a much larger scale. The shimmering glows of auroras in the sky near the North and South poles are plasmas as well, created when the Sun's solar wind—itself a plasma—interacts with charged particles in our upper atmosphere.

Plasmas in space react to changes in temperature and pressure in the same way that a balloon full of gas does on Earth. If you leave a balloon in the hot sun, its internal pressure increases as the gas molecules inside move faster. As a result, the balloon expands. In a refrigerator or freezer, the balloon shrivels as molecules slow down and gas pressure decreases. Similarly, the internal pressure of a star must maintain a constant balance. If it doesn't, the star will either collapse or expand, but in a much more dramatic fashion than our balloon. The critical factor is the amount of outward pressure produced by fusion of the nuclear fuel in the star's core. If the energy production within a star changes even slightly, dramatic evolution occurs.

Most stars fuse their hydrogen happily for billions of years before they run out of fuel. Our own Sun has a comfortable life expectancy of about 10 billion years. But the most massive stars live fast and die young. Their inward gravitational forces are so strong that nuclear fusion must proceed at a breakneck pace to support the stars' weights. Their stores of hydrogen dwindle in just a few tens of millions of years, a cosmic wink of an eye. Contractions in the inner layers then increase the internal pressures enough for other elements to fuse into heavier offspring: Helium creates carbon, carbon begets oxygen and neon, oxygen forges silicon, and silicon sparks iron. Iron is the end of the line, since its fusion absorbs rather than releases energy. The energy output from the star's core then drops precipitously, and the outer layers rush inward. Within seconds the core collapses and unleashes a stupendous shock wave that explodes the rest of the star.

These blasts are called supernovas. They happen about once a century in our galaxy and once a second in the universe as a whole. A typical supernova spews 100 times more energy into the cosmos in its first 10 seconds than the Sun will emit in its entire lifetime. This frenzied burst of energy triggers an orgy of alchemy. The doomed star's elements combine with a blizzard of free protons and neutrons to create cobalt, copper, gold, uranium, and the other heavy elements. The next time you glance at your wedding band or swallow that zinc supplement, offer thanks to some star that exploded in our galactic neighborhood long ago.

As generation after generation of massive stars blow up and scatter their atoms into space, the percentage of heavy elements in the gas clouds of a galaxy increases. In other words, stars relentlessly convert the universe's original supply of hydrogen and helium into the rich panoply of elements that fill out the periodic table. But don't worry; the cosmos won't run out of stellar fuel anytime soon. Even after 13 billion years, stars have burned much less than 1 percent of the primordial hydrogen and helium. New stars will continue to form for hundreds of billions of years. However, the rate of starbirth will gradually decrease as the universe grows older and matter spreads out.

Astronomers can chart subtle changes in stellar ingredients to learn how our Milky Way galaxy has evolved. Newborn stars have the highest proportion of heavy elements because they arise from matter already burned and expelled by many other stars. On the other hand, old stars that formed early in the galaxy's history consist of hydrogen, helium, and very little else. We can detect these elemental signatures as faint patterns imprinted upon the rainbows of light from stars when we examine them with spectrographs.

Surveys of the Milky Way show that the youngest stars reside in the galaxy's gas-rich spiral arms. There, density waves—the ones akin to stop-and-go traffic on our highways—trigger ongoing bursts of starbirth. The Orion Nebula and its quartet of huge baby stars, called the Trapezium, inhabit a spiral arm of the Milky Way next to our own. The oldest stars live in globular clusters, swarms of perhaps 100,000 suns that surround the center of the galaxy like moths around a street lamp. These stars are among the most primitive objects known, with ages rivaling that of the universe. We have yet to spot any stars that contain no heavy elements at all. If we see such objects, we will have found survivors from the first generation of stars after the Big Bang itself.

The creation of heavy elements inside stars has another important consequence for the cosmos: It allows rocky planets to form (page 95). Planetary systems appear to be a natural byproduct of starbirth. Smaller clumps of gas and dust collapse into planets within the thick disks that surround baby stars. However, planets around the earliest stars could draw only from hydrogen and helium. Those first planetary systems thus may have contained bodies like Jupiter but no Earths. Only after generations of massive stars had exploded in nearby parts of the galaxy could rocky planets (continued)