“For me, it is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring.”

—Carl Sagan, The Demon-Haunted World: Science as a Candle in the Dark

In 1888 the American astronomer Simon Newcomb wrote, “We are probably nearing the limit of all we can know about astronomy.” Boy, was he wrong. With the advent of photography in the nineteenth century and its continued refinement through CCDs and nonoptical telescope technology, combined with the development of the sciences in the twentieth century through today, and their engineering applications, our knowledge about the universe continues to expand at a phenomenal rate. New discoveries are being made in virtually every realm of astronomy: Every planet and moon we explore continue to reveal their secrets; we continue to discover new and fascinating properties of stars and stellar evolution; understanding the formation and evolution of galaxies, like our Milky Way, is very much a work in progress; the properties of black holes, simple as these objects are, still beguile astrophysicists; the cause of the origin of the universe has yet to be determined; and the nature of 95% of the matter and energy in the universe is still unknown to science, among many other things.

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To his credit, in 1903 Simon Newcomb acknowledged that “What lies before us is an illimitable field.” Today it is virtually impossible to keep up with all the new discoveries in astronomy, which is why astronomy textbooks such as this one must be updated so often. I have made every effort to provide your students with insights into the latest discoveries, along with the presentation of the underlying science and previous discoveries. I am also pleased to include a wide variety of modern learning techniques and new features in this sixth edition of Discovering the Essential Universe while still providing the wide range of factual topics that is the hallmark of the text.

In the realm of astronomy education, educators continue to develop methods to help students understand how to think like scientists and grasp the core concepts, even when scientific theories are at odds with students’ prior beliefs and misconceptions. I was a student at Cornell during Sagan’s hey-day there. It was memorable watching him blasting around in his orange and black Porsche 914. In the mid-1990s, just before his untimely death, he and I began discussing how to help students overcome their misconceptions about astronomy, an important theme of this book. Throughout each chapter, I have woven in explanations that will help your students identify, address, and hopefully overcome many of them.