THE UNIVERSE When we look up in the night sky we can see the stars and the moon. And because it is natural to be curious, we ask questions and we want answers. When our view was limited by what our eyes could see, the sky was our Universe. Then the telescopes deepened our view, photography enhanced it, and spectroscopy broadened it. The universe grew from a sky of stars to a realm of galaxies, to an expanding universe of galaxies. Many people believe that nature, the sun and moon, the star, even human beings never had a beginning. There is an endless, external cycle of birth, life and death that constantly repeats itself and it never began and will never end. In the Book of Genesis in the Bible, it was written that at first the world did not exist and that God is the only one who existed. So He created the world.
The universe is the totality of everything that has ever existed. It is so large that it contains billions of stars, and all of the planets, galaxies and all of space. The study of the universe is called Cosmology.
Traditional Views about the Universe
1. Geocentric Universe
Greeks believed that the earth was a sphere that stayed motionless at the center of the universe or the geocentric (Earth-centered) view. Orbiting the earth were seven wanderers (planetai in Greek) including the sun, the moon and the known planets, Mercury, Venus, Mars, Jupiter and Saturn. Greece was centered as the “Golden Age” of early astronomy.
Claudius Ptolemy created the book Almagest, the only surviving comprehensive ancient treatise on astronomy. His geocentric outlook later became the Ptolemaic system in the second century A.D. and dominated western thought for some 2000 years.
2. The Heliocentric System The view of the universe in which the sun is taken to be at the center is called the heliocentric system. It was first proposed by Aristarchus of Samos but is never gained wide support because its proponents could not explain why the relative position of stars seemed to remain despite the Earth’s changing viewpoint as it moved around the sun. Nicolas Copernicus was a Renaissance astronomer and the first person to formulate a comprehensive heliocentric cosmology which displaced the Earth from the center of the universe. Having concluded that the Earth is a planet, Copernicus reconstructed the model of the solar system with the sun ate the center and the planets Mercury, Venus, Earth, Mars, Jupiter and Saturn orbiting around it. But he retained the description of planetary motions as being a series of superimposed circular motion, mathematically equivalent to the Ptolemaic theory. Copernicus’ monumental work, de Revolutionibus Orbium Coelestium (On the Revolution of the Heavenly Spheres), was published in 1543. Johannes Kepler upheld this in his law of Planetary Motion based on the work of his former employer Tycho Brahe. Kepler’s labor laid the ground work for Sir Isaac Newton’s Law of Gravitation (1687) which became possible for astronomers to predict with great accuracy the movements and positions of the planets. Galileo Galilei in his first discoveries using the telescope for astronomical purposes has become as essential tool in planetary studies. The first telescope was made by Duthcman Hans Lippershey.
Theories on the Origin of the Universe
1. Big Bang Theory Most astronomers believe that the universe began about 15 billion years ago in a huge explosion they called the Big Bang. This theory successfully explains the expansion of the universe and the observed abundance of helium in the universe. The Big Bang Theory was first developed in 1927 by A.G.E. Femaitre and coined and revised in 1946 by George Gamow.
2. Open Universe Edwin Hubble discovered that the galaxies of the universe are moving farther apart. This means that the universe is getting bigger continually. This is called the Open Universe Theory. Alternatively, the galaxies may come together, until finally they will collide and explode. This event is called the “Big Crunch.” If the Big Crunch occurred, the sky would grow as hot as the sun. Finally everything would vanish into a black hole.
3. Steady State Proposed by Hemann Bondi, Thomas Gold, and Fred Hoyle in 1948, Steady State Theory proposes that matter is being continuously created, at the rate of a few hundred atoms per year. This would allow the density of the universe to remain constant as it expands. This violates the Law of Conservation of Matter and Energy.
4. Creation Theory Nowadays, Creation Theory earns a strong influence to scientists explaining that there was once a Creator who, through His Word, designed and created the universe and the vastness of it. According to His Word, He created everything systematically and with purpose.
What is a Galaxy? Galaxy is a collection of stars, gas, and dust bound together by gravity. The smallest galaxies may contain only a few hundred thousand stars, while the largest galaxies have thousands of billions of stars. The galaxy is so huge that it would take a beam of light about 130,000 years to cross from one end to the other even though light travels at almost 186,000 mi per second. In the early part of modern times, it was proposed that galaxies were telescopically visible fuzzy patches of light scattered among the stars. Immanuel Kant, a German philosopher in the mid-1700s, believed that galaxies contain billion of stars and, as such, was a universe in itself. In 1920, an American astronomer Edwin Hubble was able to locate within one of these fuzzy patches some unique stars that are known to be intrinsically very bright, so a clear feature of galaxies had been described.
The Milky Way Our galaxy, the Milky Way, is just one of the other galaxies in the universe. It is a barred spiral galaxy with two arms, namely the Large Magellanic Cloud and the Small Magellanic Cloud, that rotate slowly. Milky Way is a part of about 30 galaxies called the local group. Stars within our galaxy are dominant source of light at the infrared wavelength. Andromeda Galaxy is the nearest large galaxy. It is a spiral galaxy but is four times as massive and is 2 million light years away. Our galaxy is traveling through intergalactic space.

Types of Galaxy
1. Elliptical These are the most abundant group making up about 60% of the total galaxies. They appear like ellipsoidal clouds of stars, with very little internal structure apart from a denser nucleus. They virtually lack interstellar gas and dust. They are generally smaller compared with other types of galaxies ranging from ball shaped to egg shaped. The term “dwarf” has been applied to those that are so small because they are not visible at great distances.
2. Spiral
These are flat and disk shaped galaxies. At the center of a spiral galaxy is a bulge of old stars, known as the nucleus. The nucleus is surrounded by an invisible cloud of dark matter. Spiral galaxies have large arms that spiral outward. This spiral shape is caused by the rotation of the galaxy around the nucleus. Spiral galaxies have two famous spiral arms that slowly revolve round a central ball of stars.
NGC 4414 The majestic galaxy, NGC 4414 is an unbarred spiral galaxy about 62 million light-years away in the constellation Coma Berenices. NGC 4414 is a giant spiral-shaped disk of stars, with a bulbous central hub of older yellow and red stars. The outer spiral arms are considerably bluer due to ongoing formation of young, blue stars, the brightest of which can be seen individually at the high resolution provided by the Hubble camera. The arms are also very rich in clouds of interstellar dust, seen as dark patches and streaks silhouetted against the starlight.
3. Barred- Spiral This type of galaxy is a spiral galaxy with a bar like bulge in the center, extending between the core and the spiral arms. About a third of spiral galaxies have this straight bar of stars, gas, and dust extending out from the nucleus. As with other spiral galaxies, barred spiral galaxies rotate, and new stars form from the dust and gas in their arms. Astronomers believe that some elliptical galaxies containing hints of a bar and spiral might once have been barred spiral galaxies.
4. Irregular These galaxies do not have the clearly defined shape and structure of typical elliptical, lenticular, or spiral galaxies. They account for only a small percentage of known galaxies. Some irregular galaxies are the result of gravitational interactions or collisions between formerly regular galaxies. Many irregular galaxies orbit larger regular ones; the Magellanic Cloud galaxies orbiting the Milky Way are examples.
Components of the Galaxy A spiral galaxy has three main parts to its visible matter. These are the disk (containing the spiral arms), the halo, and the nucleus or central bulge. The halo and the nucleus are also collectively called as the spherical component of the galaxy because they have an approximately spherical distribution with respect to the center of the galaxy. Aside from these visible components, the galaxy also contains at least three other parts that are “invisible.” They are the galactic magnetic field, charged particles trapped in the galactic magnetic field, and a halo of “dark matter” that is of unknown composition but that makes it felt by its gravitational influence on the visible matter.

The Disk of the Galaxy Most of the gas and dust of the Milky Way is contained in the disk and often termed the interstellar medium. The gas is primarily hydrogen and helium, and the dust makes many regions of the disk opaque. The disk is not a clearly defined thing, because it depends on what objects that we use to define it, and because portions of it are blocked from our view at visible wavelengths. In addition, as one goes vertically in the disk (perpendicular to the plane), there is no sharp boundary for the disk. Rather, the density of stars gradually gets smaller. Therefore, astronomers sometimes refer to more than one disk for the galaxy. The disk of Milky Way is quite prominent and even in other spiral galaxies because of its spiral arms that contain many hot young stars and, therefore, luminous. These younger stars are often contained in associations, and open clusters.
The Halo of the Galaxy
The halo of the galaxy is rather spherical in shape and contains little gas, dust, or star formation. It is difficult to measure precisely, but the halo appears to extend beyond the disk. The clusters found in the halo are globular clusters, so the halo is population II, and contains very old stars; they may be as old as 15 billion years and are the oldest components of the galaxy. This implies that the galaxy itself is at least 15 billion years old.
The stars of the halo are on more elliptical orbits that are randomly oriented. This indicates that the halo stars pass through the disk and the nucleus of the galaxy, but spend the majority of the time far above or far below the plane of the galaxy.
The Nucleus of the Galaxy The nuclear bulge or core contains the highest density of stars in the galaxy. Although some hot young stars may be found in the nucleus, the primary population of stars is similar to that of the old stars found in the halo. Astronomers also observed that even though at visible wavelengths, dust, gas, and stars obscure the core of the galaxy; at other wavelengths, there are some pieces of evidence that violent processes may be taking place there. According to them, many galaxies may contain very massive black holes at their cores, and our own galaxy may be no exception.
The Galactic Magnetic Field and Cosmic Rays The disk of the galaxy is permeated by a magnetic field. This field is weak, being only about 1/50,000 of the strength of the Earth’s magnetic field at the surface, but it influences the motion of charged particles in the galaxy. One important consequence of the magnetic field is that it can bend the path of and even trap the high-energy charged particles called cosmic rays. Thus the galaxy is filled with cosmic rays and because of the effect of the magnetic field we cannot tell with certainty where they come from, though the strongest arguments favor supernova explosions for their source.
The Dark Matter Halo Astronomers assert that there is abundant evidence that the vast majority of matter in the universe does not show up in our telescopes. However, they can infer its presence by its gravitational influence. They refer to this as dark matter. No one clearly knows what it is, though there is fairly strong evidence that it is not the ordinary matter of stars, gas, dust, and planets. Some indirect means suggest that the dark matter halo may extend as far as 100,000 parsecs from the center of the galaxy.

Is the universe expanding? The continuous exploration of our universe gave way to the idea that the universe is expanding indefinitely in all directions. This idea has disturbed some, and alternative explanations have been proposed. Abbe Lemitre proposed the idea that the expanding universe was created at some finite time in the past, and at the beginning, all the matter now in the universe was packed together like a single primeval atom with protons, neutrons, and electrons. Until such time, these electrically charged particles jumbled together at a high temperature, pressure, and density. Then, there came a violent explosion, and the materials has been flying outward ever since, it is called the Big Bang Theory. As the temperature subsides, the primary particles combine to form all of the elements in the universe including our solar system. A group of American physicist led by George Gamow revises this theory. According to them, after the explosion, the lighter elements were formed first before the temperature had dropped below the threshold of thermonuclear reactions. Then the materials was scattered in all directions with the heavier elements being formed during the evolution of the stars. They also assert that because if the varying velocities and the positions of the outer galaxies, the universe is expanding, which must have taken place about 12 billion years ago. According to this theory, the sun was formed about 8 billion years ago. Another hypothesis states that it was formed at the same time as the earth, some 4.5 billion years ago. Various views concerning about whether the universe is expanding or not are still surfacing. Considering the Big Bang Theory, if the explosion produced sufficiently great velocities, the galaxies could move away from the earth forever, but if the velocities were less than this critical, they would gradually slow down and fall back together again. There are evidences that may lead us to believe that the outer galaxies are not moving as fast as they ought to after the original big bang. Some believe that galaxies are actually slowing down and will come back together again as a primeval atom that they were, 82 billion years after the last explosion. It means that there is a tendency that there will be another big bang. It seems, therefore, that we are on the expanding part of the cycle of an “oscillating universe.” H. Bondi, T. Gold, and F. Hoyle of England had proposed another theory in 1951. They claim that there is an entirely new cosmology that as the material in the universe expands and closed outward the Creator is continually creating new material at the center, keeping the density of galaxies the same throughout the universe, known as the Continuous Creation Theory, that suggest that there is no beginning nor end to the universe, because matter is being continually created as it is dissipated. This view also states that there will always be young, medium-aged, and old galaxies in the same proportion. Until now, there are insufficient data that have come up with any reliable conclusion. If the Big Bang theory is correct, the number of galaxies per unit volume of space ought to be decreasing, while in the Steady State Theory, the number per unit volume ought to remain the same. Data seem to indicate a thinning of material in space, but again, not conclusively. In 1960, Allen Sandage discovered an extremely bright object in the sky through the use of a radio telescope that was about 100 times as bright as any known galaxy. And it was confirmed that these were smaller than regular galaxies and were not stars in the usual sense. They are called quasi-stellar or quasars. Over thirty quasars were discovered and identified before the end of 1964. All show a large Doppler red shift in their spectra that indicates an expanding universe as suggested by the Big Bang Theory. This explains that they are receding at very high velocity, over 100,000 mi/sec. The quasars must be very far off, 3C147 means about 8.7 billion light years away.
INTRODUCTION TO THE SOLAR SYSTEM
The Solar System is made up of all the planets that orbit our Sun. In addition to planets, the Solar System also consists of moons, comets, asteroids, minor planets, and dust and gas. The sun is considered as the richest source of electromagnetic energy that is mostly in the form of heat and light in the solar system. Science believed that the solar system began about 4.6 billion years ago. Proxima Centauri is a red dwarf star about 4.3 light-years away is the nearest known star to the Sun. The whole solar system, together with the local stars visible on a clear night, orbits the center of our home galaxy, a spiral disk of 200 billion stars known as the Milky Way. The sun contains 99.85% of all the matter in the solar system. The planets, which condensed out of the same disk of material that formed the sun, contain only 0.135% of the mass of the solar system. Jupiter contains more than twice the matter of all the other planets combined. Satellites of the planets, comets, asteroids, meteoroids, and the interplanetary medium constitute the remaining 0.015%.
THEORIES ON THE ORIGIN OF THE SOLAR SYSTEM
1. The Planetisimal Theory In 1778, George Comte de Buffon, a French scientist, first proposed this theory. He supposed that the planetary system was forms from the material removed from the sun by great gravitation. Eventually the material cooled and condensed to form small bodies, which in the course of time, become planets.
2. The Dust Cloud Theory This was formulated by German physicist Carl Friedrich von Weizsacken and US chemist Harold C. Urey in 1945. It presumed that the Nebula was flattened by its rotation and the planetary in the gas molecules accelerated the lighter ones so that moist of them escaped from the nebula. Meanwhile, the matters in the disc were clumping together into bigger and bigger lumps, which became the planets and their moons.
3. The Companion Star Theory Fred Hoyle proposed that the sun once had a companion star. As this star collided with the sun, it eventually exploded and its materials were held by the sun’s gravitation. From these materials, various planets and other bodies in the solar system were formed.
4. The Nebulas Hypothesis This was proposed by Marquis de Laplace, a French mathematician and astronomer in 1976; it states that the gas and dust particles began to come together under gravity to form a denser and denser mass, which started to rotate. The more it collapsed, the faster it rotated, over time, to formed into a disc, with a bulge in the middle.
THE PLANETS A planet is a large body made of gas, metal, or rock that orbits a star. The eight planets that orbit the sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Another large body is Pluto, now classified as a dwarf planet or plutoid, they may be divided into two groups, namely, the inner or terrestrial planets, and the outer or Jovian planets. This division is based not only on their distance from the sun, but also on the physical properties of the planets.
The Satellites Also known as natural satellites, is a relatively small natural body that orbits a planet. Mars have natural satellites, namely, Phobos and Deimos. There are probably also many more planetary satellites that have not yet been discovered.
Orbits
The orbits of the planets are ellipses with the Sun at one focus, though all except Mercury and Pluto are nearly circular. The orbits of the planets are all more of less in the same plane (called the ecliptic and defined by the plane of the Earth’s orbit). With the aid of astronomical instruments, the astronomers were able to measure the orbits of the planets around the sun. The ecliptic is inclined only 7 degrees from the plane of the sun’s equator. Pluto’s orbit deviates most from the plane of the ecliptic with an inclination of 17 degrees. Aside from the planets and their satellites, there are numerous smaller bodies that inhabit the solar system like a large number of asteroids (small rocky bodies) orbiting the sun, mostly between Mars and Jupiter but also elsewhere; the comets (small icy bodies) which some and go from the inner parts of the solar system in highly elongated orbits and at random orientations to the ecliptic; and many comets beyond Neptune in the Kuiper Belt. With a few exceptions, the planetary satellite orbit in the same sense as the planets and approximately in the plane of the ecliptic, but this is not generally true for comets and asteroids.
Interplanetary Space Nearly all the solar system by volume appears to be an empty void. Far from being nothingness, this vacuum of “space” comprises the interplanetary medium. It includes various forms of energy and at least two material components: interplanetary dust, that consists of microscopic solid particles and interplanetary gas, which is a tenuous flow of gas and charged particles, mostly protons and electrons—plasma--that stream from the sun, called the solar wind.
Sizes
When everything is reduced in size by a factor of a billion, the model earth would be about 1.3 cm in diameter as the size of a grape, the moon would be about a foot of about 30 cm from the earth and the sun would be 1.5 meter in diameter as like about the height of a man and 150 meters from the earth. The diameter of Jupiter could be compared with the size of a large grapefruit estimating to 15 cm and 5 blocks away from the sun. Saturn, on the other hand, could have the size of an orange and would be 10 blocks away; Uranus and Neptune, like the size of a lemon and 20 and 30 blocks away. A human on this scale would be the size of an atom but the nearest star would be over 40,000 km away.
Classification
How do we classify the members of the solar system? Traditionally, the solar system has been divided into planets, their satellites, asteroids, and comets. However, there are different ways to classify the members of the solar system. Some classified them according to their chemical composition and point of origin.
Specifically, the nine planets can be classified as follows:
1. By composition
• Terrestrial or rocky planets – composed primarily of rock and metal and have relatively high densities, slow rotation, solid surfaces, no rings, and few satellites. These planets are Mercury, Venus, Earth and Mars.
• Jovian or gas planets – basically composed of hydrogen and helium and generally have low densities, rapid rotation, deep atmospheres, rings, and numerous satellites such as Jupiter, Saturn, Uranus, and Neptune.
• Pluto is neither a terrestrial nor a gas planet.
2. By size
• Small planets – have diameters less than 13,000 km, examples are Mercury, Venus, Earth, Mars and Pluto.
• Giant planets – have diameters greater than 48,000km, examples are Jupiter, Saturn, Uranus, and Neptune. Sometimes also referred to as gas giants.
• Mercury and Pluto are sometimes referred to as lesser planets.
3. By position relative to the sun
• Inner planets like Mercury, Venus, Earth, and Mars.
• Outer planets such as Jupiter, Saturn, Uranus, Neptune, and Pluto.
• The asteroid belt between Mars and Jupiter forms the boundary between the inner solar system and the outer solar system.
4. By position relative to the earth
• Inferior planets – show phases like the moons when viewed from the earth and they are closer to the sun than Earth. Mercury and Venus are inferior planets.
• Superior planets – farther from the sun than earth and always appear full or nearly so. Mars thru Pluto are considered as superior planets.
5. By history
• Classical planets – planets known since prehistorical times. They are visible to the unaided eye. They are Mercury, Venus, Mars, Jupiter, and Saturn.
• Modern planets – discovered in modern times visible only with telescoped. They are Uranus, Neptune, and Pluto.
What is a light- year? Astronomers use special units to represent enormous distances in spaces, they are measured in light-year. One light-year is equal to the distance traveled by a beam of light on one year (5.88 trillion mi or 9.4 trillion km). Astronomical Unit is the basic unit of distance within our solar system. One AU is the average distance from the earth to the sun (about 92,960,000 mi of 149,600,000km).

THE SUN, THE STARS, AND THE MOON
The Sun The Sun is the most prominent feature in our solar system. It is the largest object and contains approximately 98% of the total solar system mass. One hundred and nine Earths would be required to fit across the Sun's disk, and its interior could hold over 1.3 million Earths. The Sun appears to have been active for 4.6 billion years and has enough fuel to go on for another five billion years or so. At the end of its life, the Sun will start to fuse helium into heavier elements and begin to swell up, ultimately growing so large that it will swallow the Earth. After a billion years as a red giant, it will suddenly collapse into a white dwarf -- the final end product of a star like ours. It may take a trillion years to cool off completely.
What are the external features of the sun?
 Photosphere. It is the outer visible surface of the sun or other star and the source of its continuous spectrum. It has a temperature of 6,000˚C (11,000˚F). This layer has a mottled appearance due to the turbulent eruptions of energy at the surface. Sunspots are dark depressions on the photosphere with a typical temperature of 4,000˚C (7,000˚F)
 Chromosphere. The chromosphere is above the photosphere. Solar energy passes through this region on its way out from the center of the Sun. Faculae, bright luminous hydrogen clouds which form above regions where sunspots are about to form and flares are bright filaments of hot gas emerging from sunspot regions and arise in the chromospheres. Spicule, a narrow jet of rising material, also appears in the solar chromosphere.
 Prominences. Prominences are surges of glowing gas rising from the surface of the sun. The largest appears as huge arches that last from several hours before collapsing back. Prominence follows lines of magnetic force, and seen pinkish when seen at the edge of the sun during an eclipse.
 Corona. The corona is the outer part of the Sun's atmosphere, consisting of plasma (hot ionized gas). It is visible to the eye during a total solar eclipse. It is in this region that prominences appear. The outer region of the corona stretches far into space and consists of particles traveling slowly away from the sun.
What are the internal structures of the sun?
 Core. Thermonuclear reactions occur in this area, which extends out to a distance of 0.25 solar radiuses from the center.
 Radiation zone. Energy from the thermonuclear reaction in the core is transported outward through radioactive diffusion in this zone to a distance of 0.83 solar radii.
 Convection zone. Energy transported from the radioactive zone of the sun’s outer layer is transported by this zone by convection process.

Internal Structure Explained Solar energy is produced deep within the core of the sun. In this area, the temperature (15,000,000˚C; 27,000,000˚ F) and the pressure (340 billion times earth’s air pressure at sea level) are so intense that nuclear reactions take place. This reaction happens when four protons or hydrogen nuclei fuse together to form one alpha particle or helium nucleus. The alpha particle is about.7% less massive than the four protons. The difference in mass is expelled as energy and is carried out to the surface of the sun through the process known as convection, where it s released as light and heat. It takes million years before the generated energy from the sun’s core to reach its surface. Every second, 700 million tons of hydrogen is converted into helium ashes. N the process, 5 million tons of pure energy is released. Thus, as time goes, the sun is becoming lighter.
How does the sun shine?
The surface temperature of the sun is about 5,500˚C to 6,000˚C compared to its interior temperature, which is about 15 million ˚C and is gaseous throughout. Here, the atoms that make up its main gas, hydrogen, have so much energy that may break apart, coming together again as a helium gas. During this reaction, a burst of energy is given out which drives the sun.
Solar Energy and Influence The core of the sun converts hydrogen to helium at a rate of 600 million tons. Therefore, converting hydrogen to helium means that the sun loses at least a million tons of its mass every second. The sun influences an enormous volume of spaces around it. Gases streaming from the corona become the high speed solar wind. Solar wind consists of atomic particle that are magnetic, and forces the planets magnetic field into a pear shape. The heliosphere is the invisible “bubble” that contains the sun’s magnetic field. It shields the solar system from some of the cosmic radiation in space.
Is the earth eternal? The sun’s lifetime is not limited by the amount of its fuel, but by the growth of its core. When the core reaches a certain size, the sun will expand and it will start to destroy the Earth! But how? The sun will continue to shine almost unchanged for several billion years. Meanwhile, the earth will pass through ice ages and warm periods as its orbits goes through a regular cycle of slight change. In about 5 to 10 billion year’s time, the energy from the sun’s huge core will make its outer layers extended. As our star swells and becomes hotter, the water on earth will start to boil away. Life forms will be suffocated in the great heat. As the sun turns into a red giant star, the earth will be burned to cinder (imagine the flowing magma inside the volcano), and its atmosphere will be stripped away.
THE STARS
What is a star? A star is ball of hydrogen and helium with enough mass that it can sustain nuclear fusion at its core. Although all stars are born at the same time, their lifetimes and endings will be different according to the amount of material they contain. Stars are sources of light and heat.

The Life of a Star A star is born when a cloud of gas and dust shrinks into a ball. The ball gets hotter and hotter until it starts to glow as a newborn star. An average star shines for billions of years, burning up its gasses. When it runs out of fuel, it swells up into a reddish color known as red giant. At its greatest size, the diameter of a red giant may be a hundred times that of the original star, which forms the core of the red giant. The red giant then starts to shrink and becomes a white dwarf. The white dwarf slowly cools then fades to a cold black dwarf. Big stars turn into red super giants. Instead of cooling down, they explode as supernovae and for a short time, burns more brightly than a billion suns. What is left is either a neutron star or, if the star was very big, a black hole.
Properties of Stars
Distance
To measure the distance of a star from the earth, astronomers use parallax, the apparent change in the position of a star due to change in the position of the observer. The first star distance was measured in 1838.Astronomers generally express distance between stars in parsecs (parallax per second). One parsec is equal to 3.26 lights years. The first to introduce a method of measuring the distances of stars is Friedrich Wilhem Bessel.
Magnitude
Star brightness is rated in magnitude: the smaller the number, the brighter the star. In Orion, the bright Rige is magnitude 0, while Wintaka and Alnitak are both magnitude 2.
Color and Temperature Temperature is the most important factor that affects the color of stars, from the cooler red stars, warmer yellow stars, to the hottest bluish white stars.
Size
Most stars are about the same size. But some super giant stars are hundreds of times larger than sun, while some dwarf stars are no bigger than the earth. Neutron stars may be only 15 km (9 miles) across, but they are very dense.
Composition
Most stars are composed of 72% hydrogen, 25% helium, and 3% of all the other elements (iron, carbon, nickel, etc.) a spectroscope is used to study the composition of stars. Father Angelo Sechi, an Italian priest and astronomer introduced the classification of stars according to composition.
Motion
Stars move a high speeds (relative motion) in the course of a century. These movements will change slightly the shape of constellation, but have not materially disarranged the constellations. Only relative motion can be measured for absolute motion is meaningless.
What is a constellation? A constellation is a two-dimensional view of objects in three-dimensional space. To most people, a constellation is a group of stars. Actually, a constellation has a definite area of the sphere, with internationally agreed boundaries. The sky contains 88 constellations; each one represents a mythological person, venture, or object. The largest constellation is Hydra (the water snake) while the smallest is crux (the cross) the Big Dipper is not a separate constellation but is part of Ursa Major (The Great Bear)
MOON
The Moon is Earth's only known natural satellite. It is composed of relatively bright highlands and darker plains. Galileo and other early astronomers made telescopic observations, noting an almost endless overlapping of crates.
The Soviet space craft Luna 2 visited the moon in 1959. It is the only extraterrestrial body to have been visited by humans. On July 20, 1968, Neil Armstrong became the first man to step onto the surface of the moon and was followed by Edwin Aldrin, both of the Apollo 11 mission. They experienced the effects of no atmosphere. The lunar sky is always black because diffraction of light requires an atmosphere. Aside from this, they also found out the effect of gravitational differences. The moon’s gravity is one-sixth that of the earth. Thus, if a man weighs 180 lb on earth, he weighs only 30 lb on the moon.
Interior of the Moon The Apollo missions left seismometers on the lunar surface that have allowed us to deduce the general features of the Lunar interior by studying the seismic waves generated by "moonquakes" and occasional meteor impacts. It has a crust about 65 km thick, a mantle about 1000 km thick, and a core that is about 500 km in radius. A limited amount of seismic data suggests that the outer core may be molten. There does appear to be some amount of differentiation, but not on the scale of that of the Earth. It has no magnetic field to speak of, but magnetization of Lunar rocks suggests that it may have had a larger one earlier in its history. Although there is a small amount of geological activity on the Moon, it is largely dead geologically.
Seismic data also reveal the past of the moon. The was heavily bombarded early in its history, which caused many of the original rocks of the ancient crust to be thoroughly mixed, melted, buried, or obliterated. As the intense meteor bombardment associated with debris left over from the formation of the Solar System continued, most of the craters that we now see on the surface of the Moon were formed by meteor impact. Since the moon has neither an atmosphere nor any water, the components in the soils don not weather chemically as they would on earth. Rocks aging more than 4 billion years old still exist there, yielding information about the early history of the solar system that is unavailable on earth. Geological activity on the moon consists of occasional large impacts and the continued formation of the regolith, this implies that it is considered dead. With such an active early history of bombardment and a relatively abrupt end of heavy impact activity, the moon is considered fossilized in time.
The Surface Materials of the Moon The Moon rocks and soil have three major types: the regolith, the maria, and the terrae. The regolith contains rocks, fragments of minerals from the original bedrock, and glassy particles formed during the impacts, it is caused by micrometeorite bombardment that thoroughly pulverized the surface rocks into fine-grained debris. It can be found over the entire moon, with the exception of steep crater and valley walls. It is 2 to 8 meter (7 to 26 feet) thick on the maria and may exceed 15 meters on the terrae, depending on how long the bedrock underneath it has been exposed to meteoritic bombardment. The major products of volcanic processes on the Moon are evident to the Earth-bound observer in the form of the lunar maria. These are large flows of basaltic lava that correspond to low-albedo surfaces covering nearly a third of the near side. Most scientists believed that they were lava-filled plains, since they possessed lava flow patterns and collapses attributed to lava tubes. They average only a few hundred meters in thickness but are so massive they frequently deformed the crust underneath them that created fault like depressions and raised ridges. Terrae or highlands were described as the brighter, older, and generally higher standing terrain occupying most of the Moon's surface. They are much more densely populated by large craters. Seismic data shows that nearly all of the highland breccias and impact melts are formed about 4.0 to 3.8 billion years ago. The intense bombardment may have begun 4.6 billion years ago, which is the estimated time of the moon’s origin. The three principal theories were: 1.) Co-accretion, which asserted that the moon and the earth formed at the same time from the solar nebula; 2.) Fission, which believed that the moon split off the earth; and 3.) Capture, which viewed that the moon is formed elsewhere and was subsequently captured by the earth. New and detailed information from the moon rocks led to the impact theory that held the earth collided with a very large object and that the moon was formed from the ejected material. There are still details to be worked out, but the impact theory is now widely accepted.
The Moon is 384,403 kilometers (238,857 miles) away from the Earth. Its diameter is 3,476 kilometers (2,160 miles). Both the rotation of the Moon and its revolution around Earth takes 27 days, 7 hours, and 43 minutes. This synchronous rotation is caused by an unsymmetrical distribution of mass in the Moon, which has allowed Earth's gravity to keep one lunar hemisphere permanently turned toward Earth. Optical librations have been observed telescopically since the mid-17th century. In the summer of 1994, the moon was very extensively mapped by the little spacecraft Clementine and again in 1999 by Lunar Prospector.
The Gravity of the Moon The gravitational force between the earth and the moon causes some interesting effects and the most obvious is the tides. The moon’s gravitational attraction is stronger on the side of the earth nearest to the moon and weaker on the opposite side. Since the earth, particularly the oceans, is not perfectly rigid, it is stretched out along the line toward the moon, producing two small bulges on the earth’s surface. The effect is much stronger in the ocean water that is why water bulges higher that is why there are two high tides per day. Because Earth's Moon rotates around the Earth and shows us nearly the exact same side it is considered to be in "synchronous rotation". In its early days it was not synchronous but because of Earth's gravity it has slowed down the Moon's rotational speed, then there was no longer an off center torque on the moon, and a stable situation was achieved.Very small but real liberations are caused by the effect of the sun’s gravity and the eccentricity of earth’s orbit, perturbing the moon’s orbit and allowing cyclical preponderances of torque in both east-west and north-south directions. The moon has no global magnetic field. But some of its surface rocks exhibit permanent magnetism, indicating that there may have been a global magnetic field early in the moon’s history. With no atmosphere and no magnetic field, the moon’s surface is exposed directly to the solar wind. Over its 4 billion year lifetime, many hydrogen ions from the solar wind have become embedded in the moon’s regolith. Thus, samples of regolith returned by the Apollo missions proved valuable in studies of the solar wind. This lunar hydrogen may also be of use someday as rocket fuel.
THE PLANETS AND OTHER HEAVENLY BODIES A planet is a large body made of gas, metal, or rock that orbits a star. Moving in nearly the same speed around the sun are the nine planet and their satellites. Planets are most crystalline and are made visible to us by reflected sunlight. All of them are formed probably at the same time, from the same cloud of gas and dust around the sun, but there are great differences between them.
The Terrestrial Planets There are four terrestrial planets in our Solar System: Mercury, Venus, Earth, and Mars. They are called terrestrial because they have a compact, rocky surface like the earth’s. The planets Venus, Earth, and Mars have significant atmospheres, while Mercury has almost none.
The Jovian Planets Jovian planets are also known as gas giants. There are four Jovian planets: Jupiter, Saturn, Uranus, and Neptune. These four planets also comprise the outer planets.
Mercury
Mercury is the innermost planet in the Solar System. It is a dead, air less world that whirls through space in the merciless glare of the sun. It can be observed in the sky with unaided eyes only when the sun is well below the horizon. It is the closest planet to the sun, and the second smallest in the solar system.
In Roman mythology Mercury is the god of commerce, travel and thievery, the Roman counterpart of the Greek god Hermes, the messenger of the Gods. The planet probably received this name because it moves so quickly across the sky.
The only spacecraft to have visited Mercury was Mariner 10. It flew three times in March 1974 and 1975, and was photographed half, within 470 miles off the planet surface.
Mercury’s orbit is highly eccentric; at perihelion, it is only 46 million km from the sun, but at aphelion, it is 70 million. The perihelion of its orbit precedes around the sun at a very slow rate. The Doppler radar observations in 1965 suggested that Mercury rotated three times in two years. Mercury is the only body in the solar system known to have an orbital rotational resonance with other ratio 1:1.
The surface of Mercury consists of cratered terrain and smooth plains and many deep craters similar to those on the moon. The craters formed when meteors or small comets crashed into the planet. The largest known crater is Caloris Basin, with a diameter of 1300 km (800 miles).
Like the other terrestrial planets Mercury is made mostly of rock and metal, its surface appears to be much like that of the moon. It reflects approximately 6 %of the sunlight it receives, about the same as the moon's surface reflects. Like the moon, Mercury is covered by a thin layer of minerals called silicates in the form of tiny particles.
Although Mercury is not tidally locked to the Sun, its rotational period is tidally coupled to its orbital period. Mercury rotates one and a half times during each orbit. Because of this 3:2 resonance, a day on Mercury (sun rise to sun rise) is 176 Earth days long.
Mariner 10 showed that Mercury has a magnetic field that is 1% as strong as Earth's. This magnet field is inclined 7 degrees to Mercury's axis of rotation and produces a magnetosphere around the planet. The source of the magnetic field is unknown. It might be produced from a partially molten iron core in the planet's interior. Another source of the field might be from remnant magnetization of iron-bearing rocks which were magnetized when the planet had a strong magnetic field during its younger years. As the planet cooled and solidified remnant magnetization was retained.
Even before Mariner 10, Mercury was known to have a high density. Its density is 5.44 g/cm3 which are comparable to Earth's 5.52g/cm3 density. In an uncompressed state, Mercury's density is 5.5 g/cm3 where Earth's is only 4.0 g/cm3. This high density indicates that the planet is 60 to 70 percent by weight metal, and 30 percent by weight silicate. This gives a core radius of 75% of the planet radius and a core volume of 42% of the planet's volume.
Mercury's history of formation is similar to that of Earth's. About 4.5 billion years ago the planets formed. This was a time of intense bombardment for the planets as they scooped up matter and debris left around from the nebula that formed them. Early during this formation, Mercury probably differentiated into a dense metallic core, and a silicate crust. After the intense bombardment period, lava flowed across the surface and covered the older crust. By this time much of the debris had been swept up and Mercury entered a lighter bombardment period. During this period the intercrater plains formed. Then Mercury cooled. Its core contracted which in turn broke the crust and produced the prominent lobate scarps. During the third stage, lava flooded the lowlands and produced the smooth plains. During the fourth stage micrometeorite bombardment created a dusty surface also known as regolith. A few larger meteorites impacted the surface and left bright rayed craters. Other than the occasional collisions of meteorites, Mercury's surface is no longer active and remains the same as it has for millions of years.
Could water exist on Mercury?
It would appear that Mercury could not support water in any form. It has very little atmosphere and is blazing hot during the day, but in 1991 scientists at Caltech bounced radio waves off Mercury and found an unusual bright return from the north pole. The apparent brightening at the North Pole could be explained by ice on or just under the surface. But is it possible for Mercury to have ice? Because Mercury's rotation is almost perpendicular to its orbital plain, the North Pole always sees the sun just above the horizon. The insides of craters would never be exposed to the Sun and scientists suspect that they would remain colder than -161 C. These freezing temperatures could trap water out gassed from the planet, or ices brought to the planet from cometary impacts. These ice deposits might be covered with a layer of dust and would still show bright radar returns.
Venus
Venus, the jewel of the sky, was once known by ancient astronomers as the morning star and evening star. Early astronomers once thought Venus to be two separate bodies. Venus, which is named after the Roman goddess of love and beauty, is veiled by thick swirling cloud cover. The surface of Venus is extremely dry. There is no liquid water on its surface because the scorching heat would cause any to boil away. Roughly two-thirds percent of the Venusian surface is covered by flat, smooth plains that are marred by thousands of volcanoes, ranging from about 0.5 to 150 miles (0.8 to 240 kilometers) wide, with lava flows carving long, winding canals up to more than 3,000 miles (5,000 kilometers) in length, longer than on any other planet.
Six mountainous regions make up about one-third percent of the Venusian surface. One mountain range, called Maxwell, is about 540 miles (870 kilometers) long and reaches up to some 7 miles (11.3 kilometers) high, making it the highest feature on the planet.
Venus also possesses a number of surface features unlike anything on the Earth. For example, Venus has coronae, or crowns — ringlike structures that range from roughly 95 to 360 miles (155 to 580 kilometers) wide. Scientists believe these formed when hot material beneath the crust rises up, warping the planet’s surface. Venus also has tesserae, or tiles — raised areas in which many ridges and valleys have formed in different directions.
An ancient name for Venus was even Lucifer. This name did not carry any fiendish connotations, however — Lucifer means "light-bringer," and when seen from Earth, Venus is brighter than any other planet or even any star in the night sky because of its highly reflective clouds and its closeness to our planet. A Venusian day is 243 Earth days and is longer than its year of 225 days. Oddly, Venus rotates from east to west. To an observer on Venus, the Sun would rise in the west and set in the east.Until just recently, Venus' dense cloud cover has prevented scientists from uncovering the geological nature of the surface. Developments in radar telescopes and radar imaging systems orbiting the planet have made it possible to see through the cloud deck to the surface below. Four of the most successful missions in revealing the Venusian surface are NASA's Pioneer Venus mission (1978), the Soviet Union's Venera 15 and 16 missions (1983-1984), and NASA's Magellan radar mapping mission (1990-1994). As these spacecraft began mapping the planet a new picture of Venus emerged.
Venus' surface is relatively young geologically speaking. It appears to have been completely resurfaced 300 to 500 million years ago. Scientists argue how and why this occurred. Venus topography consists of vast plains covered by lava flows and mountain or highland regions deformed by geological activity. Maxwell Montes in Ishtar Terra is the highest peak on Venus. The Aphrodite Terra highlands extend almost half way around the equator. Magellan images of highland regions above 2.5 kilometers (1.5 miles) are unusually bright, characteristic of moist soil. However, liquid water does not exist on the surface and cannot account for the bright highlands. One theory suggests that the bright material might be composed of metallic compounds. Studies have shown the material might be iron pyrite (also known as "fools gold"). It is unstable on the plains but would be stable in the highlands. The material could also be some type of exotic material which would give the same results but at lower concentrations. Archnoids are circular to elongated features similar to coronae. They may have been caused by molten rock seeping into surface fractures and producing systems of radiating dikes and fractures.
Earth
From the perspective we get on Earth, our planet appears to be big and sturdy with an endless ocean of air. From space, astronauts often get the impression that the Earth is small with a thin, fragile layer of atmosphere. For a space traveler, the distinguishing Earth features are the blue waters, brown and green land masses and white clouds set against a black background.
Earth is the third planet from the Sun at a distance of about 150 million kilometers (93.2 million miles). It takes 365.256 days for the Earth to travel around the Sun and 23.9345 hours for the Earth rotate a complete revolution. It has a diameter of 12,756 kilometers (7,973 miles), only a few hundred kilometers larger than that of Venus. Our atmosphere is composed of 78 percent nitrogen, 21 percent oxygen and 1 percent other constituents. Earth is the only planet in the solar system known to harbor life. Our planet's rapid spin and molten nickel-iron core give rise to an extensive magnetic field, which, along with the atmosphere, shields us from nearly all of the harmful radiation coming from the Sun and other stars. Earth's atmosphere protects us from meteors, most of which burn up before they can strike the surface. Explorer 1 discovered an intense radiation zone, now called the Van Allen radiation belts. This layer is formed from rapidly moving charged particles that are trapped by the Earth's magnetic field in a doughnut-shaped region surrounding the equator. Other findings from satellites show that our planet's magnetic field is distorted into a tear-drop shape by the solar wind. We also now know that our wispy upper atmosphere, once believed calm and uneventful, seethes with activity -- swelling by day and contracting by night. Affected by changes in solar activity, the upper atmosphere contributes to weather and climate on Earth.
Besides affecting Earth's weather, solar activity gives rise to a dramatic visual phenomenon in our atmosphere. When charged particles from the solar wind become trapped in Earth's magnetic field, they collide with air molecules above our planet's magnetic poles. These air molecules then begin to glow and are known as the auroras or the northern and southern lights.
Mars
Mars (Greek Ares) was named after the Roman god of was because it appears like the color of spoiled blood. The name of the month March is derived from Mars. Mars is the fourth planet from the Sun and is commonly referred to as the Red Planet. The rocks, soil and sky have a red or pink hue. The distinct red color was observed by stargazers throughout history. It was given its name by the Romans in honor of their god of war. Other civilizations have had similar names. The ancient Egyptians named the planet Her Descher meaning the red one. The mysterious planet has intrigued sky watchers for centuries. It shines very brightly when closest to the earth, moving quickly in front of the stars, and it has a bright, reddish color. Before space exploration, Mars was considered the best candidate for harboring extraterrestrial life. Astronomers thought they saw straight lines crisscrossing its surface. This led to the popular belief that irrigation canals on the planet had been constructed by intelligent beings. With the apparent seasonal color changes on the planet's surface, life is possible in Mars. This phenomenon led to speculation that conditions might support a bloom of Martian vegetation during the warmer months and cause plant life to become dormant during colder periods. In July of 1965, Mariner 4 transmitted 22 close-up pictures of Mars. All that was revealed was a surface containing many craters and naturally occurring channels but no evidence of artificial canals or flowing water. Finally, in July and September 1976, Viking Landers 1 and 2 touched down on the surface of Mars. The three biology experiments aboard the landers discovered unexpected and enigmatic chemical activity in the Martian soil, but provided no clear evidence for the presence of living microorganisms in the soil near the landing sites. According to mission biologists, Mars is self-sterilizing. They believe the combination of solar ultraviolet radiation that saturates the surface, the extreme dryness of the soil and the oxidizing nature of the soil chemistry prevent the formation of living organisms in the Martian soil. The question of life on Mars at some time in the distant past remains open. Ending a long 20 years hence, Mars’ Pathfinder landed successfully on Mars July 4, 1997. Other instruments found no sign of organic chemistry at either landing site, but they did provide a precise and definitive analysis of the composition of the Martian atmosphere and found previously undetected trace elements. On August 6, 1996, David McKay et al., announced the first identification of organic compounds in a Martian meteorite. Authors also suggested that there may be features observed in the rock and evidence of ancient Martian microorganisms. Whether these findings are true or not, remember, “Extraordinary claims require extraordinary evidence.” Much work remains to be done before we can be confident of this most extraordinary claim. The atmosphere of Mars is quite different from that of Earth. It is composed primarily of carbon dioxide with small amounts of other gases. The six most common components of the atmosphere are:
Carbon Dioxide (CO2): 95.32%, Nitrogen (N2): 2.7%, Argon (Ar): 1.6%, Oxygen (O2): 0.13%, Water (H2O): 0.03%, Neon (Ne): 0.00025 %.
Martian air contains only about 1/1,000 as much water as our air, but even this small amount can condense out, forming clouds that ride high in the atmosphere or swirl around the slopes of towering volcanoes. Local patches of early morning fog can form in valleys. At the Viking Lander 2 site, a thin layer of water frost covered the ground each winter.
There is evidence that in the past a denser martian atmosphere may have allowed water to flow on the planet. Physical features closely resembling shorelines, gorges, riverbeds and islands suggest that great rivers once marked the planet.
The average recorded temperature on Mars is -63° C (-81° F) with a maximum temperature of 20° C (68° F) and a minimum of -140° C (-220° F).Barometric pressure varies at each landing site on a semiannual basis. Carbon dioxide, the major constituent of the atmosphere, freezes out to form an immense polar cap, alternately at each pole. The carbon dioxide forms a great cover of snow and then evaporates again with the coming of spring in each hemisphere. When the southern cap was largest, the mean daily pressure observed by Viking Lander 1 was as low as 6.8 millibars; at other times of the year it was as high as 9.0 millibars. The pressures at the Viking Lander 2 site were 7.3 and 10.8 millibars. In comparison, the average pressure of the Earth is 1000 millibars.
Jupiter
Jupiter is the fifth planet from the Sun and is the largest planet in the solar system. If Jupiter were hollow, more than one thousand Earths could fit inside. It also contains two and a half times the mass of all the other planets combined. It has a mass of 1.9 x 1027 kg and is 142,800 kilometers (88,736 miles) across the equator. Jupiter possesses 62 known satellites. The four largest are Callisto, Europa, Ganymede and Io, and were named after Galileo Galilei who observed them as long ago as 1610. The German astronomer Simon Marius claimed to have seen the moons around the same time, but he did not publish his observations and so Galileo is given the credit for their discovery.
Jupiter has a very faint ring system, but is totally invisible from the Earth. (The rings were discovered in 1979 by Voyager 1.) The atmosphere is very deep, perhaps comprising the whole planet, and is somewhat like the Sun. It is composed mainly of hydrogen and helium, with small amounts of methane, ammonia, water vapor and other compounds. At great depths within Jupiter, the pressure is so great that the hydrogen atoms are broken up and the electrons are freed so that the resulting atoms consist of bare protons. This produces a state in which the hydrogen becomes metallic. The most prominent feature of the visible cloud surface of Jupiter is a large reddish formation as “The Great Red Spot” (GRS). It is a vast whirlpool on the surface of Jupiter about twice the earth’s diameter, draws material from below as it rotates every six days. The color, which sometimes fades away for several years, may be caused by sunlight reacting with chemicals in the clouds to release red phosphorus. Auroral emissions, same as to Earth’s northern lights, were observed in the polar regions of Jupiter. They appear to be related to material from Io that spirals along magnetic field lines to fall into Jupiter’s atmosphere. Cloud-top lightning bolts, similar to super bolts in Earth’s high atmosphere, were also observed.
Saturn
Saturn is the sixth planet from the Sun and is the second largest in the solar system. Saturn is the god of agriculture. He is the Greek god, Cronus, the son of Uranus and Gaia and the father of Zeus. It is the root of the English word Saturday. Much of what is known about the planet is due to the Voyager explorations in 1980-81. Saturn is visibly flattened at the poles, a result of the very fast rotation of the planet on its axis. Its day is 10 hours, 39 minutes long, and it takes 29.5 Earth years to revolve about the Sun. The atmosphere is primarily composed of hydrogen with small amounts of helium and methane. Saturn is the only planet less dense than water. In the unlikely event that a large enough ocean could be found, Saturn would float in it. Saturn's hazy yellow hue is marked by broad atmospheric banding similar to, but fainter than, that found on Jupiter. The wind blows at high speeds on Saturn. Near the equator, it reaches velocities of 500 meters a second. The wind blows mostly in an easterly direction. The strongest winds are found near the equator and velocity falls off uniformly at higher latitudes. At latitudes greater than 35 degrees, winds alternate east and west as latitude increases.
Saturn's ring system makes the planet one of the most beautiful objects in the solar system. The rings are split into a number of different parts, which include the bright A and B rings and a fainter C ring. The ring system has various gaps. The most notable gap is the Cassini Division, which separates the A and B rings. Giovanni Cassini discovered this division in 1675. The Encke Division, which splits the A Ring, is named after Johann Encke, who discovered it in 1837. Space probes have shown that the main rings are really made up of a large number of narrow ringlets. It is thought that the rings may have been formed from larger moons that were shattered by impacts of comets and meteoroids. The ring composition is not known for certain, but the rings do show a significant amount of water. They may be composed of icebergs and/or snowballs from a few centimeters to a few meters in size. Much of the elaborate structure of some of the rings is due to the gravitational effects of nearby satellites. This phenomenon is demonstrated by the relationship between the F-ring and two small moons that shepherd the ring material .Radial, spoke-like features in the broad B-ring were also found by the Voyagers. The features are believed to be composed of fine, dust-size particles. The spokes were observed to form and dissipate in the time-lapse images taken by the Voyagers. Saturn has 30 named satellites and more continue to be discovered.
The wind blows at very high speeds and mostly in an easterly direction. It reaches velocities of 500 meters a second near the equator. The strongest winds are usually found near the equator and velocity falls off uniformly at higher latitudes. At latitudes greater than 35 degrees, winds alternate east and west as latitude increases. Pioneer II first visited Saturn in 1979 and later by Voyager 1 and 2. Cassini, now on its way will arrive in 2004.
Uranus
Uranus s named after the ancient Greek deity of the sky Uranus the father of Cronus.
It was discovered by William Herschel in March 13, 1781, expanding the known boundaries of the Solar System for the first time in modern history. Uranus was also the first planet discovered with a telescope. Uranus is distinguished by the fact that it is tipped on its side. Its unusual position is thought to be the result of a collision with a planet-sized body early in the solar system's history. Voyager 2 found that one of the most striking influences of this sideways position is its effect on the tail of the magnetic field, which is itself, tilted 60 degrees from the planet's axis of rotation. The magnetotail was shown to be twisted by the planet's rotation into a long corkscrew shape behind the planet. The magnetic field source is unknown; the electrically conductive, super-pressurized ocean of water and ammonia once thought to lie between the core and the atmosphere now appears to be nonexistent. The magnetic fields of Earth and other planets are believed to arise from electrical currents produced in their molten cores.
Pluto
Although Pluto was discovered in 1930, limited information on the distant object delayed a realistic understanding of its characteristics. Pluto is usually farther from the Sun than any of the eight planets; however, due to the eccentricity of its orbit, it is closer than Neptune for 20 years out of its 249 year orbit. Pluto crossed Neptune's orbit January 21, 1979, made its closest approach September 5, 1989, and remained within the orbit of Neptune until February 11, 1999. This will not occur again until September 2226. In roman mythology, Pluto (Greek Hades) is the god of the underworld. He is the brother of Zeus. Pluto is far from the sun. In 1978, careful Earth-based observation indicated that the image of Pluto had a slight bulge. This was interpreted as evidence for a previously unknown moon, named Charon. The adjacent image (Ref) shows subsequent higher quality ground-based observation, and early Hubble Space Telescope observations that show conclusively the existence of Charon. The orbit is show in the inset. Pluto’s average density lies between 1.8 and 2.1 grams per cubic centimeter. It is concluded that Pluto is 50% to 75% rock mixed with ices. Charon's density is 1.2 to 1.3 g/cm3, indicating it contains little rock. The differences in density tell us that Pluto and Charon formed independently, although Charon's numbers derived from HST data are still being challenged by ground based observations. Pluto and Charon's origin remains in the realm of theory. The icy surface is 98% nitrogen (N2). Methane (CH4) and traces of carbon monoxide (CO) are also present. The solid methane indicates that Pluto is colder than 70 Kelvin. Pluto's temperature varies widely during the course of its orbit since Pluto can be as close to the sun as 30 AU and as far away as 50 AU. There is a thin atmosphere that freezes and falls to the surface as the planet moves away from the Sun. The path toward its discovery is credited to Percival Lowell who founded the Lowell Observatory in Flagstaff, Arizona and funded three separate searches for "Planet X." Lowell made numerous unsuccessful calculations to find it, believing it could be detected from the effect it would have on Neptune's orbit. Dr. Vesto Slipher, the observatory director, hired Clyde Tombaugh for the third search and Clyde took sets of photographs of the plane of the solar system (ecliptic) one to two weeks apart and looked for anything that shifted against the backdrop of stars. This systematic approach was successful and Pluto was discovered by this young (born 4 Feb 1906) 24 year old Kansas lab assistant on February 18, 1930. Pluto is actually too small to be the "Planet X" Percival Lowell had hoped to find. Pluto's was a serendipitous discovery.
The Minor Planets Minor planet is a term used to refer to a celestial object – that is not a planet or comet – which orbits the Sun. Found in 1801, Ceres, also known as a dwarf planet, was the first minor planet discovered. The term minor planet has been in use since the 1800′s. Planetoids, asteroids, and minor planets have all been used interchangeably, but the situation became even more confusing when the International Astronomical Union (IAU) committee reclassified minor planets and comets into the new categories of dwarf planets and small solar system bodies. At the same time, the IAU created a new definition of what a planet is, and Pluto was reclassified as a dwarf planet. Hydrostatic equilibrium – the ability to maintain a roughly spherical shape – is what separates dwarf planets from the more irregularly shaped small solar system bodies. The names become even more confusing because the IAU still recognizes the use of the term minor planets.
Comets
Comets are small, fragile, irregularly shaped bodies composed of a mixture of non-volatile grains and frozen gases. They have highly elliptical orbits that bring them very close to the Sun and swing them deeply into space, often beyond the orbit of Pluto. Comet structures are diverse and very dynamic, but they all develop a surrounding cloud of diffuse material, called a coma, that usually grows in size and brightness as the comet approaches the Sun. Usually a small, bright nucleus (less than 10 km in diameter) is visible in the middle of the coma. The coma and the nucleus together constitute the head of the comet.
As comets approach the Sun they develop enormous tails of luminous material that extend for millions of kilometers from the head, away from the Sun. When far from the Sun, the nucleus is very cold and its material is frozen solid within the nucleus. In this state comets are sometimes referred to as a "dirty iceberg" or "dirty snowball," since over half of their material is ice. When a comet approaches within a few AU of the Sun, the surface of the nucleus begins to warm, and volatiles evaporate. The evaporated molecules boil off and carry small solid particles with them, forming the comet's coma of gas and dust.
As the comet absorbs ultraviolet light, chemical processes release hydrogen, which escapes the comet's gravity, and forms a hydrogen envelope. This envelope cannot be seen from Earth because its light is absorbed by our atmosphere, but it has been detected by spacecraft.
The Sun's radiation pressure and solar wind accelerate materials away from the comet's head at differing velocities according to the size and mass of the materials. Thus, relatively massive dust tails are accelerated slowly and tend to be curved. The ion tail is much less massive, and is accelerated so greatly that it appears as a nearly straight line extending away from the comet opposite the Sun. The following view of Comet West shows two distinct tails. The thin blue plasma tail is made up of gases and the broad white tail is made up of microscopic dust particles.
Newly Discovered Comets
Comet Kouhoutek is considered to be part of the Kuiper belt variety of comets. These are comets made from frozen volatile gasses such as methane. The comets composition is largely ice and reflects the difference of the Kuiper belt compared to the Inner Solar system asteroid belt and its counterpart the Oort cloud.
Comet 1993a Mueller- taken on October 6, 1993 with a 288mm f/5.2 Schmidt-Cassegrain telescope. The comet has a coma diameter of 3' and a fan-shaped tail, up to 7' long.
Comet West -John Loborde took the image on March 9, 1976, this shows two distinct tails. The thin blue plasma tail is made up of gases and the broad white tail is made up of microscopic dust particles
Hale-Bopp is a long-period comet that was discovered in 1995 and that reached perihelion in Spring, 1997. Because it was much brighter than comets normally is when it was first discovered outside the orbit of Jupiter, there was considerable speculation that in the Spring of 1997 Hale-Bopp would be a spectacular sight.
What are meteors, meteoroids, and meteorites? The term meteor comes from the Greek meteoron, meaning phenomenon in the sky. It is used to describe the streak of light produced as matter in the solar system falls into Earth's atmosphere creating temporary incandescence resulting from atmospheric friction. This typically occurs at heights of 80 to 110 kilometers (50 to 68 miles) above Earth's surface. The term is also used loosely with the word meteroid referring to the particle itself without relation to the phenomena it produces when entering the Earth's atmosphere. A meteoroid is matter revolving around the sun or any object in interplanetary space that is too small to be called an asteroid or a comet. Even smaller particles are called micrometeoroids or cosmic dust grains, which includes any interstellar material that should happen to enter our solar system. A meteorite is a meteoroid that reaches the surface of the Earth without being completely vaporized. Meteorites have proven difficult to classify, but the three broadest groupings are stony, stony iron, and iron. The most common meteorites are chondrites, which are stony meteorites. Radiometric dating of chondrites has placed them at the age of 4.55 billion years, which is the approximate age of the solar system. They are considered pristine samples of early solar system matter, although in many cases their properties have been modified by thermal metamorphism or icy alteration. Some meteoriticists have suggested that the different properties found in various chondrites suggest the location in which they were formed. Enstatite chondrites contain the most refractory elements and are believed to have formed in the inner solar system. Ordinary chondrites, being the most common type containing both volatile and oxidized elements, are thought to have formed in the inner asteroid belt. Carbonaceous chondrites, which have the highest proportions of volatile elements and are the most oxidized, are thought to have originated in even greater solar distances. Each of these classes can be further subdivided into smaller groups with distinct properties. Other meteorite types which have been geologically processed are achondrites, irons and pallasites. Achondrites are also stony meteorites, but they are considered differentiated or reprocessed matter. They are formed by melting and recrystallization on or within meteorite parent bodies; as a result, achondrites have distinct textures and mineralogies indicative of igneous processes. Pallasites are stony iron meteorites composed of olivine enclosed in metal. Iron meteorites are classified into thirteen major groups and consist primarily of iron-nickel alloys with minor amounts of carbon, sulfur, and phosphorus. These meteorites formed when molten metal segregated from less dense silicate material and cooled, showing another type of melting behavior within meteorite parent bodies. Thus, meteorites contain evidence of changes that occurred on the parent bodies from which they were removed or broken off, presumably by impacts, to be placed in the first of many revolutions.
Particles found in highly correlated orbits are called stream components and those found in random orbits are called sporadic components. It is thought that most meteor streams are formed by the decay of a comet nucleus and consequently are spread around the original orbit of the comet. When Earth's orbit intersects a meteor stream, the meteor rate is increased and a meteor shower results. A meteor shower typically will be active for several days. A particularly intense meteor shower is called a meteor storm. Sporadic meteors are believed to have had a gradual loss of orbital coherence with a meteor shower due to collisions and radiative effects, further enhanced by gravitational influences. There is still some debate concerning sporadic meteors and their relationship with showers.

Examples of Meteorites
Chondrite Meteorite -This meteorite fell in 1970 in Cherokee County, Oklahoma. It is subclassed as H Group or an olivine-bronzite chondrite. Meteorites are bits of rock that are captured by a planet's gravity and pulled to the surface. This meteorite is of a type named chondrite and is thought to have formed at the same time as the planets in the solar nebula, about 4.55 billion years ago.
Achondrite Meteorite - Discovered at Norton County, Kansas, this type of meteorite is known as an achondrite. It has a basaltic composition and was probably formed when an asteroid melted about 4.5 billion years ago. The asteroid broke up some time later and this small piece of the asteroid was captured by Earth's gravity and fell to the ground.
Iron meteorite - found at Victoria Land, Antarctica. This type of meteorite gets its name because it is mostly made of the elements iron and nickel. This sample is probably a small piece from the core of a large asteroid that broke apart.
Vesta Meteorite - This meteorite is assumed to be a sample of the crust of the asteroid Vesta, which is only the third solar system object beyond Earth where scientists have a laboratory sample (the other extraterrestrial samples are from Mars and the Moon). The meteorite is unique because it is made almost entirely of the mineral pyroxene, common in lava flows. The meteorite's mineral grain structure also indicates it was once molten, and its oxygen isotopes are unlike oxygen isotopes found for all other rocks of the Earth and Moon. The meteorite's chemical identity points to the asteroid Vesta because it has the same unique spectral signature of the mineral pyroxene.