Task. Scan the text and compare the open and closed universe theories.
In recent years some exciting hypotheses have been advanced about the origin and evolution of the universe. Most scientists have by now accepted what is called the big bang theory. About 18 billion years ago, they say, a primordial fireball exploded and immediately began to expand, until eventually clouds of gas and dust formed and condensed, coalescing into galaxies of stars. It has been estimated that the universe now consists of about 100 billion galaxies, each one with about. 100 billion stars, all rushing away from each other as the universe continues to expand.
Our galaxy, the Milky Way, began to form about 10,000 million years ago as an enormous flattened spiral consisting of dust, gas and stars. The spiral is thicker at the centre, where the stars are more crowded. About 4,500 million years back, near the edge of the galaxy, the solar system came into being. Some of the nuclei in a cloud of gas and dust condensed, and as they were rotating they gathered up surrounding matter, growing ever larger and more solid. The central nucleus became the sun and the others developed into the planets in the solar system, one of them being our own planet, Earth. The earth began as a mass of liquid fire, but after cooling sufficiently, the surface solidified to form a rocky crust, and clouds condensed as rain, forming seas. The crust, however, has remained a thin layer, in places only a few kilometres thick. Under it lies a thicker layer of hotter denser rocks called the mantle, while the temperature of the still partially molten inner core has been calculated by some scientists to be as much as 5,538 degrees centigrade. The crust, more over, has been continuing to contract and shift, slowly changing the landscapes of the earth.
The story of the universe so far is thus one of continuous expansion and evolution. Some scientists believe that this process will continue for ever. According to the open universe theory, the universe will continue to expand indefinitely, because it doesnt contain enough matter to control the fleeing galaxies by gravitational attraction. On the other hand, according to the closed universe theory, the universe will eventually contract, because the gravitational force is supported by matter in stars and gas clouds that has not yet been detected. Astronomers have already discovered individual instances of this kind of contraction in what they call black holes; these are collapsed stars, which suck in all surrounding matter into a mass of infinitesimal volume but with immense gravity. If there had not been enough surrounding matter, black holes could not have operated in this way. Thus, it is argued, the whole universe may eventually contract into an incredibly dense cluster about 100 billion years from now. Will the universe simply cease to exist, or will it expand again into its present form or into a completely different form?
Ex. 1. Put the sentences in the logical order.
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1. Our galaxy, a huge spiral-shaped collection of stars, formed 10 billion years ago.
2. The gravitational pull of the unknown matter will prevent the Universe from growing beyond a certain size.
3. The universe was born in unimaginable explosion 18 billion years ago.
4. As the earth was cooling a rocky crust was forming and ocean came into existence.
5. The universe is continuously expanding and evolving, and it will expand forever.
6. The spiral arms of the galaxies are where star birth is taking place.
7. The earth began as a mass of liquid fire.
8. The universe had a definite beginning. Will it then have a definite end?
Ex. 2. Complete the chart below.
| Theory | Pros | Cons | Further Development of the Universe |
Ex. 3. Discuss in groups the problem raised in the text.
Text D
Galaxies
Task. Scan the text. Choose the one best alternative to each question following it. Answer all questions on the basis of what is stated or implied in the text.
Galaxies are not evenly distributed throughout the universe. A few are found alone, but almost all are grouped in formations termed galactic clusters. These formations should not be confused with stellar clusters, globular clusters of stars that exist within a galaxy. The size of galactic clusters varies enormously, with some clusters containing only a dozen or so members and others containing as many as 10.000. Moreover, galactic clusters themselves are part of larger clusters of clusters, termed superclusters. It is surmised that even clusters of superclusters are possible.
Our galaxy, the Milky Way, is part of a galactic cluster called the Local Group, which has twenty members and is typical in terms of the types of galaxies it contains. There are three large spiral galaxies: Andromeda, the largest galaxy in the group the Milky Way, the second-largest galaxy: and the Triangulum Spiral, the third largest. There are also four medium-sized spiral galaxies, including the Large Cloud of Magellan and the Small Cloud of Magellan. There are four regular elliptical galaxies: the remainder are dwarf ellipticals. Other than our own galaxy, only Andromeda and the Clouds of Magellan can be seen with the naked eye, and the Clouds are visible only from the Southern Hemisphere.
In the vicinity of the Local Group are several clusters, each containing around twelve members. The nearest cluster rich in members is the Virgo Cluster, which contains thousands of galaxies of all types. Like most large clusters, it emits X rays. The Local Group, the small neighboring clusters, and the Virgo Cluster form part of a much larger cluster of clusters the Local Supercluster.
The existence of galactic clusters presented a riddle to scientists for many years the missing mass problem. Clusters are presumably held together by the gravity generated by their members. However, measurements showed that the galaxies did not have enough mass to explain their apparent stability. Why didnt these clusters disintegrate? It is now thought that galaxies contain great amounts of dark matter, which cannot be directly observed but which generates gravitational pull. This matter includes gas, dust, burnt-out stars, and even black holes.
1. Which of the following does the text mainly discuss?
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a) clusters and superclusters of galaxies
b) an astronomical problem that has never been solved
c) a recent development in astronomy
d) the incredible distance between galaxies
2. The word evenly (given in the text in Italics) is closest in meaning to
a) uniformly
b) predictably
c) relatively
d) paradoxically
3. What conclusion can be made about galaxies that are not found in clusters?
a) They have never been observed.
b) They are larger than other galaxies.
c) They are not actually galaxies but parts of galaxies.
d) They are outnumbered by galaxies that do occur in clusters.
4. The word globular (given in the text in Italics) is closest in meaning to
a) immense
b) spherical
c) dense
d) brilliant
5. The author would probably characterize the existence of clusters of superclusters as
a) impossible
b) surprising
c) theoretical
d) certain
6. According to the text, in what way is the Local Group typical of galactic clusters?
a) in its size
b) in the number of galaxies it contains
c) in its shape
d) in the types of galaxies that make it up
7. In the Local Group, which of the following types of galaxies are most numerous?
a) large spirals
b) medium-sized spirals
c) regular elliptical
d) dwarf elliptical
8. All of the following are visible from somewhere on Earth without a telescope EXCEPT
a) the Clouds of Magellan
b) Andromeda
c) the Triangulum Spiral
d) the Milky Way
9. According to the passage, the Local Group and the Virgo Cluster have which of the following in common?
a) Both are rich in galaxies.
b) Both emit X rays.
c) Both are part of the same supercluster.
d) Both are small clusters.
10. The word riddle (given in the text in Italics) is closest in meaning to
a) tool
b) puzzle
c) theory
d) clue
11. Which of the following is NOT true about the dark matter?
a) It is impossible to observe directly.
b) It may include black holes.
c) It helps explain the missing mass problem.
d) It is found in the space between galaxies.
12. As used throughout the text the word members (given in the text in Italics) refers to
a) stars
b) galaxies
c) scientists
d) clusters
Dialogue
Ex. 1. Do you agree or disagree with the following statements? Write agree (A) or disagree (D) in the first column of the table.
| My opinion | The Doctors opinion | |
| a) You mustnt get too much sun. | ||
| b) The sun is good for us at any time of the day. | ||
| c) The sun can make our skin get older more quickly. | ||
| d) We can get skin cancer from too much sun. | ||
| e) The sun can make us feel better. | ||
| f) The sun can help us slim. | ||
| g) The sun can protect us from coughs and colds. | ||
| h) Direct sunlight makes us feel sleepy. | ||
| i) The sun makes us produce fewer white blood cells. | ||
| j) People with fair skin shouldnt go in the sun. |
Ex. 2. Now read the dialogue to see if Dr. Williams agrees with them or not, and write A or D in the second column.
Is the Sun Good or Bad for Us?
Interviewer: Well, Mrs Robins, to answer the questions posed in your letter we are lucky to have in the studio today Doctor James Williams. Doctor Williams, is the sun good or bad for us?
Dr Williams: Mrs Robins, there are two answers to this: the sun can be either good or bad for you. Now, this depends on how much sun you get and at what time of the day.
Interviewer: What about the negative effects of the sun, Doctor? We, we hear that its usually the result of prolonged or repeated exposure.
Dr Williams: Yes, thats right. Now, the negative effects are, of course, it increases the ageing process of the skin. And it also increases your chances of developing skin cancer. But, the sun in moderate doses has positive effects too.
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Interviewer: What are those?
Dr Williams: Well, scientific experiments have shown that, we feel better; it can help us slim; it can protect us from coughs, colds and other illnesses.
Interviewer: Now, how does it do all those things?
Dr Williams: Well, it makes us feel better because we have a gland in the brain which produces a substance called melatonin. Now, a lot of melatonin makes us feel sleepy. Now, in the sunlight, ultraviolet rays enter the body through the eyes and make the gland produce less melatonin, so therefore we feel brighter.
Interviewer: Oh, I see. What about keeping slim?
Dr Williams: Ah, yes, ultraviolet rays help us to burn up calories. Sunlight makes the body use oxygen more efficiently. So therefore, we process our food better. And therefore we need less.
Interviewer: Now, you mentioned that sunlight can protect us against illnesses. Ive heard of that.
Dr Williams: Indeed, indeed. Experiments show that exposure to sunlight makes us produce more white blood cells, especially the cells responsible for fighting off disease. So, with correct doses of sunlight we increase our chances of not catching coughs, colds or other illnesses.
Interviewer: So, to sum up, Doctor Williams, what is your advice? Go in the sun or not?
Dr Williams: Oh, yes, yes, absolutely, but, not too often, and not for too long, especially between midday and three pm when the sun is at the highest. If you think of countries which have a very good climate, places we tend to go to for our holidays, they have a very good tradition, the natives of the siesta.
Interviewer: Yes, of course.
Dr Williams: And especially if you have fair skin, you must be very, very careful not to burn.
Interviewer: Thank you very much, Doctor Williams.
Ex. 3. Compare and discuss your answers.
Ex. 4. Write a brochure entitled Is the sun good or bad for us? You could mention any of the points Dr. Williams made. Use the box to help you.
Listening Comprehension
Text Stars
Part A. Pre-listening activities
Task 1. Make sure that you know the following words.
| essential , roughly constellation (the Great Bear) enormous , to be related to , to maintain , , common dwarf , |
Task 2. Before listening answer the questions.
1. Why do scientists study stars?
2. What types of stars do astronomers classify?
3. What constellations do you know? What are they famous for?
4. How do you understand the term Milky Way?
Part B. Listening activities
Task 1. As you listen to the tape, make brief notes to help you answer the following questions.
1. Is there enormous variety in the mass different stars contain?
2. How do stars compare in mass with the sun?
3. What are the differences between stars?
4. What are stars little bigger than the earth called?
5. On the basis of what is the main classification of stars focused on?
Task 2. Listen to the tape once more and
A) fill in the gaps.
1. Stars contain amounts of matter.
2. There are no any stars with a mass less or more of the mass of the sun.
3. The name of the largest star is
4. Its diameter is
5. Stars must have high internal temperatures to
6. Surface temperature is related to
b) note down the temperature of:
very hot stars
intermediate stars
cooler stars
c) note down the colours of:
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very hot stars
intermediate stars
cooler stars
Part C. After Listening Activities
Task 1. Discuss in pairs.
1. Stars size and mass.
2. Surface and internal temperatures of stars.
Task 2. Summarize the information about the properties of stars (in writing).
Revision
Ex. 1. Here is an article about Uranus. Discuss with a partner the best way to translate the words in brackets.
Uranus
The first planet to be discovered since ancient times was Uranus. In 1781 William Herschel, an amateur astronomer, was carrying out a survey of the sky when he spotted a greenish disc. At first he thought it was a comet, but its movement showed it to be a planet ( ) than Saturn. Astronomers were able to work out that Uranus is another gas giant, () than Jupiter and Saturn but still ( ) than Earth. Herschel himself discovered two moons circling the planet, and later astronomers found three more. One of ( ) things about Uranus is that it is tipped up, effectively circling the sun on its side. In 1977, scientists found a set of dark, narrow rings around the planet. And that was the sum total of our knowledge of Uranus until Voyager 2 reached the planet in 1986.
Ex. 2. Fill in the text with the appropriate word from the box.
| crescent, satellites, rotation, cycle, illuminated(2), approximately, average, revolution, circles, celestial, axis, shadow |
Although closer to the earth than any other (1) ___ body, the moon is nevertheless an (2) ___ of 384,400 km (almost 240,000 mi) away. Its diameter of 3,476 km (92,160 mi) a little more than a quarter of the earths diameter places it among the largest (3) ___ in the solar system. The moon (4) ___ the earth every 271/3 days and, like the earth, turns on its (5) ___ as it revolves. In the case of the moon the (6) ___ keeps pace exactly with the (7) ___ so the moon turns completely around only once during each circuit of the earth. This means that the same face of our satellite is always turned toward us and that the other side remains hidden from the earth, though not from spacecraft. The steady pull of the earths gravity on one side of the moon has made this side bulge slightly but measurably toward the earth.
Each month the moon completes its familiar (8) ___ of phases. First there is a thin (9) ___ in the western sky at sunset which grows and moves eastward (relative to the stars): then a half-moon, until after 2 weeks the full moon rises in the east at sunset; next the moon wanes, becoming a thin crescent that rises just before the sun; finally it disappears altogether a few days before its next appearance as a (10) ___.
These different aspects represent the amounts of the moons (11) ___ surface visible to us in different parts of its orbit. When the moon is full, it is on the opposite side of the earth from the sun, so the side facing us is fully (12) ___. In the dark of the moon (or new moon), it is moving (13) ___ between us and the sun, so the side toward the earth is in (14) ___.
Ex. 3. Make complete sentences out of the following notes. Then arrange the sentences into one paragraph. Make sure that the paragraph makes sense, and that the sentences follow each other logically.
The/part/galaxy/stars/Milky Way/all/of/our/of/the/are
______________________________________________________
analysing/the/scientists/to find out/starlight/nature/the/of/the/befo-re/techniques/most/physical/stars/of/were discovered/didnt expect
_________________
member/immense/stars/of/our/a/an/of/sun/aggregate/is
_________________
birth/evolution/we/its/star/trace/to/death/maturity/through/a/from/its/the/in fact/of/ are able to
_________________
aggregates/are/universe/there/galaxies/and/many/the/in/these/are called
_________________
we/deal/have/however/information/of/on/a/detailed/stars/of/thousands/
great/today
_________________
others/vast/virtually/each/empty/the/by/reaches/space/from/of/is separated
_________________
sun/the/other/even/telescope/to/a/of/light/point/powerful/star/no/
most/the/than/more/than/as/appears
_________________
Ex. 4. Translate the text into Russian (in writing).
The Lunar Surface
The moon was hardly a mystery even before the voyages of Apollo 11 and of the other manned spacecraft that followed it there. Even a small telescope reveals the chief features of the lunar landscape: wide plains, jagged mountain ranges, and innumerable craters of all sizes. Each mountain stands out in vivid clarity, with no clouds or haze to hide the smallest detail. Mountain shadows are black and sharp-edged. When the moon passes before a star, the star remains bright and clear up to the moons very edge. From these observations we conclude that the moon has little or no atmosphere. Water is likewise absent, as indicated by the complete lack of lakes, oceans, and rivers.
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But there is still no substitute for direct observation and laboratory analysis, and each spacecraft that has landed on the moon and returned to earth, manned or unmanned, has brought back information and samples of the greatest value. The lack of a protective atmosphere and of running water to erode away surface features means that there is much to be learned on the moon about our common environment in space, both past and present. And from the composition and internal structure of the moon hints can be gleaned of its origin and past history, which may well bear upon those of the earth as well. Thus the study of the moon is also a part of the study of the earth, doubly justifying the effort of its exploration.
With the help of no more than binoculars it is easy to distinguish the two main kinds of lunar landscape, the dark, relatively smooth maria and the lighter, ruggedly mountainous highlands. Mare means sea in Latin, but the term is still used even though it has been known for a long time that these regions are not covered with water.
The maria are large, dark, smooth regions on the lunar surface that consist of lava pulverized by meteorites.
They are not perfectly smooth, but are marked by small craters ranging up to 236 km in diameter. Most of the craters are circular with raised rims that are steeper on the inside than on the outside, and some have mountain peaks at their centers. Craters resembling those on the moon are produced on the earth both by volcanic activity and by meteoric impact. There is no question that some of the lunar craters are of meteoric origin. However, present evidence points to a volcanic origin far in the past for the majority of the craters.
Ex. 5. Translate into English using the vocabulary of the Unit.
I.
1. .
2. , .
3. 7 .
4. .
5. .
6. , .
7. . .
8. 88 .
9. .
10. .
11. , . , .
12. .
13. , .
14. .
II. , , , , , . XVII , . , , , , .
III. , . 6 XV . , 8 . , . 100 , , , . , . .
Active Vocabulary
accompany v
approach v
approximately a
attraction n
axis n
celestial body
cluster n
galactic ~
stellar ~
consist of v
syn. be composed of
be made up of
contract v
contraction n
crescent n
dominate v
evaporate v
expand v (), ()
expansion n ,
gravity n
the law of universal ~
include v
luminosity n
luminous a
matter n
mean a
origin n 1. , ; 2.
plane n
property n ,
provide v
pull v
reflect v
revolve v (around the sun) ( )
rotate v (on ones axis) ( )
syn. circle _(the sun)
orbit _(the sun)
move around
satellite n
Additional Reading
The Planets
The planets seem to fall naturally into two categories. The inner planets of Mercury, Venus, Earth, and Mars are solid, relatively small, and rotate fairly slowly on their axes. The outer planets of Jupiter, Saturn, Uranus, and Neptune are gaseous, large, and rotate fairly rapidly. Although relatively little is known about Pluto, it seems to resemble the inner planets more than the outer ones despite its status as the outermost one of all.
Mercury
Mercury, smallest of the planets, has a crater-pocked surface much like that of the moon but lacking the extensive lava flows so prominent there. The Mariner 10 spacecraft detected a weak magnetic field around Mercury but no atmosphere. A bleak place, but an interesting one because the combination of a high density and little surface melting in the past suggests a quite different geologic history from that of the earth. Surface temperatures on the sunlit side are 3000 C or so, and because there is no atmosphere to transfer or retain heat, the temperature drops at night to about - 1750C.
Venus
In size and mass the planet Venus resemble s the earth more closely than any other member of the suns family. Apart from the sun and the moon, Venus is the brightest object in the sky, and is even visible in daylight. Venus has the distinction of spinning backward on its axis; that is, looking downward on its north pole, Venus rotates clockwise, whereas the earth and the other planets rotate counterclockwise. The rotation of Venus is extremely slow, so that a day on that planet represents 243 of our days.
The surface of Venus is obscured by thick layers of clouds. The dense atmosphere is mainly carbon dioxide, with a little nitrogen and a trace of water vapor also present. At the surface, atmospheric pressure is a hundred times that of the earth. On the earth carbon dioxide is an important absorber of radiation from the earth that prevents the rapid loss of heat from the ground after sunset. Venus, blanketed more effectively by far than the earth, retains more heat; estimates based on data radioed back by spacecraft suggest an average surface temperature of about 4300 C, enough to melt lead. Since the temperature is so high, the existence of life on Venus seems impossible.
Mars
The reddish planet Mars has long fascinated astronomers and laymen alike, for it is the only other known body on which surface conditions seemed suitable for life of some kind. Yet Martian climates are exceedingly severe by our standards, and the thin atmosphere does little to screen solar ultraviolet radiation. If life exists on Mars, it is adapted to an environment that would soon destroy most earthly organisms.
Mars rotates on its axis in a little over 24 h; its revolution about the sun requires nearly 2 years; and its axis is inclined to the plane of its orbit at nearly the same angle as the earths. These facts mean that the Martian day and night have about the same lengths as ours and that Martian seasons are 6 months long and at least as pronounced as ours. Over half again farther from the sun than the earth, Mars receives considerably less light and heat. Its atmosphere, largely carbon dioxide, is extremely thin, so little of the suns heat is retained after nightfall. Daytime temperatures in summer rise to perhaps 300 C, but at nightfall drop to perhaps 750C.
Another difficulty that life must face on Mars is the scarcity of water. A trifle is certainly there, as water vapor in the atmosphere and possibly in the white polar caps as well, but apparently not a great deal. The polar caps, which increase in area in winter and decrease in area in summer, are believed to be almost entirely frozen carbon dioxide (dry ice). However, water may well have once been more abundant on mars than it is today. Some surface features photographed by the Mariner 9 spacecraft early in 1972 strongly suggest erosion by running water within the past million years or so. The earths surface water probably was vented from volcanoes early in its history, and there seems no reason why the same process should not have occurred on Mars, whose surface is dotted with extinct volcanoes.
The fact that most terrestrial life requires liquid water and oxygen plus protection from solar ultraviolet radiation does not necessarily mean that life of some kind could not develop in their absence. Certain bacteria on the earth are known whose life processes require carbon dioxide, not oxygen, so an oxygen-containing atmosphere is not indispensable, at least for primitive forms of life. Conceivably organisms could exist which can thrive on water gleaned from traces of it in the minerals of surface rocks. And shells of some sort might protect Martian creatures from ultraviolet radiation. The absence of indications of life in photographs taken thousands of miles away from the Martian surface is in itself not significant; at such distances terrestrial life would probably not be apparent to a visitor from elsewhere. (And a closer look might suggest that the car is the most conspicuous form of life on earth.)
The pictures radioed back by Mariner 9 as it orbited Mars showed a host of intriguing geologic structures, many apparently of recent origin. The Martian landscape is extremely varied: there are regions pocked with huge craters, regions broken up into irregular short ridges and depressions, vast lava flows, channels that look as though they were carved by running water, even peculiar areas that seem to indicate glacial activity. Though rainstorms are absent at least these days - violent winds periodically drive great clouds of dust around the planet. The surface markings so obvious through the telescope do not seem to coincide with the topographical features found by Mariner 9, and some of these markings are known to change color with the Martian seasons. Perhaps the dust storms also follow the seasons and are responsible for the color changes; perhaps some form of vegetation is the cause; perhaps the true explanation lies elsewhere.
Early in this century the Italian astronomer Giovanni Schiaparelli and the American Percival Lowell reported that the surface of Mars was covered with networks of fine lines, popularly called canals (a poor English translation of the Italian canali, meaning cannels). The apparent straightness and geometric patterns of these canals were considered evidence of the work of intelligent beings. But the pictures radioed back by the various spacecraft to pass near Mars show no signs of canals, though there do seem to be several regions where a number of craters are approximately in line. Probably the canals are optical illusions; certainly the existence of Martian creatures advanced enough to be capable of digging actual canals is highly unlikely.
Jupiter
The giant planet Jupiter, like Venus, is shrouded in clouds. The clouds occur in bands of changing color yellow, red, brown, blue, purple, gray and there are some semipermanent markings, such as the Red Sport some tens of thousands of kilometers across. The latter make possible a determination of the planets period of rotation. This turns out to be less than 10 h, which means that points on Jupiters equator travel at the enormous speed of 45,000 km/h; the earths equatorial speed is only 1,670 km/h. Because of its rapid rotation, Jupiter bulges much more at the equator than the earth does.
The four satellites of Jupiter that Galileo discovered over 3 centuries ago are conspicuous objects in a small telescope. The largest is as big as Mercury, and the smallest is about the size of the moon. The other eight satellites are very small (25 to 250 km in diameter), and one of them escaped detection until 1951.
Jupiters volume is about 1,300 times that of the earth, but its mass is only 300 times as great. The resulting low density only a third more than that of water means that Jupiter cannot be composed of a mixture of rock, iron, and nickel as is the earth. Like the other giant planets (Saturn, Uranus, and Neptune), Jupiter must consist chiefly of hydrogen and helium, the two lightest elements. Probably Jupiter does not have an actual surface; instead, its atmosphere gradually becomes thicker and thicker with increasing depth until it becomes a liquid. A terrestrial analogy might be the slushy surface of a snowbank on a warm winter day.
Jupiters interior is believed to be very hot, about 500,0000 C according to some estimates, but not hot enough for nuclear reactions to occur in its hydrogen content whose release of energy would turn Jupiter into a star. But if Jupiters mass were 30 times greater, the increased internal pressure would push the temperature to 20 million0 C, and the result would be a miniature star.
Jupiters atmosphere apparently contains such gases as ammonia, methane, and water vapor as well as hydrogen and helium. As mentioned earlier, laboratory experiments show that when a mixture of these gases is exposed to energy sources such as are usually present in a planetary atmosphere (for instance lightning, ultraviolet light, streams of fast ions), the various organic compounds characteristic of life are formed. It seems entirely possible some biologists think probable that some form of life has evolved in the dense lower atmosphere of Jupiter. It is interesting that simple microorganisms such as bacteria and yeasts are able to survive when exposed to gas mixtures that simulate the Jovian atmosphere at temperatures and pressures comparable to those on Jupiter.
The American spacecraft Pioneer 10 passed close to Jupiter late in 1973 after a journey that lasted 20 months and covered over a billion kilometers. Of the wealth of information radioed back, a few items are especially notable. For example, Jupiter has a complex magnetic field about 8 times stronger than the earths, and this field traps high-energy protons and electrons from the sun in belts that extend many Jovian radii outward, (The Van Allen belts around the earth are similar, but 10,000 times weaker). Another important finding confirmed that Jupiter radiates over twice as much energy as it receives from the sun, which means that it has powerful internal sources of energy; by contrast, the atmospheres of Venus, Earth, and Mars are in balance, and radiate only as much energy as they get from the sun. It has been suggested that Jupiter is still contracting gravitationally, and in this contraction potential energy is turned into heat just as compressing air in a tire pump warms up the air.
Saturn
In its setting of brilliant rings, Saturn is the most beautiful of the earths kindred. The planet itself is much like Jupiter: similarly flattened at the poles by rapid rotation, similarly possessing a dense atmosphere, its surface similarly hidden by banded clouds. Farther from the sun than Jupiter, Saturn is considerably colder; ammonia is largely frozen out of its atmosphere, and its clouds consist mostly of methane.
The famous rings, two bright ones and a fainter inner one, surround the planet in the plane of its equator. This plane is somewhat inclined to Saturns orbit. Hence, as Saturn moves in its leisurely 29-year journey around the sun, we see the rings from different angles. Twice in the 29-year period the rings are edgewise to the earth; in this position they are practically invisible, which suggests that their thickness is small, perhaps 20 km as compared with the 270,000-km diameter of the outer ring.
The rings are not the solid sheets they appear to be but instead consist of myriad small bodies ranging in size from boulders a meter or more across to dust particles, each of which revolves about Saturn like a miniature satellite. No satellite of substantial size can exist close to its parent planet because of the disruptive effect of tide-producing forces, which are proportionately less the farther distant the satellite. The Roche limit is the minimum radius that a satellite orbit must have if the satellite is to remain intact; the limit is named in honor of E.A.Roche, who investigated the origin of Saturns rings a century ago. For Saturn the Roche limit is calculated to be 2,4 times the planets radius, and in fact the outer rim of the outer ring is 2,3 radii from the center of Saturn and the closest satellite never approaches closer that 3,1 radii from the center. Saturn has 10 ordinary satellites outside the rings; the innermost of these was discovered in 1966.
Uranus and Neptune
The two outermost planets, Uranus and Neptune, owe their discovery to the telescope. Uranus was found quite by accident in 1781, during a systematic search of the sky by the great English astronomer William Herschel. It is just barely visible to the naked eye, and in fact had been identified as a faint star on a number of sky maps prepared during the preceding hundred years. Herschel suspected Uranus to be a planet because, through the telescope, it appeared as a disk rather than as a point of light. Observations made over a period of time showed its position to be changing relative to the stars, and its orbit was determined from these data. The discovery of Neptune in 1846 was made as the result of predictions based on it gravitational effect on other planets.
Uranus and Neptune are large bodies, each with a diameter about 3 ½ times that of the earth. In most of their properties Uranus and Neptune resemble Jupiter and Saturn. Their atmospheres are largely methane, which accounts for their greenish color, with some hydrogen present as well. Because these planets are so far from the sun, their surface temperatures are below 2000 C, and any ammonia present would be frozen out of their atmospheres.
Stellar Evolution
A star shines because it is a large, compact aggregate of matter that contains abundant hydrogen. A body of this sort cannot avoid being luminous because of the energy liberated in the conversion of its hydrogen into helium. We may imagine as the starting point in a stars history a stage when its matter was an irregular mass of cool, diffuse gas and small, solid particles. Gravitation in such a mass would concentrate it into a smaller space. The gradual contraction would heat the gas, much as the gas in a tire pump is heated by compression. At length the temperature would grow high enough for hydrogen to be converted into helium, and the mass would begin to glow brightly. From this time on the tendency to contract would be counterbalanced by the pressure of radiation from the hot interior, so shrinking would stop and the star would maintain a nearly constant size. The diameter of a star is thus determined by equilibrium between gravitational forces pulling its material inward and forces due to radiation pushing its material outward.
A star does not shine because some occult force has started I shining; it shines because it has a certain mass and a certain composition. If we could somehow build a star by heaping together sufficient matter of the right composition, it would start to shine of its own accord.
A star consumes its hydrogen rapidly if it is large, slowly if it is small. A fairly small star like our sun makes its supply of hydrogen last for a period of the order of 10 billion years; probably the sun is now about halfway through this part of its career. When the hydrogen supply at last begins to run low in a star like the sun, the life of the star is by no means ended but enters its most spectacular phase. Further gravitational contraction makes the interior still hotter and other nuclear reactions become possible - particularly reactions in which atoms of heavier elements are made by a combination of helium atoms. These reactions, once started, give out so much energy that the star expands to become a giant. Energy is now being poured out at a prodigious rate, so the stars life as a giant is much shorter that the earlier part of its existence.
Eventually the new energy-producing reactions run out of fuel, and again the star shrinks although probably not without a few last brief flare-ups, which we see from the earth as novae (new stars) that shine brilliantly for a week or two and then subside into insignificance. The shrinking ultimately reduces the star to the white dwarf state. As a slowly contracting dwarf the star may remain luminous for billions of years more with its energy now coming from the contraction, from nuclear reactions involving elements heavier than helium, and from proton-proton reactions in a very thin outer atmosphere of hydrogen.
Stars much more massive than the sun have somewhat different histories. Eventually they become unstable and explode violently, emitting enormous amounts of material. Such explosions we observe as supernovae, flare-ups 10,000 or more times as luminous as ordinary novae. Having lost perhaps half its mass, a star of this kind can then subside like its smaller brethren into a dwarf star.
Today astronomers believe that the residual dwarfs of supernovae are different from ordinary white dwarfs because of the large mass of their parent stars. These hypothetical dwarfs are calculated to have densities far in excess of ordinary dwarfs, with masses comparable to that of the sun packed into spheres perhaps 15 km (9 mi) in diameter. The matter of such a star would weigh billions of tons per cubic inch. (If the earth were this dense, it would fit into a large apartment house). Under the pressures that would be present the most stable form of matter is the neutron. Pulsars, which emit brief, intense bursts of radio waves at regular intervals, are believed to be rotating neutron stars with magnetic fields that lead to radio emission in narrow beams; as a pulsar rotates, its beams swing with it to produce the observed fluctuations. A notable pulsar is located at the center of the Crab nebula, which is the remnant of a supernova that was seen in A.D. 1054 and has been expanding and glowing brightly ever since.
UNIT IV
| THE EARTH |
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