depending on whether it is to the east or west of the Sun as viewed from the Earth. The planet may rise as much as four hours before the Sun, and may sets as much as four hours after it.

Venus revolves around the Sun once every 225 days in an orbit that is very nearly circular. And as it revolves, it is quite far indeed. But at its closest, when it is between the Sun and Earth, it is only some 26million miles away.

Venus like Mercury, shows a complete of phases, or shapes, to an observer on Earth armed with a small telescope or good binoculars. When the planet is at the farthest part of its orbit from the Earth, it appears as a disk. When Venus is between the Sun and the Earth, it is seldom visible. About 35days before and after this time, it appears as a crescent and is at its brightest - two and on-half times brighter than when it is sees as disk.

The surface of Venus is obscured by the planet's thick clouds, and so is invisible to optical instruments. For centuries, astronomers could only guess what lay beneath this veil. Some conjured tales of swamps, forests, and strange creatures.

Starting in 1962 the United States and the Soviet Union sent more than 20 probes to the planet for a true view. The U.S pioneer-Venus 1 craft and soviet Venera 15 and 16 orbiters used radar to pierce the thick clouds and make low-resolution maps of the planet's surface. The mapping revealed mainly rolling upland plains, some lowland plains, and two highland areas. One called Aphrodite Terra, is about half the size of Africa. The other highland, Ishtar Terra, is about the size of the United States, and contains named Maxwell Montes.

Other Venera probes actually traveled through the clouds, landed softly on the surface, and transmitted the first color pictures of the planet's surface. Venusian crust showed that it contained basaltic rock similar to that associated on Earth with recent volcanic activity.

The U.S spacecraft Magellan launched in May 1989, has radar-mapped most of the Venusian surface with far better resolution than previous craft. Magellan cracks and fissures, rugged mountains, and bizarre "pancake domes" formed by hot lava welling up from beneath the surface.

Magellan also photographed giant craters formed when large hunks of rock from space crashed into the planet. Interestingly, Even the smallest craters found in Venus are quite huge-over 2.5 miles in diameter. Only very large hunks of space rock can survive the fiery passage through Venus' thick atmosphere and reach surface. Perhaps the biggest surprise from Magellan's pictures is the apparent lack of erosion on Venus. The mountains, craters, and other surface features appear rough and unweathered, almost as if they were newly formed. A major reason for this is that Venus is  bone-dry. Water on Earth smooths down the surfaces features. Rivers, for example, gradually change mountains into valleys. But with surface temperatures of 896F (480C), Venus is too hot for water. If Venus did have water init past, it must have quickly evaporated.

Though there appear to be no signs that water ever flowed across Vebus' surface, Magellan's pictures show long channels cut by of hot lava. Volcanoes must have resurfaced Venus in its recent past, because the planet has far fewer craters than do Mars or the Moon. Scientists suspect that Venus volcanic face-lift came within the past few hundred million years.

Scientist are also trying to understand the geologic forces responsible for producing the volcanoes, mountain belts, and other surface features of Venus. On Earth the crust is divided into separate moving plates. Their movement, a process known as plate tectonics, is responsible for mountain building, seafloor spreading, and earthquakes. The crust of Venus does not appear to work in this way. Instead, up-wellings of hot, light material and down-wellings of cooler, dense material may pull and shove the surface crust.

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Will the universe continue expanding? Will it just stop or even begin to contract?

The answer depends on the amount of mass that the universe contains, according to work done in the 1920's by Russian's mathematician Alexander Friedman. If the universe mass exceeds a certain crucial value, then gravity should eventually stop everything from flying away from everything else-just as, if you throw a ball straight up, you can depends on gravity to stop the ball and return it to you.

With enough mass the universe will eventually succumb to the attractive force of gravity and collapse once again into a single point - a fate often called the Big Crunch. But if there isn't enough mass, the universe will just keep on expanding. Scientists are still trying to find evidence for which of these fates our universe faces. Some say our universe alternates between Big Bang and Big Crunches in a perpetual display of the power of gravity.

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Will the universe continue expanding? Will it just stop or even begin to contract?

The answer depends on the amount of mass that the universe contains, according to work done in the 1920's by Russian's mathematician Alexander Friedman. If the universe mass exceeds a certain crucial value, then gravity should eventually stop everything from flying away from everything else-just as, if you throw a ball straight up, you can depends on gravity to stop the ball and return it to you.

With enough mass the universe will eventually succumb to the attractive force of gravity and collapse once again into a single point - a fate often called the Big Crunch. But if there isn't enough mass, the universe will just keep on expanding. Scientists are still trying to find evidence for which of these fates our universe faces. Some say our universe alternates between Big Bang and Big Crunches in a perpetual display of the power of gravity.

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Not happy with either the Big Bang or the steady state theory? A minority of astronomers are formulating other views of the creation of the universe. One model comes from the mind of a Nobel Laureate Hannes Alfven, a Swedish plasma physicist. Called the Plasma Universe, his model starts by noting that 99% of the observable universe (including the stars) is made of plasma. Plasma, an ionized gas that conduct electricity, is sometimes called the fourth state of matter. This theory state that the Big Bang never happened, and that the universe is crisscrossed by gigantic electric currents and huge magnetic fields.

Under this view the universe has existed forever, chiefly under the influence of an electromagnetic force. Such a universe has no distinct beginning and no predictable end. In the Plasma Universe, galaxies come together slowly over a much greater time span than in the Big Bang theory, perhaps talking as long as 100 billion years.

Little of the evidence for the Plasma Universe comes from direct observations of the sky. Instead, it comes from laboratory experiments. Computer simulations of plasma subjected to high-energy fields reveals patterns that look like simulated galaxies. Using actual electromagnetic fields in the laboratory, researchers have also been able to replicate the plasma patterns seen in galaxies. While still a minority view, the Plasma Universe is gaining favor with younger, more laboratory-minded astronomers who value hard empirical evidence over mathematical proofs.

Meanwhile, another group of astronomers is developing a Steady-State theory that actually conforms to astronomical observations. Like its predecessor, this steady-state theory propose a universe with no beginning and no end. Rather, matter is continuously created via succession of "Little Bangs," perhaps associated with mysterious quasars. In this new theory, galaxies would form at a rate determined by the pace at which can the universe expands. These theorists can be even account for the cosmic background radiation: they maintain that the microwaves are actually coming from a cloud of tiny iron particles-not some primordial explosion.

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The Big Bang is not the only proposed theory concerning our universe's origin. In the 1940s a competing hypothesis arose, called the Steady State theory. Some astronomers turned this idea simply because, at the time, there wasn't enough information to test the Big Bang. British astrophysicist Fred Hoyle and others argued that the universe was not only uniform in space-an idea called the Cosmological Principle - but also unchanging in time, a concept called perfect cosmological principle. This theory didn't depend on a specific event like the Big Bang. Under the Steady State theory, stars and galaxies may change, but on the whole the universe has always looked the way it does now, and it always will. 

The Big Bang predicts that galaxies recede from 1 another, space becomes progressively emptier. The Steady State theorists admit that the universe is expanding, but predict that new matter continually comes to life in the space between the receding galaxies. Astronomers propose that this new material was made up of atom of hydrogen, which slowly coalesced in open space to form new stars.

Naturally, continuous creation of matter from empty space has met with criticism. How can you get from nothing? The idea violates a fundamental of law of physics: the conservation of matter. According to this law, matter can neither be created nor destroyed, but only converted into the forms of matter or into energy. But skeptical astronomers have found it hard to directly disprove the continuous creation of matter, because the amount of matter formed under the Steady State theory is so very tiny: about one atom every billion years for every several cubic feet of space.

The Steady State Theory fails, however, in 1 important way. If matter is continuously created everywhere, then the average age of stars in any section of the universe should be the same. But astronomers have found that not to be true.

Astronomers can figure out how old a galaxy or star is by measuring its distance from Earth. The farther away from the Earth an object is, the longer it has taken light from the object to travel across space and reach Earth. That means the most distant objects we can see are also the oldest.

For example, take a quasar's, the small points of light that give off enormous amounts of radio energy. Because the light from quasars i shifted so far to the red end of the spectrum, astronomers use Hubble's law to calculate that this powerhouses lie at a great distance from Earth, and hence are very old. But quasars exist only at these great distances -  none are found quasars. If the Steady State theory were true there ought to be but young and old quasars. Since astronomers haven't found quasars that formed recently, they conclude the universe must have changed over time. The discovery of quasars has put the Steady State theory on unsteady ground.     

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How and when did the universe begin? No other specific question is more fundamental or provokes such spirited debate among researchers. After all, no 1 was around when universe was begun, so who can say what happened?

Since early part of 1900s, one explanation of the origin and fate of the universe, the Big Bang Theory, has dominated the discussion. Proponents of the Big Bang maintain that, around 20 billion years ago, all the matter and energy in the known cosmos was crammed into a tiny, compact point. In fact, according to this theory, matter and energy back then were the same thing, and it was impossible to distinguish one from the other.

Adherents of Big Bang believe that this small but incredibly dense point of primitive matter/energy exploded. Within seconds fireball ejected matter/energy in all directions at velocities approaching the speed of light. At some later time maybe seconds later, maybe years later energy and matter became separate entities. All different elements in the universe today developed from what spewed out of this original explosion. 

Big Bang theorist claim that all of the galaxies, stars, and planets still retain the explosive motion of the moment of creation and are moving away from each other at great speed. This supposition came from an unusual findings about our neighboring galaxies. In 1929 astronomers Edwin Hubble, working at the Mount Wilson Observatory in California, announced that all of the galaxies he had observed were receding from us, and from each other, at speeds of up to several thousand miles per second.      

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If the universe started out as a dense "soup" of particles of matter and energy, then how did it get "lumpy" enough for galaxies to form? Did they start out as a huge clouds of gas that contracted - or, instead, as small clumps of stars that merged?

Even more puzzling, how did galaxies coalesce into vast clusters? 

The largest such structure, called the "Great Wall" of galaxies, is nearly 500 million light-years long. According to current theories, the universe simply hasn't been around long enough to build such unimaginably vast structures.

1 idea is that the universe contains infinitely long but infinitesimally thin "cracks" called cosmic strings. Like crystal growing on a piece of thread, matter might have accumulated along these strings early in the universe's history to eventually form great filamentary clusters of galaxies. There is no direct evidence for cosmic strings as yet, however.

A more widely accepted possibility is that galaxies may just be "puddles" or "trace sediments" in a "haze" of invisible dark matter that may have been clumpy before galaxies formed. This material was first suspected when astronomers measured  its ghostly gravitational pull on galaxies within distant cluster.

Today, astronomers believe that this dark or missing matter may account for up to 90% of the mass of the universe. Since it is invisible, its identity remain unknown. Scientist have proposed possibilities that range from subatomic particles to black holes.

If the universe has enough dark matter, its gravitational pull will eventually slow the expansion of the universe, and pull it back onto itself.  If that were to be happen someday, all matter would be compressed into a super-dense fireball similar to that from which the universe emerged 15 to 20 billion years ago. 

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