ORIGINS

We explore the overall structure of the Solar System and how ideas about it were developed over many centuries, providing basic information for theories of its formation.

Ancient peoples knew of fi ve planets – Mercury, Venus, Mars, Jupiter, andSaturn. When we observe these planets in the sky, we join the people oflong ago in noticing that the positions of the planets vary from night tonight with respect to each other and with respect to the stars. In this chapter, wewill discuss the motions of the planets, and you will learn how major fi gures inthe history of astronomy explained these motions. These explanations have led totoday’s conceptions of the Universe and of our place in it.

A BRIEF SURVEY OF THE SOLAR SYSTEM

Our ideas of the Solar System took shape before even the telescopewas invented, but nowadays we have visited almost all of the SolarSystem with spacecraft (■ Fig. 5–1 ). Let us fi rst give an overview ofthe Solar System before discussing the historical fi gures who startedus on our current path. Discussion of the individual Solar-Systemobjects occurs in later chapters.

ORIGINS

We find that the properties of giant planets differ from those of Earth because they formed much farther out from the center of the solar nebula. Comparison of features on these planets and their moons leads to a greater understanding of the origin of Earth’s properties.

Jupiter, Saturn, Uranus, and Neptune are giant planets; they are also calledthe jovian planets. They are much bigger, more massive, and less densethan the inner, terrestrial planets; see A Closer Look 7.1: Comparative Datafor the Major Worlds. Their internal structure is entirely different from that ofthe four inner planets. All the giant planets have atmospheres that are almostentirely hydrogen and helium. The small percentage of heavier elements is greaterin Uranus and Neptune than in Jupiter and Saturn.

In this chapter, we also discuss a set of moons of these giant planets, someof which range in diameter between 1/2 and 1/4 the size of the Earth, as largeas Mercury or the dwarf planet Pluto. Close-up space observations have shownthat these moons are themselves interesting objects for study.

ORIGINS

We comment on “Origins” and “Structure and Evolution” as organizing themes.

Astronomy is in a golden age, fi lled with the excitement of new discoveriesand a deeper understanding of the Universe, our home – and what anenthralling universe it is!

We have explored most of the planets in the Solar System, revealing an astonishinglywide variety of terrains and moons. We have discovered planets orbitingother stars, increasing our confi dence that life exists elsewhere. We havesolved many of the mysteries surrounding stellar birth and death, revealing amongother things how the chemical elements inside our bodies, like calcium andoxygen, formed inside stars. With the Hubble Space Telescope, the Chandra X-rayObservatory, and the Spitzer Space Telescope we have examined galaxies shortlyafter their birth, deducing important clues to the origin and evolution of our ownMilky Way Galaxy. (When we refer to our Milky Way Galaxy, we say “our Galaxy”or “the Galaxy” with an uppercase “G.” When we refer to other galaxies, we usea lowercase “g.”)

We have witnessed explosions of stars halfway across the visible Universe whosepower is so tremendous that it rivals a galaxy containing ten billion normal stars.Indeed, we are lucky that none have recently occurred too close to Earth, for wewouldn’t survive. We have detected black holes – strange objects whose gravitationalpull is so strong that nothing, not even light, can escape. And, most recently, we havefound strong evidence that what we thought was “empty space” actually contains adark, gravitationally repulsive kind of energy that is causing the Universe to expandfaster and faster with time. The origin of this “dark energy” is a complete mystery,but an understanding of it may revolutionize physics.

ORIGINS

We explore the significance of quasars as an early phase in the origin and evolution of large galaxies, such as our Milky Way Galaxy. We find that quasars allow us to study the distribution of matter, both visible and dark, in the vast space between us and them.

Quasars, and the way in which they became understood, have been oneof the most exciting stories of the last half century of astronomy. Firstnoticed as seemingly peculiar stars, quasars turned out to be some of themost powerful objects in the Universe, and represent violent forces at work (seethe Chapter Opener). We think that giant black holes, millions or even billions oftimes the Sun’s mass, lurk at their centers. A quasar shines so brightly becauseits black hole is pulling in the surrounding gas, causing the gas to glow vividlybefore being swallowed.

Our interest in quasars is further piqued because many of them are amongthe most distant objects we have ever detected in the Universe. Since, as welook out, we are seeing light that was emitted farther and farther back in time,observing quasars is like using a time machine that enables us to see theUniverse when it was very young. We fi nd that quasars were an early stage inthe evolution of large galaxies. As time passed, gas in the central regions wasused up, and the quasars faded, becoming less active. Indeed, we see examplesof active galaxies relatively near us, and in some of these the presence ofa massive black hole has been all but proven.

ORIGINS

Discoveries of planetary systems around other stars are providing insights into the process by which planets form. The exoplanets are also possible abodes for life outside our own Solar System.

Astronomers have long speculated about the origin of our Solar System.They have noted regularities in the way the planets orbit the Sun and inthe spacing of the planetary orbits. But until recently, astronomers havebeen limited to studying one planetary system: our own.

In 1600, Giordano Bruno was burned, naked, at the stake, in part for wonderingabout the “plurality of worlds,” to use the term of that day. Finally, afterhundreds of years of wondering whether they existed, astronomers discoveredanother planetary system in 1992, though it was around a very strange type ofstar. And in the last two decades, planetary systems galore have been discoveredaround stars like the Sun. While twenty years ago we knew of only the handfulof planets orbiting our Sun, as of this writing (2012) we know of about 1000planets around other stars, as well as more than 2000 additional good candidates(see the Chapter Opener), the latter mostly from NASA’s Kepler spacecraft(■ Fig. 9–1 ).

ORIGINS

We discover how the heavy elements in the Earth, so necessary for life itself, came into existence through nuclear reactions in stars. We discuss the ultimate fate of our Sun, billions of years into the future. And, we learn about the origins of interesting classes of stars, including red giants, white dwarfs, and neutron stars, as well as the origins of exploding stars.

The more massive a star is, the shorter its stay on the main sequence. Themost massive stars may be there for only a few million years. A star likethe Sun, on the other hand, is not especially massive and will live on themain sequence for about ten billion years. Since it has taken over four billionyears for humans to evolve, it is a good thing that some stars can be stable forsuch long times. A few types of stars explode violently at the ends of their lives(see the Chapter Opener), but our Sun will expire in a less sensational manner.

In this chapter, we will fi rst discuss what will happen when the Sun dies:It will follow the same path as other single lightweight stars, stars born withup to about 10 (but possibly as low as 8) times the mass of the Sun. Theywill go through planetary nebula (■ Figs. 13–1, 13–2) and white-dwarf stages.Then we will discuss the death of more massive stars, greater than about 8 or10 times the Sun’s mass, which we can call heavyweight stars. (We use theboxing terms “lightweight” and “heavyweight” colloquially, though we are referringto mass rather than weight.) The most massive stars go through spectacularstages. Some wind up in such a strange final state – a black hole – that wedevote the entire next chapter to it.

ORIGINS

Throughout their existence, humans have used the apparent positions of celestial objects to define the day, month, year, and seasons, as well as for navigation.

T he Sun, the Moon, and the stars rise every day in the eastern half of thesky and set in the western half. If you leave your camera on a tripod withthe lens open for a few minutes or hours in a dark place at night, you willphotograph the “star trails” – the trails across the photograph left by the individualstars. In this chapter, we will discuss the phases of the Moon and planets, andhow to fi nd stars and planets in the sky. Stars twinkle; planets don’t twinkle asmuch (■ Fig. 4–1 ). We will also discuss the motions of the Sun, Moon, and planets,as well as of the stars in the sky.

THE PHASES OF THE MOON AND PLANETS

From the simple observation that the apparent shapes of the Moonand planets change, we can draw conclusions that are importantfor our understanding of the mechanics of the Solar System. Inthis section, we shall see how the positions of the Sun, Earth,and other Solar-System objects determine the appearance of theseobjects.

The phases of moons or planets are the shapes of the sunlitareas as seen from a given vantage point. h e fact that the Moongoes through a set of such phases approximately once every monthis perhaps the most familiar astronomical observation, aside fromthe day/night cycle and the fact that the stars come out at night(■ Fig. 4–2c , 4–3 ). In fact, the name “month” comes from the word“moon.” h e actual period of the phases, the interval between a particularphase of the Moon and its next repetition, is approximately 29 1 / 2 Earthdays. h is type of period that depends on the orientation with respect toa third body (in this case, the Sun) is known as a synodic period .

ORIGINS

The birth of the Universe itself is probably the most fundamental of all questions about origins. The behavior of the Universe at very early times greatly affected its contents, producing the matter of which we are made and the relative proportions of the light elements that dominate all others. It is possible that many universes exist, yet only a small fraction might have conditions suitable for life as we know it.

Distant galaxies (see the Chapter Opener ) are moving away from us; the Universe is expanding. But, by itself, the expansion of the Universe does not prove that there was a big bang; indeed, one could postulate that the Universe had no beginning in time and will have no end. The fatal blow to this “steady-state theory” was the discovery of a faint radio glow that pervades all of space and was produced when the Universe was very young. The existence of a nearly uniform amount of helium and deuterium (heavy hydrogen) throughout the Universe provides additional evidence for a very hot, dense phase in its early history.

But how did the Universe achieve a uniform temperature, and why is spaceso nearly fl at? These troubling issues led to a magnifi cent but still unprovenhypothesis: The early Universe apparently went through a stage of phenomenallyrapid expansion, doubling its size many times in just a tiny fraction of asecond. Such a process may have created the Universe from essentially nothing!And if it happened once, it might well happen again; perhaps there existmultiple “universes” physically disconnected from ours.

ORIGINS

The formation of stars like the Sun is critical to our existence. With its large supply of fuel, the Sun exists for a long time as it produces energy through nuclear fusion, making possible the development of advanced life on Earth.

Even though individual stars shine for a relatively long time, they are noteternal. Stars are born out of the gas and dust that exist within a galaxy(see the Chapter Opener); they then shine brightly on their own for a longtime. Eventually, they die. Though we can directly observe only the outer layers ofstars, we can deduce that the temperatures at their centers must be millions ofkelvins. We can even fi gure out what it is deep down inside that makes the starsshine.

To determine the probable life history of a typical star, we observe starshaving many different ages and assume that they evolve in a similar manner.However, we must take into account the different masses of stars; some aspectsof their evolution depend critically on mass.

ORIGINS

Gravity is the dominant force shaping the structure and evolution of the Universe. Black holes provide a laboratory for testing the most extreme predictions of Einstein’s general theory of relativity, our best existing description of gravity.

The peculiar forces of electron and neutron degeneracy pressure (an effectof quantum physics) support dying lightweight stars and some heavyweightstars against gravity. The strangest case of all occurs at the death of themost massive stars, which contained much more than 10 and up to about 100solar masses when they were on the main sequence. After these stars undergosupernova explosions, some may retain cores of over 2 or 3 solar masses. Nothingin the Universe is strong enough to hold up the remaining mass against the forceof gravity, so it collapses.

The result is a black hole, in which the matter disappears from contact withthe rest of the Universe (Chapter Opener and ■ Fig. 14–1). Later, we shalldiscuss much more massive black holes than those that result from the collapseof a star. Most astronomers now agree that black holes are widespread and veryimportant in our Universe.

ORIGINS

The small, rocky, terrestrial planets and the Moon show many similarities and differences among their interiors, surfaces, and atmospheres. By studying these objects, we can begin to understand the formation and evolution of the Earth, whose properties seem uniquely suitable for the development of complex life.

Mercury, Venus, Earth, and Mars share many similar features. Smallcompared with the huge planets beyond them, these inner planets alsohave rocky surfaces surrounded by relatively thin and transparent atmospheres,in contrast with the larger, gaseous/liquid planets. Together, we call thesefour the terrestrial planets (from the Latin “terra,” meaning earth), which indicatestheir significance to us in our attempts to understand our own Earth. In this chapter,we discuss each of these rocky bodies, as well as their moons. (See A CloserLook 6.1: Comparative Data for the Terrestrial Planets and Their Moons.)

Venus and the Earth are often thought of as “sister planets,” in that theirsizes, masses, and densities are about the same (■ Fig. 6–1 ). But in manyrespects they are as different from each other as the wicked stepsisters werefrom Cinderella. The Earth is lush; it has oceans of water, an atmospherecontaining oxygen, and life. On the other hand, Venus is a hot, forebodingplanet with a uniform temperature of 735 K (865°F), a planet on which lifeseems unlikely to develop. Why is Venus like that? How did these harsh conditionscome about? Can it happen to us here on Earth?

ORIGINS

Knowledge of the age, geometry, structure, and evolution of the Universe is fundamental to our overall understanding of our place in the Cosmos. An amazing discovery is that the expansion of the Universe is now speeding up, not slowing down as expected if normal attractive gravity dominates throughout.

Cosmology is the study of the structure and evolution of the Universe on itsgrandest scales (see the Chapter Opener). Some of the major issues studiedby cosmologists include the Universe’s birth, age, size, geometry, andultimate fate. We are also interested in the birth and evolution of galaxies, topicsalready discussed in Chapter 16.

As in the rest of astronomy, we are trying to discover the fundamental laws ofphysics, and use them to understand how the Universe works. We do not claimto determine the purpose of the Universe or why humans, in particular, exist;these questions are more in the domain of theology, philosophy, and metaphysics.However, in the end we will see that some of our conclusions, which aresolidly based on the methods of science, nonetheless seem to be untestablewith our present knowledge; they are at the boundary of what is generallyconsidered to be science.