Excerpt from MAGNIFICENT UNIVERSE

INTRODUCTION

Before setting off, let's establish where we are. Our planet, the Earth, circles the Sun, from which it receives the light and warmth that keep it alive. Since its birth, the Earth has gone around the Sun 4.6 billion times. Eight other planets also orbit the Sun, and most have one or more moons revolving around them. The planets Mercury and Venus lie so close to the Sun that they are too hot for life, while Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto reside so far away that they are too cold. Smaller bodies--asteroids, comets, meteoroids--also scamper around the Sun. This is our solar system, which we visit in Chapter 1, "The Planets."

Mighty though it is, the solar system's centerpiece--the Sun--is simply a star, similar to the points of light that twinkle at night, and sunlight is really just very strong starlight. The Sun looks so different from other stars because the Earth huddles so close to it. Other stars lie so far away that in order to express their distances, we resort to a gargantuan unit, the light-year--the distance a beam of light speeds through in a year. Shrink the cosmos so that the Earth lies a mere inch from the Sun, and on the same scale even remote Pluto would reside within just forty inches; but a single light-year is so immense it would span an entire mile, and the nearest star system to the Sun, Alpha Centauri, would reside over four miles away, corresponding to a true distance of over four light-years. Indeed, if we traveled to Alpha Centauri, the Sun would look like a bright star; if we traveled a few dozen light-years farther, the Sun would look like a faint star; and if we traveled beyond fifty-five light-years, it would disappear from view. But stars would still shine around us--stars red, orange, yellow, white, and blue; stars large and small; stars young and old; stars with planets that may harbor living beings who see our Sun as just another star in the sky. In Chapter 2, "The Stars," we explore how stars are born, live, and die.

These stars speckle our Galaxy, the Milky Way, a giant spiral housing hundreds of billions of stars--two dozen stars for every person on Earth. The Sun is 27,000 light-years from the Galaxy's center, about halfway between the center and the Milky Way's luminous edge. Just as the Moon orbits the Earth and the Earth orbits the Sun, so the Sun orbits the center of the Milky Way, pulling the Earth and the other planets with it. Live eighty years and you will travel through 360 billion miles of space--even if you never leave the town in which you were born.

Beyond the shores of the Milky Way shine other galaxies, at least ten of which revolve around our own. The nearest giant galaxy to ours, Andromeda, lies 2.4 million light-years away. The Milky Way, Andromeda, their satellite galaxies, and a handful of other galaxies populate the Local Group, a collection of over thirty galaxies held together by gravity. The Local Group in turn belongs to the Local Supercluster, a vast assemblage of galaxies that stretch across 100 million light-years of space. Here and elsewhere galaxies sport a delightful variety of shapes--fuzzy ellipticals, stunning spirals, amorphous irregulars--which we meet in Chapter 3, "The Galaxies."

The universe today is bigger than it was yesterday, because it is expanding. By running the clock backward, astronomers can deduce that the original universe was small and hot, having arisen in a huge explosion called the big bang, 10 to 15 billion years ago. In Chapter 4, "The Universe," we examine the entire cosmos--its birth, its present expansion, and its future fate.

From Ken Croswell's Magnificent Universe. Copyright by Ken Croswell. All rights reserved.

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