Superclusters and Clusters of Galaxies
Our Milky Way Galaxy
Our Solar System
The Sun: Our Daytime Star
Mercury: The Closest to The Sun
Venus: The Hot Planet
Earth: Our Unique Planet
Mars: The Red Planet
Jupiter: King of the Planets
Saturn: The Ringed Giant
Uranus: The Tilted Giant
Neptune: The Last Giant
Pluto: The last Planet
Probably, you are reading this book in a room with four walls in your home, and you feel that you are not moving. Your place exists on a tiny planet called Earth. The Earth is one of nine planets orbiting around the Sun. The sun is a typical star of intermediate size and luminosity. The Sun is one of the 100 billion stars traveling together that makes up a spiral galaxy called the Milky Way. Galaxies are generally not isolated in space but are often members of small or moderate-sized groups, which in turn form large clusters of galaxies. The Milky Way is one of a small group of about 20 galaxies that astronomers call the Local Group. The spiral galaxy Andromeda is also a member of the Local Group. The Large, Small, and Mini Magellanic Clouds are nearby satellite galaxies, but each is small and faint, with about 100 million stars. Further work revealed a concentration of clusters of galaxies that are called superclusters. Ours is called the Local or Virgo supercluster. There seem to be no observations to lead astronomers to assume any structure larger than superclusters. This grouping from the solar system to the supercluster systems is based upon the observation that each system travels in concert or as a group.
Figure 2.2 - Big Bang
Now, let us try to see how much your actual speed in the universe is. According to classical mechanics, persons in a uniformly moving train behave as they would be if the train were at rest. With the windows covered, it would be impossible to tell whether the train was moving. To an observer on the ground outside the train, the persons inside the train would seem to have the same speed as that of the train. However, if a person inside the train started running in the same direction as the train, the outside observer would see that person moving faster than the train. As an example, if the train is moving with a speed of 100 Km/hr, and the person is moving with a speed of 10 km/hr, then the observer would see that person moving with 110 km/hr (100+10=110). Therefore, it can be concluded that the actual speed of the person inside a train can be found by adding his speed in the train to the speed of the train. In the same way, let’s find your speed by adding the following:
Speed at the equator of Earth due to its rotation about its axis = 0.44 km/sec
Orbital speed of Earth about the Sun = 29.8 Km/sec
Speed of the Sun relative to the center of the Milky Way = 250 Km/sec
From that, we conclude that your speed relative to the center of the Milky Way is 280 km/sec or 174 mi/sec! That is an incredible speed. And, you don’t even know it or feel it!
Superclusters and Clusters of Galaxies
Figure 2.4 - Virgo Cluster is an extremely dense cluster of galaxies
An unknown process created the density differences in the universe. They become marked only after some two billion years. Where density was greater, Protogalaxies, the first celestial bodies, were formed. The process of expansion of the universe had made it possible for galaxies to form in considerable numbers. The resulting galaxies would have an immense range of sizes - from one hundred times the size of our galaxy down to a hundred-thousandth of it. According the existing model of the Big Bang, the largest structure appeared first. Protogalaxies are assumed to be formed first after 2 billion years from the Big Bang. Protogalaxies shrank to become galaxies.
Overall, the distribution of clusters and superclusters in the universe is not uniform. Instead, superclusters of tens of thousands of galaxies are arranged in long, stringy filaments, around large voids. The Great Wall, a galactic filament discovered in 1989, stretches across more than 500 million light-years of space. Clusters of Galaxies include from two to thousands galaxies. Superclusters or clusters of Galaxies are detected by groups of galaxies that travel together.
A Galaxy is a massive group of hundreds of millions of stars, all gravitationally interacting, and orbiting about a common center. All the stars visible to the unaided eye from Earth belong to the Earth's galaxy, the Milky Way. The sun with its associated planets is just one star in this galaxy.
The most distant galaxies known, near the edge of the observable universe, are faint blue objects called "blue fuzzies" because of their appearance on photographic plates. The images were obtained by aiming a telescope at apparently blank regions of the sky and using a solid-state charge-coupled detector to gather the very faint light, then processing the images by means of a computer. The galaxies, moving away from Earth at about 88 percent of the speed of light, may have been formed about 2 billion years after the origin of the universe.
A Muslim Persian astronomer, Al-Sufi (903-36), is credited with first describing the spiral galaxy seen in the constellation Andromeda. By the middle of the 18th century, only three galaxies had been identified. In 1780, the French astronomer Charles Messier (1730-1817) published a list that included 32 galaxies. These galaxies are now identified by their Messier (M) numbers; the Andromeda galaxy, for example, is known among astronomers as M31.
In the early years of the twentieth century, it was already known from the work of the English astronomer William Huggins that nebulae (Latin for "clouds") were of two kinds: hazy patches of gases with spiral or elliptical shapes. Since 1900 galaxies have been discovered in large numbers by photographic searches. Galaxies at enormous distances from Earth appear so tiny on a photograph that they can hardly be distinguished from stars. The largest known galaxy has about 13 times as many stars as the Milky Way.
When viewed or photographed with a large telescope, only the nearest galaxies exhibit individual stars. For most galaxies, only the combined light of all the stars is detected. Galaxies exhibit a variety of forms. Some have an overall globular shape, with a bright nucleus surrounded by a luminous structureless disk. Such galaxies, called ellipticals, contain a population of old stars, usually with little apparent gas or dust, and few newly formed stars. Elliptical galaxies come in a large range of sizes, from giant to dwarf. In contrast, spiral galaxies are flattened disk systems containing not only some old stars but also large populations of young stars, much gas and dust, and molecular clouds that are the birthplace of stars. Often the regions containing bright young stars and gas clouds are arranged in long spiral arms that can be observed to wind around the galaxy. Generally a halo of faint older stars surrounds the disk; a smaller nuclear bulge often exists, emitting two jets of energetic matter in opposite directions.
Other disklike galaxies, with no overall spiral form, are classified as irregulars. These galaxies also have large amounts of gas, dust, and young stars, but no arrangement of a spiral form. They are usually located near larger galaxies, and their appearance is probably the result of a tidal encounter with the more massive galaxy. Some extremely peculiar galaxies are located in close groups of two or three, and their tidal interactions have caused distortions of spiral arms, producing warped disks and long streamer tails.
Our Milky Way Galaxy
Earth is a planet orbiting one of 100 billion stars that constitute a large spiral galaxy. William Herschel, a pioneer of the study of the skies, first introduced the concept of these islands of stars in the 1780s using large telescopes. On the basis of counting the stars visible in different directions, he concluded that the sun lay at the center of a star system that was flat and elongated in shape. Astronomers have known this system as the Galaxy - from "galaxias" -, which is what the Milky Way was called in ancient Greece.
Figure 2.5 - Milky Way Galaxy. The flattened disk of the Milky Way
Galaxy shows the galaxy’s spiral arms and the sun’s modest place in the
The observations of radio telescopes have confirmed that this system is fully disklike with four spiral arms. They are the Centaurus, Sagittarius, Perseus, and the Orion arms. The Sun lies in the Orion arm of the Galaxy.
At the center of the Galaxy, as of all spirals, there is a concentrated core of stars. These form a bulge at least 20,000 light-years in diameter and some 3,000 light-years thick. The Sun is 30,000 light-years from the center of the Galaxy. The Sun, about halfway out towards the edge moves at a speed of 250 Km/sec and takes no less than 200 million years to complete each rotation, a period sometimes called the cosmic year. Since its formation, the Sun has managed only twenty round trips. If we assume that the cosmic year is divided into 365 cosmic days, and the cosmic day is divided into 24 cosmic hours, then humans were created on Earth, perhaps, only 2 cosmic hours ago!
Our Solar System
The first astronomers, long ago, noticed that there were five special "stars" that gradually moved through the sky. They became known as the "wanderers" or planets. Planets shine with a steady light, but real stars often twinkle. All the planets visible in the night sky are members of the Sun’s family, or the solar system. The five planets that can be seen without a telescope are Mercury, Venus, Mars, Jupiter, and Saturn. After the invention of the telescope, astronomers found three more distant planets. Uranus was discovered in 1781, Neptune in 1846, and Pluto in 1930. All nine planets travel in orbits around the sun.
Figure 2.6 - Sun and planets of the solar system
Johannes Kepler studied the motion of the planets. In 1609 he discovered that the orbits of the planets are slightly stretched circles, called ellipses. A circle has one focal point called the center, while an ellipse has two focal points. For each planetary orbit, the sun is at one of the focal points. Thus the distances of the planets from the sun change by small amounts as they travel in their orbits. Kepler discovered how the planets moved, but it was Newton who realized that the force of gravity holds the planets in their orbits. If the Sun’s gravity did not constantly keep pulling at the planets, they would fly away into the depths of space.
Now we know that our solar system consists of the sun; the nine planets and their satellites; the asteroids, comets, and meteoroids; and interplanetary dust and gas. The planets are commonly divided into two groups: the inner planets (Mercury, Venus, Earth, and Mars) and the outer planets (Jupiter, Saturn, Uranus, Neptune, and Pluto). The inner planets are small and are composed primarily of rock and iron. The outer planets (except Pluto) are much larger and consist mainly of hydrogen, and helium. The sun consists of 73.5% hydrogen, and 25% helium.
The dimensions of this system are specified in terms of the mean distance from the Earth to the sun, called the astronomical unit (AU). One AU is 150 million km (about 93 million mi). The most distant known planet, Pluto, has an orbit at 39.44 AU from the sun. Comets achieve the greatest distance from the sun; they have highly eccentric orbits ranging out to 50,000 AU or more. This solar system is the only planetary system known to exist, although in the 1980s a number of relatively nearby stars were found to be encircled by swarms of orbiting material of indeterminate size or to be accompanied by suspected brown dwarfs. Many astronomers think it is likely that solar systems of some sort are numerous throughout the universe.
If one could look down on the solar system from far above the North Pole of Earth, the planets would appear to move around the sun in a counterclockwise direction. All of the planets except Venus and Uranus rotate on their axes in this same direction. The entire system is remarkably flat—only Mercury and Pluto have obviously inclined orbits. Pluto's orbit is so elliptical that it is sometimes closer to the sun than Neptune is.
Current theories connect the formation of the solar system with the formation of the sun itself, about 4.7 billion years ago. The fragmentation and gravitational collapse of an interstellar cloud of gas and dust, triggered perhaps by nearby supernova explosions, may have led to the formation of a primordial solar nebula. The sun would then form in the densest, central region. At larger distances from the center of the solar nebula, gases condense into solids such as are found today from Jupiter outward. The association of planet formation with star formation suggests that billions of other stars in our galaxy may also have planets. The high frequency of binary and multiple stars, as well as the large satellite systems around Jupiter and Saturn, attest to the tendency of collapsing gas clouds to fragment into multibody systems.
The satellite systems mimic the behavior of their parent planets, but many more exceptions are found. Jupiter, Saturn, and Neptune each have one or more satellites that move around the planets in retrograde orbits (clockwise instead of counterclockwise), and several satellite orbits are highly elliptical. Jupiter, moreover, has trapped two clusters of asteroids (the so-called Trojan asteroids).
Asteroids are small rocky bodies that move in orbits primarily between the orbits of Mars and Jupiter. Numbering in the thousands, asteroids range in size from Ceres, which has a diameter of 1000 km (620 mi), to microscopic grains. Some asteroids are perturbed into eccentric orbits that can bring them closer to the sun. If the orbits of such bodies intersect that of the Earth, they are called meteoroids. When they appear in the night sky as streaks of light, they are known as meteors, and recovered fragments are termed meteorites. Laboratory studies of meteorites have revealed much information about primitive conditions in our solar system. The surfaces of Mercury, Mars, and several satellites of the planets (including Earth's moon) show the effects of an intense bombardment by asteroidal objects early in the history of the solar system. On Earth this record has eroded away, except for a few recent impact craters.
Some meteors and interplanetary dust may also come from comets, which are basically collections of dust and frozen gases about 5 to 10 km (about 3 to 6 mi) in diameter. Comets orbit the sun at distances so great that they can be perturbed by stars into orbits that bring them into the inner solar system. As comets approach the sun, they release their dust and gases to form a spectacular coma and tail. Under the influence of Jupiter's strong gravitational field, comets can sometimes adopt much smaller orbits. The most famous of these is Halley's comet, which returns to the inner solar system at 75-year periods. Its most recent return was in 1986.
The Sun: Our Daytime star
Figure 2.7 - Sun-Planets sizes
The Sun lies on the spiral Orion arm of our Milky Way galaxy, which is marked by dark and bright nebulae from which new stars are constantly born. The sun is a typical star of intermediate size and luminosity. Yet if the Sun is indistinguishable among its celestial neighbors, it is immense by terrestrial standards. It diameter is 1,392,000 Km, more than 109 times the equatorial diameter of the Earth. Its volume is 1,303,600 times that of our planet. In light-years, the Sun’s average distance to the Earth is 8.3 light-minutes (which means that light from the Sun reaches the Earth in 8.3 minutes), whereas the distance to the nearest star, Proxima Centauri, is 4.28 light-years, 250,000 times greater than the distance from the Sun to the Earth.
Sunlight and other radiation are produced by the conversion of hydrogen into helium in the sun's hot, dense interior. The temperature of the Sun’s core is about 15 million degrees in the Kelvin scale. Although this nuclear fusion is destroying 600 million metric tons of hydrogen each second, the sun is so massive (2 E+30 kg, or 4.4 E+30 lb.) that it can continue to shine at its present brightness for 6 billion years. This stability has allowed life to develop and survive on Earth.
The table below shows the dimensions and properties of the solar planets:
|Distance from Sun (million km)||57.8||108||149.6||228||778||1430||2871||4500||5900|
|Diameter (1000 Km)||4.88||12.1||12.8||6.79||143||120||51.8||49.5||2.3|
|Orbital speed km/sec||47.9||35.0||29.8||24.1||13.1||9.64||6.81||5.43||4.74|
|Inclination of orbit in degrees *||7.00||3.39||1.00||1.85||1.30||2.49||0.77||1.77||17.2|
* (Earth = 1), ** (Water = 1)
Mercury: The Closest to The Sun
Figure 2.8 - Mercury
Mercury can be observed either before dawn or after sunset as a bright, silvery, starlike object. Since its orbit lies within the Earth’s, Mercury presents phases similar to our moon. Mercury is seen only when it is furthest away on the opposite side from the Sun. Mercury is never further from the Sun than 69.7 million km, and with its elliptical orbit, it gets as close as 45.9 million km.
Because it is close to the sun, it gets 4.7 times more heat, light and other radiation per unit area than the Earth. Its surface temperature can reach 467 deg. C. At night the temperature plunges down to –200 deg. C because there is no blanket of atmosphere to trap the heat. Mercury resembles the Earth in its internal structure. It is surprisingly dense because it has an unusually large iron core, twice as much as the Earth’s. With only a transient atmosphere, Mercury has a surface that is covered with craters caused by heavy bombardment by asteroids early in its history. The spacecraft Mariner 10 detected a very weak magnetism around Mercury.
Venus: The Hot Planet
Figure 2.9 - Venus
Venus is one of the easiest planets to pick out in the sky, and is sometimes called the evening star. If you observe Venus with even a small telescope, you will see that it has phases like the Moon. When Galileo discovered this behavior he realized that Venus must orbit the Sun at a closer distance than the Earth. Venus is the twin-planet of the Earth. It is close in size, weight, and internal composition of the Earth. Like the Earth, it has similarly sized, dense nickel-iron core and a rocky mantle. However, their atmospheres are very different. Venus has a carbon dioxide atmosphere 90 times thicker than that of Earth, causing an efficient greenhouse effect by which its atmosphere is heated. The resulting surface temperature is the hottest of any planet - about 477 deg. C (about 890 deg. F). When the Earth first formed, it may have been rather like Venus is today. Life on Earth has broken down the dense carbon dioxide atmosphere that once existed here as well.
Earth: Our Unique Planet
Figure 2.10 - Earth
Earth is the third in distance from the sun and the fifth largest of the planets in diameter. It is the only planet known to support life, although some of the other planets have atmospheres and contain water. Earth prospers with life, sustained by very complex systems that provide light, air, heat, water, and food all in exquisite balance. It shows evidence of having been created specially to accommodate living things. The mean distance of the Earth from the sun is 149,503,000 km (92,897,000 mi.). This is exactly the right distance. If the Earth were much closer to the Sun or farther away from it, the temperatures would be too hot or too cold for life.
Figure 2.11 - Moon
The Earth and its satellite, the moon, move together in an elliptical orbit about the sun. The eccentricity of the orbit is slight, so that the orbit is virtually a circle. The approximate length of the Earth's orbit is 938,900,000 km (583,400,000 mi), and the Earth travels along it at a velocity of about 106,000 km/hr (about 66,000 mph). This speed is precise enough to offset the gravitational pull of the Sun and keep the Earth at the proper distance. If the speed were decreased, the Earth would be pulled towards the sun, and could become a wasteland like Mercury. If the Earth’s orbital speed were increased, it would move farther away from the sun, and could become an icy waste like Pluto. The Earth rotates on its axis once every 23 hr 56 min. 4.1 sec (based on the solar year). A point on the equator therefore rotates at a rate of a little more than 1600 km/hr (about 1000 mph). This provides regular periods of light and darkness. But what if the Earth rotated on its axis once every year, it would mean that the same side of the Earth would be facing the sun all year long. That side would become like a furnace desert, while the side away from the Sun becomes an icy wasteland. Few, if any, living creatures could survive in those extreme temperatures.
Man has been able to study the surface of his own planet for as long as the Earth has been inhabited. Yet, it is strange to think that before orbiting the spacecraft had actually returned colored pictures of the Earth, no one had predicted accurately what it would look like from space. Now we know the Earth as a beautiful blue and white planet. From beneath the spiraling patterns of brilliant white clouds, the familiar shapes of the continents loom into view.
Many factors make the Earth unique in the solar system. It is the only planet with substantial amounts of water. The oceans cover more than three quarters of the surface. This huge amount of water coupled with the presence of oxygen in the atmosphere is a powerful force of erosion. Shifting weather behavior and long term changes in climate rapidly wear down the continental rocks. Glacier, wind and rain smooth mountains. Mighty rivers engrave channels through the rocks and the lowland plains, carrying sand from one place and laying it down in another.
Volcanoes and earthquakes are mechanisms that permit the Earth to release pressure that builds up internally as the rocks beneath our feet slowly slide. Earthquakes are sudden, unpredictable, and fatal in many parts of the globe, but they teach geologists about the inner structure of the Earth. Vibrations spreading out from an earthquake are measured all over the Earth. The manner in which these vibrations travel shows that the Earth is made of five parts:
1. The atmosphere is the gaseous envelope that surrounds the solid body of the planet. Although it has a thickness of more than 1100 km (more than 700 mi.), about half its mass is concentrated in the lower 5.6 km (3.5 mi.). The atmosphere contains the right proportions of the gases that are essential for life. Some of those gases, by themselves, are deadly, but because air contains safe proportions of these gases, we can breathe them without harm. One such gas is oxygen, making up 21 % of the air we breathe. Without it all life would vanish in minutes. But too much oxygen would endanger our existence. Pure Oxygen becomes toxic if breathed too long. In addition, the more oxygen there is, the more easily things burn, also, combustible materials would become highly flammable, and there would be fires everywhere. Thank God, oxygen is diluted with other gases, mainly nitrogen, which makes up 78 % of the atmosphere. But nitrogen is much more than just a dilutant. During thunderstorms, millions of lightening bolts occur on Earth every day. This lightening causes the oxygen to combine with nitrogen. The compounds produced are carried to the Earth, by rain, and plants make use of them as fertilizer. Carbon dioxide makes up less than 1% of the atmosphere. This percentage is just the right amount for the plants to survive, giving off oxygen in return. Humans and animals breathe in oxygen and exhale carbon dioxide. An increase in the percentage of the carbon dioxide is harmful to humans and animals. A decreasing percentage could not support plant life. What a marvelous, precise, self-sustaining cycle has been arranged for plant, animal, and human life! The atmosphere does more than sustain life. It serves as a protective shell too. About 15 miles above ground, a thin layer of ozone gas filters out harmful radiation from the Sun. Without this layer, such radiation could destroy life on Earth. Also the atmosphere shields the Earth from bombardment by meteors. Most meteors never reach the ground because they burn up in their descent through the atmosphere, appearing to us as falling stars. Otherwise, millions of meteors would strike all parts of the Earth, resulting in extensive damage in life and property.
2. The hydrosphere is the layer of water that, in the form of the oceans, covers approximately 70.8% of the surface of the Earth. The hydrosphere consists mainly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters. The average depth of the oceans is 3794 meter (12,447 ft), more than five times the average height of the continents. The mass of the oceans is approximately 1,350,000,000,000,000,000 (1.35 E+18) metric tons, or about 1/4400 of the total mass of the Earth.
3. The lithosphere consisting mainly of the cold, rigid, rocky crust of the Earth extends to depths of 100 km (60 mi.). The rocks of the lithosphere have an average density of 2.7 and are almost entirely made up of 11 elements, which together account for about 99.5% of its mass. The most abundant is oxygen (about 46.60% of the total), followed by silicon (about 27.72%), aluminum (8.13%), iron (5.0%), calcium (3.63%), sodium (2.83%), potassium (2.59%), magnesium (2.09%) and titanium, hydrogen, and phosphorus (totaling less than 1%). In addition, 11 other elements are present in trace amounts of from 0.1 to 0.02%. These elements, in order of abundance, are carbon, manganese, sulfur, barium, chlorine, chromium, fluorine, zirconium, nickel, strontium, and vanadium. The elements are present in the lithosphere almost entirely in the form of compounds rather than in their free state. These compounds exist almost entirely in the crystalline state, so they are, by definition, minerals. The lithosphere comprises two shells—the crust and upper mantle—that are divided into a dozen or so rigid tectonic plates. The crust itself is divided in two. The upper crust, of which the continents consist, is made up of rocks whose average chemical composition is similar to that of granite. The lower crust, which forms the floors of the ocean basins, is made of darker, heavier rocks.
4. The mantle: The mantle and core are the heavy interior of the Earth, making up most of the Earth's mass. The dense, heavy interior of the Earth is divided into a thick shell, the mantle, surrounding an innermost sphere, the core. The mantle extends from the base of the crust to a depth of about 2900 km (1800 mi.). Except for the zone known as the asthenosphere, it is solid, and its density, increasing with depth. The upper mantle is composed of iron and magnesium silicates, as typified by the mineral olivine. The lower part may consist of a mixture of oxides of magnesium, silicon, and iron.
5. The core: Seismological research has shown that the core has an outer shell about 2225 Km. (1380 mi.) thick. This shell is probably rigid, and studies show that its outer surface has depressions and peaks, the latter forming where warm material rises. In contrast, the inner core, which has a radius of about 1275 Km. (795 mi.), is solid. Both core layers are thought to consist largely of iron, with a small percentage of nickel and other elements. Temperatures in the inner core may be as high as 6650 deg. C (12,000 deg. F).
Mars: The Red Planet
Figure 2.12 - Mars
Viewed through a telescope, Mars looks like a rusty-red disk. Its surface has various light and dark parts, as well as white ice caps at the north and south poles. Like our Earth, Mars experiences a cycle of seasons - while one half of the planet has summer, the other half has winter. The markings on Mars and their changes led astronomers to speculate for many years that Mars might have simple plant life. Spacecrafts Viking I and Viking II both landed on Mars in 1976. They have taken thousands of photographs from space so that a very great deal is now known about what Mars is really like. Mars has a core of iron and iron compounds with a diameter of 3,000 Km. It also has a mantle of silicate materials. The whole planet is a great desert. The red color is typical of desert rocks that are found in many places on Earth and comes from the rusting or oxidation of iron. Even the sky looks red on Mars, due to red dust in the air. Sometimes great dust storms develop, and about every ten years there is such a huge hurricane that the whole planet becomes engulfed in choking dust. Meteorites that crashed onto the surface from space created many craters. Volcanic activity too has contributed to the scenery on Mars. Mars is an inhospitable place. This cold and dry world has an atmosphere that is very thin compared to our Earth. The Martian air consists mainly of carbon dioxide, so people or animals could not breathe it. There is almost no oxygen. We know now that there are no plants on Mars.
Jupiter: King of the Planets
Figure 2.13 - Jupiter
Jupiter is one of the giant planets that include Saturn, Uranus, and Neptune. They have the major share of all the planets’ mass. The giant planets have over a hundred times as much material as the tiny planets circling the inner solar system. The outer planets are mainly made of light gases such as hydrogen and helium, whereas the inner planets are made of rocks and iron. These giant planets are considerably larger than the inner planets. Jupiter, for example, is eleven times the diameter of the Earth and it has a volume over one thousand times as large. These giants are not so dense as the Earth either, for their densities are closer to that of the water than of rock.
All of the giants spin rapidly on their rotation axes. Jupiter takes less than ten hours to make a single spin. This high speed twirling makes the planets bulge out at their equators. A further interesting feature of the outer planets is the many moons, over thirty in all. Jupiter and Saturn each have a moon that is slightly bigger than the planet Mercury. And Saturn, of course, has intrigued us for centuries with its splendid system of rings.
At times Jupiter outshines all the stars in the night sky - only Venus gets brighter. Jupiter shows a variety of features, some of which can be seen by small telescope. The dark and light colored bands of clouds are well known. The Great Red Spot (GRS) is the dominant feature of the Southern Hemisphere of Jupiter, which has been observed through telescopes ever since the 1650s. Pictures from the spacecraft Voyager I show the circulating nature of the GRS and show small puffy features within the spot itself. It rotates counterclockwise with a period of 6 days and is currently about 26,200 km long by 13,800 km wide.
Jupiter emits radio waves and has a large and strong magnetic field. Like that of the Earth, this field is a dipole, similar to a bar magnet. Because of this magnetic field, Jupiter possesses a magnetosphere, which extends into the space around the planet. Jupiter’s magnetosphere is very different from that of the Earth. First, Jupiter’s magnetic field is about 100 times larger than that of the Earth and, second, the effect on it of the solar wind is some 25 times less because Jupiter is much further from the sun.
Another odd fact about Jupiter is that it sends out more heat than it receives from the Sun! This is because Jupiter is still shrinking, by about one centimeter each year. This shrinking releases heat energy.
In the year 1610 Galileo discovered Jupiter’s four main moons. They are named Io, Europa, Ganymede and Callisto. All four are easily spotted even with binoculars.
Saturn: The Ringed Giant
Figure 2.14 - Saturn
Saturn is similar to Jupiter in many ways, except that it has a magnificent series of rings. Saturn has ten satellites. One of them, Titan, is nearly 6000 Km across, which makes it the largest moon in the solar system. Titan has its own atmosphere, made up of methane and ammonia. Saturn is not even as dense as water - a lump of Saturn matter would float on the sea.
Galileo first saw Saturn’s ring system in July 1610 with his newly developed telescope. His instrument was not good enough to show the rings clearly, and all Galileo could report was that Saturn appeared to be a triple planet. When he was observing it some seven years later, it was near its edge and the rings were invisible to him. Saturn, he said, seemed to have swallowed its own children. The Dutch astronomer Huygens, who managed to observe that there was, indeed, a ring around Saturn, finally cleared the mystery in 1655. But the true nature of the rings was not discovered until two centuries later, in 1856, when James Clerk Maxwell analyzed the evidence and showed that the gravitational field of Saturn would tear any solid ring to pieces. Maxwell concluded, therefore, that the rings could be composed of tiny particles in orbit around the planet. Subsequent studies, including results obtained from the Voyager probes, confirm Maxwell’s conclusion.
The rings are a magnificent spectacle with no more than 1,000 \meter thick. They extend outwards from 7,000 km above the cloudy surface of Saturn to more than 74,000 km. The most accurate counts to date show that there are at least 10,000 rings.
How the rings were formed is something of a mystery. There are two competing theories. The first is that they are the debris left behind after a satellite was torn apart by the gravitational forces of Saturn. The second is that they consist of material that failed to combine into a satellite at the time the planet was forming. The second theory is now thought to be the more likely.
Uranus: The Tilted Giant
Figure 2.15 - Uranus
The astronomer William Herschel discovered Uranus on the 13th of March 1781. Although Herschel had intended to work as a musician, he found astronomy fascinating. He taught himself about the skies and in 1773 he made his own reflecting telescope. With this he started to look at the stars. As his enthusiasm and knowledge grew, he built larger and larger telescopes
Herschel decided to make a map of the stars and to record their positions and brightness. During one of these careful searches, he found an entirely new planet. This greatly surprised scientists who had not suspected that there were any more planets. Herschel wanted to name his new planet after King George III who then reigned England. Eventually, however, it was agreed to call the planet Uranus. The choice of Uranus was made because in mythology Uranus was the father of Saturn, and Saturn was the father of Jupiter.
Uranus has a diameter four times larger than the Earth and it takes 84 years to orbit the Sun. Occasionally, one can just about catch a glimpse of Uranus with the naked eye on a very dark night if one knows where to look. With an up-to-date chart of planet positions, one can find it with a small telescope. One odd feature about Uranus is that it is a planet lying on its side. The rotational axis is tipped over at an angle of 98 Degree. This means that the seasons on Uranus must be very strange indeed. For several Earth years, the Sun does not shine at all in one hemisphere, while the other is continuously bathed in sunlight.
Neptune: The Last Giant
Figure 2.16 - Neptune
Astronomers located Neptune in 1846 after a remarkable piece of detective work by mathematicians. After many years of careful observation, Uranus puzzled observers. It did not keep to the path around the Sun that astronomers predicted on the basis of Newton’s law of gravity. Something kept knocking it off course. English and French mathematicians realized that another planet might be tugging Uranus to one side. These wizards computed where the unseen planet must be. An observatory in Berlin worked on these calculations, and they found a new dot of light: planet Neptune had been found.
Pluto: The last Planet
Figure 2.17 - Pluto