1-4 By observing galaxies, astronomers learn about the origin and fate of the universe

Stars are not spread uniformly across the universe but are grouped together in huge assemblages called galaxies. Galaxies come in a wide range of shapes and sizes. Our Sun is just one star in a galaxy we call the Milky Way (see the figure that opens this chapter). A typical galaxy, like our Milky Way, contains several hundred billion stars. Some galaxies are much smaller, containing only a few million stars. Others are monstrosities that devour neighboring galaxies in a process called “galactic cannibalism.”

Our Milky Way Galaxy has arching spiral arms like those of the galaxy shown in Figure 1-9. These arms are particularly active sites of star formation. In recent years, astronomers have discovered a mysterious object at the center of the Milky Way with a mass millions of times greater than that of our Sun. It now seems certain that this curious object is an enormous black hole.

Figure 1-9: R I V U X G
A Galaxy This spectacular galaxy, called M63, contains about a hundred billion stars. M63 has a diameter of about 60,000 light-years and is located about 35 million light-years from Earth. Along this galaxy’s spiral arms you can see a number of glowing clumps. Like the Orion Nebula in our own Milky Way Galaxy (see Figure 1-7), these are sites of active star formation.
(2004–2013 R. Jay GaBany, Cosmotography.com)

Some of the most intriguing galaxies appear to be in the throes of violent convulsions and are rapidly expelling matter. The centers of these strange galaxies, which may harbor even more massive black holes, are often powerful sources of X-rays and radio waves.

Even more awesome sources of energy are found still deeper in space. Often located at distances so great that their light takes billions of years to reach Earth, we find the mysterious quasars. Although in some ways quasars look like nearby stars (Figure 1-10), they are among the most distant and most luminous objects in the sky. A typical quasar shines with the brilliance of a hundred galaxies. Detailed observations of quasars imply that they draw their energy from material falling into enormous black holes.

Figure 1-10: R I V U X G
A Quasar The two bright starlike objects in this image look almost identical, but they are dramatically different. The object on the left is indeed a star that lies a few hundred light-years from Earth. But the “star” on the right is actually a quasar about 9 billion light-years away. To appear so bright even though they are so distant, quasars like this one must be some of the most luminous objects in the universe. The other objects in this image are galaxies like that in Figure 1-9.
(Charles Steidel, California Institute of Technology; and NASA)

Galaxies and the Expanding Universe

The motions of distant galaxies motivate the ideas of the expanding universe and the Big Bang

The motions of distant galaxies reveal that they are moving away from us and from each other. In other words, the universe is expanding. Extrapolating into the past, we learn that the universe must have been born from an incredibly dense state some 13.7 billion years ago. A variety of evidence indicates that at that moment—the beginning of time—the universe began with a cosmic explosion, known as the Big Bang, which occurred throughout all space.

Thanks to the combined efforts of astronomers and physicists, we are making steady advances in understanding the nature and history of the universe. This understanding may reveal the origin of some of the most basic properties of physical reality. Studying the most remote galaxies is also helping to answer questions about the ultimate fate of the universe. Such studies suggest that the expansion of the universe will continue forever, and it is actually gaining speed.

8

The work of unraveling the deepest mysteries of the universe requires specialized tools, including telescopes, spacecraft, and computers. But for many purposes, the most useful device for studying the universe is the human brain itself. Our goal in this book is to help you use your brain to share in the excitement of scientific discovery.

In the remainder of this chapter we introduce some of the key concepts and mathematics that we will use in subsequent chapters. Study these carefully, for you will use them over and over again throughout your own study of astronomy.