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To most normal people, astrophysics—the science of stars, galaxies, and the universe we live in—would seem to be a topic suited to a university-level textbook, and so the idea of a guide to astrophysics for the amateur astronomer may not, on first appearance, make any sense. However, let me assure you that anyone can understand how a star is born, lives its life, and dies, how galaxies are thought to evolve and what their shape can tell us about their origins and age, and even how the universe began and how it may end. In fact, very little mathematics is needed, and when it is used, it is only a matter of multiplication, division, subtraction, and addition!
What is more, there are many wonderful objects that can be observed in the night sky that will illustrate even the most obtuse astrophysics concepts. All one needs is a willingness to learn and a dark night sky. Learning about, say, the processes that give rise to star formation, or what happens to a very large star as it dies, or even why some galaxies are spiral in shape whereas others are elliptical can add another level of enjoyment and wonder to an observing session. For instance, many amateur astronomers are familiar with the star Rigel, in the constellation Orion, but how many of you know that it is a giant star, with a mass more than 40 times that of the Sun, and it is nearly half a million times more luminous than the Sun? How many know that the closest large galaxy, M31 in Andromeda, has a supermassive black hole lurking at its center with a mass more than 50 million times that of the Sun? Or that the Orion Nebula, regarded by many as the premier nebula in the sky, is in fact an enormous stellar nursery where stars are actually being born as you read this book? Knowing details such as these can add another level of enjoyment to your observing sessions.
Each section of this book addresses a specific aspect of astrophysics. The first part focuses on the concepts needed for a complete understanding of the remainder of the book, and as such will be divided into specific topics, such as the brightness, mass, and distance of stars, and so on. Then we will look at the tools of an astronomer, namely spectroscopy. It is true to say that nearly all of what we know about stars and galaxies was and is determined by this important technique. We shall spend a fair amount of time looking at something called the Hertzsprung-Russell diagram; if ever a single concept or diagram could epitomize a star’s life (and even a star cluster’s life), the H-R diagram, as it is known, is the one to do it. It is perhaps the most important and useful concept in all of stellar evolution, and it is fair to say that once you understand the H-R diagram, you understand how a star evolves.
Moving on to the objects themselves, we start with the formation of stars from dust and gas clouds, and conclude with the final aspect of a star’s life, which can end in a spectacular event known as a supernova, resulting in the formation of a neutron star and perhaps a black hole!
On a grander scale, we delve into galaxies, their shapes (or morphology, as it is called), distribution in space, and origins. The topics covered are chosen specifically so that examples of objects under discussion can be observed; thus, at every point in our journey, an observing section will describe the objects that best demonstrate the topics discussed. Many of the objects, whether they be stars, nebulae, or galaxies, will be visible with modest optical instruments, and many with the naked eye. In a few exceptional cases, a medium-aperture telescope may be needed. Of course, not all observable objects will be presented, but just a representative few (usually the brightest examples). These examples will allow you to learn about stars, nebulae, and galaxies at your own pace, and they will provide a detailed panorama of the amazing objects that most of us observe on a clear night. For those of you who have a mathematical mind, some mathematics will be provided in the specially labelled areas. But, take heart and fear not—you do not have to understand any mathematics to be able to read and understand the book; it is only to highlight and further describe the mechanisms and principles of astrophysics. However, if you are comfortable with maths, then I recommend that you read these sections, as they will further your understanding of the various concepts and equip you to determine such parameters as a star’s age and lifetime, distance, mass, and brightness. All of the maths presented will be simple, of a level comparable to that of a 4th-year school student, or an 8th-grader. In fact,
to make the mathematics simpler, we will use rough (but perfectly acceptable) approximations and perform back-of-the-envelope calculations, which, surprisingly, produce rather accurate answers!
An astute reader will notice immediately that there are no star maps in the book. The reason for this is simple: in previous books that I have written, star maps were included, but their size generated some criticism. Some readers believed the maps were too small, and I tend to agree. To be able to offer large and detailed star maps of every object mentioned in this book would entail a doubling of its size, and probably a tripling of cost. With the plethora of star-map software that is available these days, it is far easier for readers to make their own maps than to present any here.
A final point I wish to emphasize here is that the book can be read in several ways. Certainly, you can start at the beginning and read through to the end. But if you are particularly interested in, say, supernovae and the final stages of a star’s life, or in galaxy clusters, there is no reason that you should not go straight to that section. Some of the nomenclature might be unfamiliar, but I have attempted to write the book with enough description that this should not be a problem. Also, many of you will undoubtedly go straight to the observing
lists. Read the book in the way that is most enjoyable to you. Without further ado, let us begin a voyage of discovery...
