The more massive a star is, the shorter its stay on the main sequence. The most massive stars may be there for only a few million years. A star like the Sun, on the other hand, is not especially massive and will live on the main sequence for about 10 billion years. Since it has taken over four billion years for life and humans to evolve, it is a good thing that some stars can be stable for such long times. A few types of stars explode violently at the ends of their lives (see the Chapter Opener), but our Sun will expire in a less sensational manner.

Astronomy is in a golden age, filled with the excitement of new discoveries and a deeper understanding of the Universe, our home – and what an enthralling universe it is!

We have explored all the planets in the Solar System, revealing an astonishingly wide variety of terrains and moons. We have realized that there are even more dwarf planets, not to mention hundreds of thousands of smaller objects in our Solar System. We have discovered planets orbiting other stars, increasing our confidence that life exists elsewhere. We have solved many of the mysteries surrounding stellar birth and death, revealing among other things how the chemical elements inside our bodies, like calcium and oxygen, formed inside stars.

Distant galaxies are moving away from us; the Universe is expanding. But, by itself, the expansion of the Universe does not prove that there was a big bang; indeed, one could postulate that the Universe had no beginning in time and will have no end. The fatal blow to this “steady-state theory” was the discovery of a faint radio glow that pervades all of space and was produced when the Universe was very young. The existence of a nearly uniform amount of helium and deuterium (heavy hydrogen) throughout the Universe provides additional evidence for a very hot, dense phase in its early history.

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 can study processes that undoubtedly take place in more distant stars. We will first discuss the quiet Sun, the solar phenomena that typically appear every day. Afterward, we will discuss the active Sun, solar phenomena that appear nonuniformly on the Sun and vary over time.

We have discussed the eight planets, Pluto and the other plutoids, asteroids, and some of the moons in the Solar System, and have found most of them to be places that seem hostile to terrestrial life-forms. Yet a few locations besides Earth – most notably Mars, with its signs of ancient running water, and Europa and Enceladus (see the Chapter Opener), with liquid water below their icy crusts – have characteristics that suggest life may have existed there in the past, or might even be present now or develop in the future.

Everybody knows that astronomers use telescopes, but not everybody realizes that 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 first discuss the telescopes that astronomers use to collect visible light, as they have for hundreds of years. Then we will see how astronomers now also use telescopes to study gamma rays, x-rays, ultraviolet, infrared, and radio waves. We will see how the James Webb Space Telescope, at this writing scheduled for launch in 2021, is expected to transform many areas of astronomy.

Jupiter, Saturn, Uranus, and Neptune are giant planets; they are also called the jovian planets. They are much bigger, more massive, and less dense than the inner, terrestrial planets; see A Closer Look 7.1: Comparative Data for the Major Worlds. Their internal structure is entirely different from that of the four inner planets. All the giant planets have atmospheres that are almost entirely hydrogen and helium. The small percentage of heavier elements is greater in Uranus and Neptune than in Jupiter and Saturn.

Mercury, Venus, Earth, and Mars share many similar features. Small compared with the huge planets beyond them, these inner planets also have rocky surfaces surrounded by relatively thin and transparent atmospheres, in contrast with the larger, gaseous/liquid planets. Together, we call these four the terrestrial planets (from the Latin “terra,” meaning earth), which indicates their significance to us in our attempts to understand our own Earth. In this chapter, we discuss each of these rocky bodies, as well as their moons.

At the beginning of the twentieth century, the nature of the faint, fuzzy “spiral nebulae” was unknown. In the mid-1920s, Edwin Hubble showed that they are distant galaxies like our own Milky Way Galaxy, and that the visible Universe is far larger than was previously thought. Galaxies are the fundamental units of the Universe, just as stars are the basic units of galaxies.

The thousands of stars in the sky that we see with our eyes, and the millions more that telescopes reveal, are glowing balls of gas. Their bright surfaces, marking the location outside of which the gas is transparent, send us the light that we see. Though we learn a lot about a star from studying its surface, we can never see through to a star’s opaque interior, where the important action goes on.

The peculiar forces of electron and neutron-degeneracy pressure (an effect of quantum physics) support dying lightweight stars and some heavyweight stars against gravity. The strangest case of all occurs at the death of the most massive stars, which contained much more than 10 and up to about 100 solar masses when they were on the main sequence. After these stars undergo supernova explosions, some may retain cores of over 2 or 3 solar masses. Nothing in the Universe is strong enough to hold up the remaining mass against the force of gravity, so it collapses.