Radioactive decay offers a powerful tool with which we can gauge the ages of various objects. For example, carbon-14 is a radioactive type of carbon that accumulates in living tissues in a known proportion compared with the stable and much more common carbon-12. When an organism dies, it stops adding carbon to itself. The carbon-14 begins to decay with a half-life of 5,700 years, while the carbon-12 remains exactly as it was. Thus, the ratio of carbon-14 to carbon-12 in an old object reveals how long ago it died. Archeologists and paleontologists use this technique, called radiocarbon dating, to estimate the ages of human and other animal remains. They also apply radiocarbon dating to long-dead plants and trees, as well as cloth, paper, and charcoal found at historical digs.

Geologists and planetary scientists use a similar approach to measure the ages of rocks and meteorites. But their elements have considerably longer lifetimes. Potassium-40 has a half-life of 1.26 billion years, while that of uranium-238 is 4.46 billion years. Studies of those elements have shown that meteorites—remnants of the early solar system—are about 4.6 billion years old. The narrow range of ages calculated for these objects suggests that they all formed within an interval no longer than 100 million years.

Radioactive dating of rocks from the Moon shows that it formed at about the same time as Earth. But the manner of that formation remains a topic of hot debate. Some astronomers propose that the Moon was a large asteroid captured gravitationally by the young Earth. Other theories hold that they coalesced side by side or that Earth spun quickly enough in its youth to cast a large chunk of itself into orbit. However, various clues now point to an even more striking origin (page 96). Analysis of rocks collected by Apollo astronauts suggests that the Moon's composition is similar to that of Earth's mantle, the thick rocky layer beneath the crust. More recent space missions indicate that the Moon also has a small iron core, composing just a small percentage of its mass. The best explanation seems to be that a Mars-sized object smashed into Earth while it was still forming. The impact flung an enormous disk of molten rocky debris from Earth's mantle into space, along with a small amount of iron. The Moon then assembled from the debris, leaving Earth battered but no longer alone in the solar system. Computer simulations show that this violent process is physically plausible and may be relatively common in young planetary systems.

The rocks that compose Earth and the Moon are products of stars that exploded long ago in our region of the Milky Way. But when stars die, they do more than simply shed matter that enriches the galaxy. They also leave behind the most bizarre states of matter in the universe: ultracompact objects with intense gravitational fields. These stellar cinders are unimaginably dense, denser than anything we could possibly create on Earth. We study them by observing the exotic radiation they emit and their violent interactions with other matter in the neighborhood.

Three fates are possible for the cores of dying stars. Astrophysicists have coined simple phrases that describe the essence of each type of object: white dwarf, neutron star, and black hole. The members of this heavyweight trio share some attributes. For example, the crushing power of gravity forges them in the hearts of stars that have consumed their nuclear fuel. They all attract matter like powerful drains, often with explosive results. Furthermore, each consists of "degenerate" matter. This has nothing to do with socially objectionable behavior. Rather, degeneracy means that the usual spaces between atoms are squeezed into nothingness.

The key difference among the three objects is the degree to which such squeezing occurs. The strength of the cosmic vise is set by the size of the doomed star: Massive stars create more compact remnants. Imagine crushing your car in a futuristic automobile compactor with three settings. The white dwarf setting might spit out a cube of metal and glass the size of a sugar cube but with the mass of your original car. Point the dial to neutron star and you would get a 1-ton speck the size of an amoeba. If you really hated your car and wanted it compressed to black hole densities, it would vanish from sight entirely, collapsed within a volume smaller than an atomic nucleus.

To achieve these dramatic states, stars must overcome the forces of repulsion that control ordinary matter. We can devise a mental picture of how that happens by shrinking our Superdome model of the atom to a more manageable size. Consider the atoms in everyday solid matter—a brick, for instance—as Ping-Pong balls. On that scale each atom's nucleus would be like a grain of talcum powder in the center of the (continued)