SCIENCE101: 2012

THE BIRTH OF EARTH

EARLY THEORIES

One of the earliest scientific theories of the origin of the Earth may be traced to the German philosopher Immanuel Kant. In 1755, he proposed that the solar system was the result of a rotating cloud of gas called a Nebula. Gravity would have condensed some of the particles within the cloud into planet like globs, he suggested. The rotation of the primordial gas cloud can still be felt today, he added, in the spinning motion of the Sun and that of the planets, which rotate both around the Sun and about their own axes.

EARTH

Astronomers estimate that it was about 4.6 billion years ago that Earth solidified as a full fledged planet and began to evolve towards its present-day form. The next step in this evolution was its differentiation into three regions known as the core, mantle and crust. This occurred more than 4 billion years ago.

The core, at Earth's center, is iron-rich and very dense. Surrounding the core is a broad layer called the mantle, which is less dense. Earth's outermost layer, the crust, is thin and considerably lighter, made primarily of oxygen, silicon, aluminum, and iron.

There are 2 competing theories to explain how Earth became differentiated. The first propose that during its first "moments" of life, Earth was an evenly mixed hodgepodge of materials melted into a liquid. This allowed iron and nickel to sink to the core, while the lighter elements rose to form the crust.

Many scientist believe that necessary heat required to turn that early "hodgepodge" molten came from three sources. First, the abundant radioactive elements contained in the young planet were only just beginning to break down, and this process release tremendous energy. Second, each time a stray meteorite or other bit of debris struck the young planet, tremendous heat was generated. Finally, the mass of the young planet would itself generate heat as it pressed inward on its own core. According to scientist calculation, these three factors together could have raised Earth's temperature by as much as 2,000f (1,100c).

Some scientist argue that even this huge temperature rise would not have been enough to have melted the early Earth. Instead, they say, differentiation had already occurred before the planet had been born. When Earth first began to form, they explain, the densest materials accreted next, and the elements that make up the core were added as "icing" on the cake. Calculation of such a scenario show that in a cooling cloud of gas and dust, it would indeed be the heavy elements that would first condense into a more solid form, followed by lighter and lighter elements.

Whatever the actual mechanism, the differentiation of the early Earth is responsible for the subsequent formation of the continents, oceans, and atmosphere.

HOW OLD IS EARTH?

In the 16th century, scientists estimated Earth's age in thousands of years, based in part on the fossils they had found and in part on biblical interpretation. Today scientist estimate the planet's age in billions of years, using radiometric dating as the universal timepiece. How can they be sure?

THE FOSSIL RECORDS : The antiquity of our planet first became apparent when people recognized fossils as the remains of ancient organisms, rather than fanciful rocks oddities. When digging into a rock bed, they also noticed that different types of fossils could be found at succeeding depths.

By the beginning of the 17th century, geologists had established that the rock formation in which fossils were found had been deposited in more or less distinct layers, or strata. They further accepted that the oldest layers that is, those deposited first-generally lay at the bottom of the sequence. Each subsequent, or younger, layer was then deposited on top. Therefore, in places where earth movements had not toppled and mixed up the rock beds, the distinct layers could be read as a time scale from top to bottom, youngest to oldest.

By the mid 1800s, geologist had laboriously identified nearly all the types of fossils that could be found in the exposed rock layers of Europe. They then classified the fossils in a definite order, oldest to youngest. Thus, a geologist or collector could classify newly unearthed rocks and rock layer as younger or older than other based on types of fossils contained within them. But no one could say by how many years.

EARTH'S CRUST

A patch of bare rock that interrupts the more pleasing pattern of a lush meadow or green forest reveals the true nature of Earth's Crust, the solid outer layer of the planet. The soil of the land, upon which life flourishes, is an exceedingly thin film. So, too, is the layer of ooze that carpets the ocean floor. Beneath the blanket of these sediments is bedrock, the layer of solid rocks that makes up Earth's crust. When compared to the planet's actual diameter, Earth's crust is itself relatively thin, extending downward for some 22 miles (35kilometer) under the continents and about 4 miles (6kilometers) beneath the ocean floor.

There are 2 main types of crust: the CONTINENTAL CRUST, which forms the landmasses and consist primarily of low-density rock such as granite; and the OCEAN CRUST, which is a thin layer of volcanic rock called basalt that lies under the sea.

The ocean crust is younger than the crust that underlies the continents. This is because new crust is constantly formed along a ridge that runs through the middle of the world's oceans. The new crust material wells out of Earth's interior and cools as it emerges from the mid-oceanic ridge to spread outward. It is this creation of new oceanic crust that is believed to power the slow movement of the continents across the face of Earth.

The Rocky crust of Earth both the oceanic and the continental crust - is made up of three kinds of rock: IGNEOUS, SEDIMENTARY, and METAMORPHIC.

IGNEOUS rocks forms when molten rocks cools either within Earth's crust or on its surface. The crust of the primitive Earth is believed to have once consisted entirely of igneous rock.

SEDIMENTARY rock is made from small fragments of eroded rock and the remains of tiny sea animals and plants that have become compacted and cemented together over thousands of years. Most sedimentary rocks are formed on the ocean floor.

METAMORPHIC rock is derived from pre-existing rock that sinks deep into Earth and is subjected to intense heat or pressure or both, causing the characteristics of the rock to change.

PLATE TECTONIC

Earth's crust is broken into segments called Tectonic Plates. The major and minor plates move very slowly, carrying along the continents and ocean floor that lie on top of them.

Jigsaw puzzle - that's what may people think of when they look at a map of the world. Push Europe and Africa over across the Atlantic Ocean, and the coastlines fit - fit very well - against the coastlines of the Americas. Move India, Australia, and Antarctica around, and their coastlines also fit together - like piece of a giant jigsaw puzzle.

When the Americans were discovered and mapped some five centuries ago, scientist noticed that the opposing coast along the Atlantic Ocean had shapes that would fit together. They proposed that, early in Earth's history, the continents had been joined, and that later they had been violently torn apart. in the 19th century, this idea was supported by studies of geology and life-forms on both sides of the Atlantic. The studies revealed many similarities between species, suggesting that they had been intermixing and interbreeding across the continents as recently as 150 million years ago. Studies such as the led Alfred Lothar Wegener, a German meteorologist, to propose in 1912 the theory of CONTINENTAL DRIFT.

CONTINENTAL DRIFT

Wegner believed that the opening of the Atlantic and Indian oceans was not due to an earlier cataclysm, but rather had occurred slowly and gradually. He bolstered his argument with measurements from recent surveys of the distance between Greenland and Europe. The measurement suggested that the two landmasses were moving away from each other at a perceivable rate. Wegner further theorizes that, because Earth is rotating sphere, there exists a force that pushes the continents toward the equator.

The continents, he believed, plow through the rocks of the seafloor like ship through water. As a continents moves, he added, coastal mountain range pile up like low waves along the land's leading edge.

PLATE TECTONICS

By 1965, investigations led to the proposal that Earth's surface was broken into 7 large plates and several small plates. It was further suggested that these plates are rigid, and that their boundaries are marked by earthquakes and volcanic activity. In recent years, satellite pictures have documented the existence of plate boundaries. An especially visible example is the San Andreas Fault in California.

Plates interact with one another at their boundaries by moving toward, away, or along side each other. Faults are examples of boundaries where two plates slide horizontally past each other. Mid-ocean ridges mark boundaries where plates are forced apart as new ocean floor is being created between them. Mountains, volcanic-island arcs, and ocean trenches occur at the boundaries where plates are colliding, causing one plate to slide beneath the other. The network of crustal plates and the geologic activity caused by their movements is referred to as PLATE TECTONICS.

ARCS AND TRENCHES

The theory of plate tectonics may also be used to explain circular island arc and oceanic trenches. Where ocean floor is being carried down freely into the interior, it is likely to do so along circular arcs. Geometry dictates this pattern, a concept illustrated when a person pushes his or her thumb into a dead tennis ball: the resulting depression is circular. This principles may explain the origin of circular volcanic-island arcs, such as the Aleutians.

If, on the other hand, a continental plate pushes past and over an oceanic plate, the ocean floor will be forced down into the interior of the Earth along an offshore trench. The deep trenches of the coasts of Peru and Chile are prime example.

MOUNTAIN BUILDING

The Theory of plate tectonics also explains how and when mountains ranges such as ANDES and the CASCADES arose.

Both this ranges lie at the converging boundaries of 2 plates. As these 2 plates collide, portion of Earth's crust are uplifted and folded, causing a great compressing and thrusting of rock. Earthquakes and volcanoes continue to occur int the area of these new and active mountain ranges.

Earth's crust is broken into segments called Tectonic Plates. The major and minor plates move very slowly, carrying along the continents and ocean floor that lie on top of them.

Jigsaw puzzle - that's what may people think of when they look at a map of the world. Push Europe and Africa over across the Atlantic Ocean, and the coastlines fit - fit very well - against the coastlines of the Americas. Move India, Australia, and Antarctica around, and their coastlines also fit together - like piece of a giant jigsaw puzzle.

When the Americans were discovered and mapped some five centuries ago, scientist noticed that the opposing coast along the Atlantic Ocean had shapes that would fit together. They proposed that, early in Earth's history, the continents had been joined, and that later they had been violently torn apart. in the 19th century, this idea was supported by studies of geology and life-forms on both sides of the Atlantic. The studies revealed many similarities between species, suggesting that they had been intermixing and interbreeding across the continents as recently as 150 million years ago. Studies such as the led Alfred Lothar Wegener, a German meteorologist, to propose in 1912 the theory of CONTINENTAL DRIFT.

CONTINENTAL DRIFT

Wegner believed that the opening of the Atlantic and Indian oceans was not due to an earlier cataclysm, but rather had occurred slowly and gradually. He bolstered his argument with measurements from recent surveys of the distance between Greenland and Europe. The measurement suggested that the two landmasses were moving away from each other at a perceivable rate. Wegner further theorizes that, because Earth is rotating sphere, there exists a force that pushes the continents toward the equator.

The continents, he believed, plow through the rocks of the seafloor like ship through water. As a continents moves, he added, coastal mountain range pile up like low waves along the land's leading edge.

PLATE TECTONICS

By 1965, investigations led to the proposal that Earth's surface was broken into 7 large plates and several small plates. It was further suggested that these plates are rigid, and that their boundaries are marked by earthquakes and volcanic activity. In recent years, satellite pictures have documented the existence of plate boundaries. An especially visible example is the San Andreas Fault in California.

Plates interact with one another at their boundaries by moving toward, away, or along side each other. Faults are examples of boundaries where two plates slide horizontally past each other. Mid-ocean ridges mark boundaries where plates are forced apart as new ocean floor is being created between them. Mountains, volcanic-island arcs, and ocean trenches occur at the boundaries where plates are colliding, causing one plate to slide beneath the other. The network of crustal plates and the geologic activity caused by their movements is referred to as PLATE TECTONICS.

ARCS AND TRENCHES

The theory of plate tectonics may also be used to explain circular island arc and oceanic trenches. Where ocean floor is being carried down freely into the interior, it is likely to do so along circular arcs. Geometry dictates this pattern, a concept illustrated when a person pushes his or her thumb into a dead tennis ball: the resulting depression is circular. This principles may explain the origin of circular volcanic-island arcs, such as the Aleutians.

If, on the other hand, a continental plate pushes past and over an oceanic plate, the ocean floor will be forced down into the interior of the Earth along an offshore trench. The deep trenches of the coasts of Peru and Chile are prime example.

MOUNTAIN BUILDING

The Theory of plate tectonics also explains how and when mountains ranges such as ANDES and the CASCADES arose.

Both this ranges lie at the converging boundaries of 2 plates. As these 2 plates collide, portion of Earth's crust are uplifted and folded, causing a great compressing and thrusting of rock. Earthquakes and volcanoes continue to occur int the area of these new and active mountain ranges.

EARTHQUOAKES

The crust Earth is constantly moving. Massive forces are continually shaping and reshaping its rocks - twisting and cracking them, thrusting then into enormous folds. These changes are generally slow, and most humans are rarely aware of them - except when they culminate in a major earthquake.

Each year, the world experiences about 200 quakes large enough to cause major damage. But earthquakes are much more common that this figure indicates. When all shocks are considered, from the greatest to the smallest year. Sizable shocks average about 10,000 each year. Worldwide, about one person in 8,000 will be killed and roughly 10 times as many will be injured by earthquakes.

CAUSES

Throughout humans history, the unpredictability and ruinous effects of earthquakes have provoked some fanciful explanations. In japan, Earth tremors were thought to be caused by the subterranean stirrings of of a giant catfish, normally restrained with a big mallet by a watchful deity.

This belief was paralleled by similar ideas in China and India-except that a big tortoise and a giant mole were the respective culprits. Around 300 B.C., the Greek philosopher Aristotle theorized that winds from above were drawn into hollow passageways deep inside Earth. Agitated by fire and seeking to escape, these winds caused quakes and sometimes erupted as volcanoes.

Modern geologic research has to led to a much clearer understanding of Earth's structure and the reasons for earthquakes. Scientists now know that deep-seated geologic forces in Earth lead to movements within its uppermost layer.

THE QUAKE ITSELF

Stresses within Earth's rock structure are the driving forces that cause displacements, or strains, int that structure. They take a long time to build, but they finally may reach a point at which even a slight addition of stress, or a small weakening of rock, causes a rupture.

Most earthquakes seem to be cause by sudden movements along preexisting faults located near plate boundaries. Because of friction and the tremendous pressure between the plates, the rocks on either side of the fault do not slide freely against each other. they are virtually locked together and do not move at first, despite building forces. Great strains build up, and the rocks bends. Eventually the plates reach their elastic limit, or breaking point. They suddenly shift to the fault surface, and "snap" into their new positions - a phenomenon called ELASTIC REBOUND. The resultant sudden jar is felt as an EARTHQUAKE.

As with any tearing or breaking action, an earthquakes begins at a specific point and spreads outward. The point of initial fracture is called the "focus"(or hypocenter) of an earthquake. The point on Earth's surface directly above the focus is called the EPICENTER. The distance from the surface to the focus varies from 3 to 25miles in shallow focus earthquakes, to more than 300miles in the deep-focus quakes characteristics of subduction zones.

The rocks on opposite sides of a fault and offset in proportion to the size of the earthquakes. The displacement varies from a fraction of an inch to 20 feet. In a small quake, the fracturing stops within a few seconds. In the largest quakes, it may continue for more than a minute.

Energy is released from the rocks at the leading edge of the fracture. Most of the released strains is dissipated by the breaking and crushing of rocks, by the moving of adjoining blocks, and by the creation of heat. A small portion of energy radiated outward in the form of seismic waves. The ground motion felt during an earthquake is caused by the arrival of these waves at the surface.

EFFECTS

The motion of Earth's surface caused by earthquakes waves can be so slight that no one feels it. Such minor quakes are detected only by sensitive instruments called SEISMOGRAPHS.

Many larger quakes cause a noticeable swaying of the ground and rattling, but they cause neither serious damage nor visible changes in the surface of Earth. The vast majority of quakes fall into these categories. Truly catastrophic quakes are few in number. When earthquakes of a cataclysmic nature do occur, nearly all human-made structures are destroyed.

THE SEISMOGRAPH

The first effective seismographs were built around 1890. Although today's instruments are much more sophisticated, the basic principle remains unchanged. A pendulum mass mounted on a spring is freely suspended from a frame attached to the ground. This construction allows the mass to be reasonably independent of the frame's motion. When the supporting frame is shaken by earthquakes waves. the inertia of the mass causes it to lag behind the motion is measured electronically by the seismometer, and recorded in a computer or on paper as a seismogram.

Since the motion of the ground at any point is three-dimensional, three directions of movement, perpendicular to one another, are recorded. One vertical and two horizontal seismometers generate three seismograms from which seismologists can calculate the movement in space.

Skilled technicians can interpret the initial arrival times of the P and S waves at the recording station. The distance from the earthquake focus to the station os calculated from the known speed of wave travel between different regions on the globe. The data from three or more recording stations, at different locations, are necessary to pinpoint the source of the earthquake. Today seismographic stations are set up all over the world. More than 20,000 earthquakes are documented every year by this worldwide network.

EARTH MAGNETIC FIELD

In the year 1600, William Gilbert, an English physician, set forth the theory that Earth has the properties of a huge magnet, on whose magnetic poles nearly coincide with its geographic poles. He also suggested that Earth's magnetic field originates mainly in the planet's deep interior. These ideas have been confirmed by many investigators.

Many explanations have been offered to account for the magnetism of our planet. One theory assumed that permanently magnetized iron was present in the deep when it was shown that Earth's core was partly fluid, and therefore could not hold permanent magnetism. It seems likely that permanent magnetism elsewhere in Earth's interior would not provide a sufficiently strong field.

In 1947, Patrick M.S. Blackett of the University of London in Britain suggested that any massive rotating body, such as Earth, generates a magnetic field solely as a consequences of the rotation. Laboratory experiments with large rotating objects, however, did not reveal any such field. A test on Earth itself carried out in deep mines also failed to support Blackett's theory.

It now seems probable that the magnetic field of Earth is generated by ordinary electric currents circulating through the planet's interior.

Sir Harold Lamb pointed out in 1893 that such currents would have to be continuously supplied from some source of energy within Earth. It is natural to suppose that this would take place in the part of Earth where there is the least electrical resistance - that is, in the fluid outer core.

In 1939, Walter Elsasser, a German-born American physicist, suggested that such a current might arise in the core when materials of different electric properties and at slightly different temperatures came into contact. This is called the THERMOELECTRIC HYPOTHESIS. Thermoelectricity is produced by the unequal heating of an electric circuit composed of 2 dissimilar metals. In 1954, Stanley K. Runcorn suggested that there might be a thermoelectric effect at the boundary between the mantle and the outer core.

The most highly developed theory of Earth's magnetism is the Dynamo Theory of Elsasser and Sir Edward Bullard. It holds that a huge natural dynamo deep within Earth converts mechanical energy would be supplied by a special type of fluid motion, called CONVECTION, carrying electric currents inside the outer core. Elsasser calculated that such a motion is possible. The dynamo theory of the origin of Earth's magnetism is considered to be the most promising of all the present time.

PLANETESIMAL HYPOTHESIS

Moulton had different explanation of the origin of the solar system. He and geologist Thomas C. Chamberlin proposed Planetesimal Hypothesis. This theory is based in part on the ideas put forth by one of Kant's contemporaries - the french naturalist Comte Georges Louis Leclerc de Buffon. Buffon suggested that the bits of matter that would become the planets were the result of a collision between the Sun and a Comet. Modern astronomers dismiss the idea that there could ever be a comet large enough to have had such an effect. But the idea became the seed of Chamberlin and Moulton's planetesimal hypothesis.

Instead of a cometary collision, they postulated that the surface of the Sun was disturbed by the gravitational pull of a passing star. The colossal tug tore chunks of gaseous material from the stellar surface. Some of this material was pulled along in the passing star's wake, but some of the dislodged chunks remained in orbit around the Sun. They cooled and solidified into hard masses that Moulton and Chamberlin dubbed Planetesimals. Many of the planetesimals orbiting in the same plane collided and combined to form larger and larger planetesimals. The bigger each planetesimal became, the greater its gravitational pull on surrounding material.

NEBULAR HYPOTHESIS

In 1796, Pierre-Simon de Laplace, a French astronomers and mathematician, refined Kant's theory, He suggested that Earth was first a gas and then liquid. Over time, according to Laplace, Earth cooled enough to have a solid crust. Laplace's theory became known as the Nebular Hypothesis.

Essentially, the nebula hypothesis states that at some point in the very distant past, a slowly spinning cloud of interstellar gas began to cool, causing it to shrink into a compact sphere; the sphere then began to spin faster in accordance with the law of conservation of angular momentum, a basic tenet of physics stating that a rotating object gets smaller, it spins faster. A familiar example is a spinning ice skater. When both arms are pulled in close to the body, the skater spin faster. The same principle applies to a spinning cloud of gas.

As the great sphere of interstellar gas rotated faster, centrifugal forces came into play, flinging some of the matter within the cloud away from its center. The various bits of far-flung matter settled into rings tracing paths around the gaseous sphere. The rings of matter cooled and condensed into nine planets that continued to rotate around the sphere, which condensed to form a Sun.

In its earliest form, the nebular hypothesis had some serious flaws. About 100 years after Laplace detailed his ideas, Scottish physicist James Clerk Maxwell and Sir James Jeans, an English physicist, noted that it would have required an unreasonable amount of gravitational pull to condense the proposed rings of matter into planets.

Forest Ray Moulton, an astronomer at the University of Chicago, pointed out an other problem with the theory. According to the law of conservation of angular momentum, the most massive part of any spinning system should be rotating at the greatest speed. But in fact, the planets in solar system rotate around the Sun at the rate faster than the Sun's own rotation. Thus, it is not possible that they were once part of the same spinning cloud.

MODERN THEORIES OF CREATION

MOToday's leading theories concerning Earth's birth have more in common with the early ideas of Kant and Laplace than those of the later collision theorists.

According to the most-accepted scenarios, the solar system began when a huge, old star exploded as a SUPERNOVA, sending heavier elements like carbon, lithium, and beryllium - as well as other debris - flying out into interstellar space, where they mixed with the abundant hydrogen there. This cocktail of gases became the NEBULA from which Earth and other planets formed.

About 5 billions years ago, this rapidly expanding nebula began to cool, contract, and spin more quickly. It became a somewhat flattened disk called a solar nebula.

Most of the mass of this nebula scrunched into its center. The resulting pressure created enough heat to set the center ablaze, creating a "protosun." Away from the center, where temperatures were cooler, whirlpools formed, and grains. Some of the grains, caught within the whirlpools, collided and stuck together. Through this process of accretion, the gathering bits and pieces built up into larger and larger objects, eventually forming planets.

The scientist long believed that sometime during the end of this process of accretion, a random collision blasted off a chunk of Earth large enough to become MOON. But in 1995, a chemical study of the Moon's surface revealed that it contains much less iron than does Earth. This strong evidence that it is not a former piece of Earth. Astronomers says that the Moon was originally a large, independent planetisimal that smashed into Earth. Following the collision, a large chunk of this planetesimal bounced back into space and settled into its present orbit. around the same time, the other planets in the solar system took shape, complete with their personal satellites, or moons.

STUDY OF THE UNIVERSE

A UNIVERSAL VIEW

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".

COSMIC EVOLUTION

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.

THE ASTRONOMICAL ZOO

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.

SOLAR SYSTEM

THE SUN

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.

MOONS

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.

ASTEROIDS

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

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.

COMETS

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.

HOW COMET APPEAR

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.

ORIGIN AND STRUCTURE

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.

VENUS

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.

BENEATH THE GODDESS VEIL
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.

MERCURY

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.

THE END OF THE UNIVERSE

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.

THE PLASMA UNIVERSE AND LITTLE BIG BANG

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.

STEADY STATE THEORY

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.

THE BIG BANG-ORIGIN OF THE UNIVERSE

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?

THE BIG BANG

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.

HOW DID GALAXY FORM

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.

Subscribe to: Posts (Atom)