ORIGINS

One of the most important questions we can pursue in our quest to determine our origin is whether life, and especially intelligent, communicating life, has arisen independently elsewhere in the Universe. Are creatures like humans a common phenomenon, or are we rare or even alone?

We have discussed the eight planets, Pluto and the other plutoids, asteroids,and some of the moons in the Solar System, and have found mostof them to be places that seem hostile to terrestrial life forms. Yet a fewlocations besides the Earth – most notably Mars, with its signs of ancient runningwater, and Europa and Enceladus (see Figure 7–25), with liquid water below theiricy crusts – have characteristics that suggest life may have existed there in thepast, or might even be present now or develop in the future. Astrobiology or exobiologyis the study of life elsewhere than Earth. It has also been suggested that“astrobiology” or “exobiology” is merely a NASA term, and that the fi eld shouldmore correctly be called bioastronomy (a part of astronomy) rather than “astrobiology,”which would arguably be part of biology.

ORIGINS

We discuss the telescopes used by astronomers to see the farthest and faintest objects, including not only the current generation of huge telescopes on the ground and major visible, x-ray, and infrared space telescopes aloft but also even better new telescopes under construction. These instruments allow us to learn about the earliest epochs of the Universe, to study how stars form, and to search for other planetary systems, among other things.

Everybody knows that astronomers use telescopes, but not everybody realizesthat the telescopes astronomers use are of very different types. Moreover,very few modern telescopes are used directly with the eye. In this chapter,we will fi rst discuss the telescopes that astronomers use to collect visible light, asthey have for hundreds of years. Then we will see how astronomers now also usetelescopes to study gamma rays, x-rays, ultraviolet, infrared, and radio waves.

THE FIRST TELESCOPES FOR ASTRONOMY

Over four hundred years ago, a Dutch optician put two eyeglass lensestogether, and noticed that distant objects appeared closer (that is,they looked magnifi ed). h e next year, in 1609, the English scientisth omas Harriot built one of these devices and looked at the Moon. Butall he saw was a blotchy surface, and he didn’t make anything of it.

ORIGINS

Our Sun is part of the Milky Way Galaxy, an enormous collection of billions of stars bound together by gravity. New stars forming from dense clouds of gas and dust in spiral arms provide clues to the Sun’s birth.

We have already described the stars, which are important parts of anygalaxy, and how they are born, live, and die. In this chapter, we discussthe gas and dust (small particles of matter) that are present to someextent throughout a galaxy. Substantial clouds of this gas and dust are callednebulae (pronounced “neb’yu-lee” or “neb’yu-lay”; singular: nebula); “nebula” isLatin for “fog” or “mist.” New stars are born from such nebulae. We also discussthe overall structure of the Milky Way Galaxy and how, from our location inside it,we detect this structure.

Excitingly, new methods of adaptive optics and extended abilities to observein the infrared have allowed detailed measurements to be made far into thecenter of our Galaxy, some 26,000 light-years from us. Two groups of scientistshave actually seen stars orbiting close to the center of the Galaxy, so close andso fast that a supermassive black hole must be there with a mass of about4 million times the mass of our Sun (see the Chapter Opener). Moreover, whatappears to be a giant blob of gas is approaching the supermassive black holeat such a speed that it should be there around 2013–14. We await the possiblefi reworks!

ORIGINS

We see how objects in the outer part of the Solar System, including comets, are left over from the origin of our Solar System and so can be studied to learn about conditions in our Solar System’s early years. We find that asteroids, meteoroids, and comets may lead to our demise, just as they produced at least one episode of major extinction in the past when they collided with Earth.

We have learned about the Solar System’s giant planets, which range insize from about 4 to 11 times the diameter of the Earth. We have seenthat our Solar System has a set of terrestrial planets, which range insize from the Earth down to 40 per cent the diameter of the Earth. This size rangeincludes the four inner planets as well as seven planetary satellites.

The remaining object that had long been called a “planet,” Pluto, is only 20per cent the diameter of Earth but is still over 2300 km across, so there ismuch room on it for interesting surface features. Recently, additional objectslike it, all but perhaps one smaller, have been found in the outer reaches ofthe Solar System. We shall see how we determined Pluto’s odd properties, andwhat the other, similar objects are.

ORIGINS

We find that galaxies, of which many billions are known to exist, are the fundamental units of the Universe. Essentially all stars, including our Sun, are within them. Understanding the birth and lives of galaxies is necessary for a complete picture of our existence.

At the beginning of the 20th century, the nature of the faint, fuzzy “spiralnebulae” was unknown. In the mid-1920s, Edwin Hubble showed thatthey are distant galaxies like our own Milky Way Galaxy (see the ChapterOpener), and that the Universe is far larger than was previously thought. Galaxiesare the fundamental units of the Universe, just as stars are the basic units ofgalaxies.

Like stars, many galaxies are found in clusters, and there are also superclustersof galaxies separated by enormous voids. By looking back in time atvery distant galaxies and clusters, we can study how they formed and evolved.Surprisingly, we now know that all these enormous structures consist largely of“dark matter” that emits little or no electromagnetic radiation.

We have completed our grand tour of the Universe. We have seen stars and planets, matter between the stars, giant collections of stars called galaxies and clusters of galaxies, and very distant objects such as quasars with curved space lensing even farther objects. We have witnessed the evolution of stars, in some cases ending with spectacular explosions leaving compact remnants the size of a city but half a million times more massive than Earth. We have pondered the properties of still more bizarre objects, black holes. We have learned how our Universe began in a hot, compressed state and has been expanding ever since – seemingly faster and faster during the past five billion years, perhaps driven by a cosmic antigravity effect. We have explored the origins of the Universe, galaxies, stars, the chemical elements, planets, and ultimately life itself. If you are thirsty for more information, as the authors would like you to be, you can consult the books listed in the Selected Readings.

Moreover, we have seen the vitality of contemporary science in general and astronomy in particular. The individual scientists who call themselves astronomers are engaged in fascinating studies, often pushing modern technologies to their limits. New telescopes on the ground and in space, new types of detectors, new computer capabilities for studying data and carrying out calculations, and new theoretical ideas are linked in research about the Universe.

Preface

Astronomy continues to flourish, with huge discoveries such as the one that the Universe’s expansion – which was long thought to be slowing down – is, astonishingly, accelerating. A generation of large optical telescopes has been built on mountaintops, and arrays of dozens of radio telescopes explore their part of the spectrum at high resolution. Still larger telescopes are being planned or built, including the Thirty Meter Telescope, the Giant Magellan Telescope, and the European Extremely Large Telescope, all for optical and near-infrared studies, as well as the Atacama Large Millimeter/submillimeter Array at an extremely high-altitude site for radio and far-infrared studies. The Hubble Space Telescope, updated with new cameras, sends down exciting data all the time, though we worry about its future and about a gap before the James Webb Space Telescope is launched. The latest space observatories transmit images made with gamma rays, with x-rays, and with infrared radiation. The overall structure of the Universe is being mapped and analyzed, with catalogues of millions of objects being compiled. Cosmology has become a mathematical, and even a statistical, science. NASA’s Curiosity rover triumphantly landed on Mars, bearing instruments to investigate Mars’s past habitability. Spacecraft are orbiting Mercury and Saturn; other spacecraft are en route to orbit Ceres and a comet. Another spacecraft is en route to Pluto and to objects beyond it. Moreover, new electronic instruments and computer capabilities, new space missions to Solar-System objects, and advances in computational astronomy and in theoretical work will continue to bring forth exciting results.

In The Cosmos: Astronomy in the New Millennium, we describe the current state of astronomy, both the fundamentals of astronomical knowledge that have been built up over decades and the incredible advances that are now taking place. We want simply to share with you the excitement and magnificence of the Universe.

ORIGINS

The stars are simply distant versions of our Sun. By studying their properties, such as surface temperature and intrinsic brightness, we can achieve a deeper understanding of the Sun and how its light and other forms of energy originate.

The properties of all stars, including the Sun, depend mostly on their mass, which in special cases can be determined from studies of binary stars. Star clusters can be used to determine the ages of stars, and to study stellar evolution.

The thousands of stars in the sky that we see with our eyes, and the millionsmore that telescopes reveal, are glowing balls of gas. Their bright surfacessend us the light that we see. Though we learn a lot about a star fromstudying its surface, we can never see through to a star’s interior, where the importantaction goes on.

In this chapter, we will discuss the surfaces of stars and what they tell us.First we explain how we tell the surface temperatures of stars and what weobserve to study them. We also explain how stars move, and how we determinetheir distances. Then we will learn about stars that come with friends: otherstars or groups of stars. Only when we fi nish these useful studies will we go onto discuss the stellar interiors, in Chapter 12.

ORIGINS

The Sun, the source of light and heat on Earth, is critical to the existence of humans. Its properties are typical of stars.

Not all stars are far away; one is very close at hand. By studying the Sun,we not only learn about the properties of a particular star but also canstudy processes that undoubtedly take place in more distant stars as well(see the Chapter Opener). We will fi rst discuss the quiet Sun, the solar phenomenathat typically appear every day. Afterward, we will discuss the active Sun, solarphenomena that appear nonuniformly on the Sun and vary over time.

We study the Sun from Earth’s surface not only from telescopes permanentlyset up on mountaintops and elsewhere but also from temporary observationsites set up to observe total solar eclipses (■ Fig. 10–1 ). Moreover, severalimportant spacecraft send back varied and high-resolution images of the Sunfrom outside our atmosphere, allowing us to study aspects of the Sun and partsof its spectrum that were previously unavailable to us.

In this chapter, we discuss mainly the outer layers of the Sun, which arevisible to us. In Chapter 12 , we will consider the deep-down source of energyof the Sun and of the other stars – nuclear fusion in the core, where temperaturesand pressures are extremely high.

ORIGINS

We discuss the basic constituents of atoms and some of the techniques that allow us to explore the Universe nearly as far back as the beginning of time. We discover from studies of light that distant stars and galaxies are made of the same kinds of elements as those found on Earth.

T he light that reaches us from the stars and planets is only one type of radiation, a certain way in which energy moves through space. Radiation in thissense results from the continuous changes in electricity and magnetism ateach point of space, so it is more formally known as electromagnetic radiation orelectromagnetic waves. Gamma rays, x-rays, ultraviolet, ordinary light, infrared, andradio waves are all merely electromagnetic radiation of different wavelengths.

In this chapter, we discuss the properties of radiation in what we call thespectrum and how analysis of this radiation enables scientists to study theUniverse. After all, we cannot touch a star! Despite having brought bits ofthe Moon back to Earth for study, we cannot yet do the same for even the nearestplanets, though we have found a few rocks from the Moon and from Mars in theform of meteorites (see Chapter 8 ). At various points in the book, we will discussother types of contact that we on Earth have with the Universe beyond, includingparticles called cosmic rays and extremely elusive particles called neutrinos.Remember that this book’s index can take you straight to any given topic, shouldyou choose to learn about it before the text formally addresses it.

Studies of atoms are important for understanding radiation and how it isgiven off. The simplest atom is hydrogen, and we will see how it gives offor takes up radiation at certain colors, which are known as spectral lines. These spectral lines sometimes are visible as bright colors;they are known as emission lines. When seen from certainangles (to be discussed later), a gas’s spectral lines canappear as gaps in a continuous band of color; they arethen known as absorption lines. (Note that “absorption”is spelled with a “p,” not with a second “b.”) We willsee how studying the spectral lines can even tell youwhether an object is moving toward or away from us andhow fast.