| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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skywatcher A Star Is Reborn
From our ever-changing world, the stars that hang cold and bright in the sky each evening appear to be paragons of permanence. Through years of flood and drought on Earth, plentiful harvests and famines, and even the rise and fall of civilizations, starlight lingers in the human mind as constant, familiar, and unchanging. But nothing, stars included, lasts forever. Over eons extending far beyond the lifespans of humans and even planets, stars are born, burn bright and hot, and then transform themselves into some of the strangest, most spectacular objects in the cosmos. During their elaborate transformations, the stars act as alchemists, forging the different elements deep in their fiery hearts. And when certain kinds of stars get old, they scatter these elements to the cosmic winds, seeding the universe with the stuff from which worlds are made. Like the soothsayers that were once present at a child’s birth, modern astronomers can foretell the life story of a star almost at the moment of its conception. But instead of reading palm lines and earlobe crinkles, scientists examine a star’s mass to prophesy its destiny. At roughly a million times the volume of the Earth and about 332,000 times its mass, the Sun—an average-sized star—is a vast aggregation of hydrogen gas, the lightest, simplest element of them all. But the Sun is so big that the combined weight of its gases crushes down on its center. As any swimmer knows, water pressure increases the deeper one dives. An ocean of gas has the same effect. The air in Earth’s atmosphere pushes down on our bodies at a consistent 14.7 pounds per square inch. Now compare that to the center of the Sun. Beneath more than 400,000 miles of hydrogen, the pressure in the center of the Sun is about 4.4 trillion pounds per square inch! These conditions are just right to cook up another element. The enormous pressure creates a temperature in the Sun of more than 14 million degrees Fahrenheit. That’s hot enough to make hydrogen atoms collide so forcefully that they may stick together, or fuse. The fusion of two hydrogen atoms transforms them into a helium nucleus. The extreme density of the Sun’s center ensures that this interaction happens often enough to be sustained for billions of years. This reaction, called thermonuclear fusion, converts a small amount of the hydrogen into energy, which radiates as heat and light. But given the vast size of the Sun, “small” is a relative term. Every second, the Sun converts 4 million tons of hydrogen into energy, equivalent to the output of 2.5 billion major power plants over the course of a year. Though it guzzles hydrogen gas at a prodigious rate, the Sun still has enough fuel on board to continue burning for a good long time (when astronomers speak of “burning” in stars, they are referring to fusion). Based on the mass of fuel that the Sun started with, astronomers have estimated that it can sustain thermonuclear fusion for a total of 10 to 12 billion years. Given the age of the Earth, and assuming that the planets formed about half a billion years after the Sun, scientists agree that our mother star has enough gas left to keep shining for another 5 billion years. During this time, the Sun’s own gravity will be constantly trying to make the star collapse in on itself. But while its fires still burn hot, the energy created by thermonuclear fusion is strong enough to counteract this force. Stars in this stable, hydrogen-burning “adulthood” are called main sequence stars. The 10-billion-year standoff will finally end when the supply of hydrogen in the Sun’s core has transformed into helium. Running low on fuel, the core will produce less energy, gravity will gain the upper hand, and the star’s core will collapse inward. Ironically, the pressure of the collapse increases the core’s temperature to roughly 180 million degrees Fahrenheit—high enough to fuse helium. Helium fusion not only requires a higher ignition temperature than hydrogen fusion, but also releases more energy. The resurgent stream of energy will cause the Sun’s shell to balloon outward, swallowing up Mercury, Venus, and perhaps even Earth. The Sun will then enter its next major life stage—its “old age”—as a red giant glowing a deep orange-red. Its core now burns helium and fuses the atoms into carbon, the basis of all Earthly life-forms. At the same time, the higher temperature of the core ignites a layer of hydrogen surrounding it, forming a hydrogen-burning shell. When the core’s helium runs low, it may start fusing carbon and forming oxygen, while a new helium-burning shell will form just inside the hydrogen shell. Over time—if the star is massive enough for the process to continue this far—shells of progressively heavier elements build up around the core, with the heaviest elements on the inside. When the amount of energy streaming outward builds to a critical point, it will expel the outermost shell of gas in a great burst. Once believed to be shaped like spheres, such planetary nebulae were named for their resemblance to the disks of planets as seen through telescopes. However, recent research using the Hubble Space Telescope has shown that the gases shoot away from the star in two jets pointing in opposite directions. Planetary nebulae such as the Ring Nebula in Lyra are now thought to be cylindrical; observers from Earth are just viewing them from one end. Left behind is the core of the star, compressed to an object about the size of Earth and yet containing about as much mass as the Sun. In this state, the star’s atoms are crammed so tightly together that their electrons have no room to move. The star’s core has now become degenerate matter. The word “degenerate” has different meanings in mathematics and astrophysics than it does in psychology, and here it refers to atomic structure rather than moral character. Since degenerate matter doesn’t exist anywhere on Earth, scientists can only estimate its density, which may be as much as one ton per cubic centimeter. The star’s naked core is now called a white dwarf; its condensed degenerate matter will contain up to 1.4 times the Sun’s mass. Never generating any new energy, it will slowly radiate its residual heat until it cools into a black dwarf—a dark cinder of its former radiant self. Stars much more massive than the Sun live fast, and die young. Consuming their fuel faster than our yellow orb, they may flame out into stellar old age in as little as 10 million years. As in the Sun, the cores of these behemoths create helium from hydrogen, and progress to synthesize carbon and oxygen. But because their furnaces burn so hot, they can work their way through the periodic table to neon, magnesium, silicon, sulfur, argon, calcium, titanium, and chromium. Just how far this process goes depends on the star’s mass. Stars with less mass than the Sun can only produce helium, while the most massive stars forge elements up to iron. Fusion’s “stop sign” is made of iron, which absorbs rather than releases energy. When iron in a star’s core builds up to more than 1.4 times the Sun’s mass, the outflow of energy gushing from the star’s center shuts off. The core implodes catastrophically in a few hours or less, collapsing into a body about 10 kilometers in diameter, the size of a small city. This collapse overcomes even the resistance of degenerate electron matter, and protons and electrons are fused into neutrons. The result is a core made of solid neutrons, called a neutron star. A cubic centimeter of a neutron star could weigh 100 million tons. The violent compression of the core generates a shock wave that rips the star’s outer envelope away in a titanic explosion known as a supernova. Within it, neutrons combine with the atomic nuclei already present in heavy atoms that have built up around the core. This creates even heavier and rarer elements, such as gold, platinum, and uranium, which are blown into space by the supernova. Still larger stars face a darker fate. Those whose collapsed cores are more than five times the mass of the Sun implode with such force that the collapse isn’t stopped by neutron degeneracy. The star that once burned so brightly compresses into a black hole. But in the throes of its creation, it too liberates its heavy atoms, seeding space with the building blocks of new stars and planets. Early in 2002, astronomers using the Chandra x-ray telescope announced the possible detection of a new type of star denser than neutron stars but not dense enough to become black holes. Dubbed quark stars, they are believed to be composed entirely of quarks, the fundamental subatomic particles that make up protons and neutrons. Such quark stars would be so dense that a teaspoonful would weigh billions of tons. Yet not all heavy elements have such violent births. The existence of a slower, quieter element-building process was confirmed in 2000, when astronomers discovered three stars in the Milky Way that contain lead—an element much heavier than iron. The presence of lead supports the theory that certain very old stars between 0.8 and 8 solar masses may create heavy elements beyond iron just before they expel their planetary nebulae. Over millions of years, the material expelled by stars gradually combines to form great molecular clouds. If these clouds, or nebulae, accumulate hundreds of thousands of times the mass of the Sun, a new cycle of star-birth can take place. As the gravity of a dense clump in the nebula slowly attracts more and more matter to itself, its mass grows, attracting still more material in a snowballing process. Eventually, the clump compresses into a smaller and smaller space. As it does, the central mass forms a hot protostar. The rest collects into a dusty, spinning cocoon that surrounds the star and then flattens into a disk. The Hubble Space Telescope has photographed dozens of these protoplanetary disks, or proplyds, within wintertime’s Orion Nebula. Proplyds may eventually condense into solid, metal-rich bodies such as planets (to astronomers, “metals” are virtually anything heavier than helium). But not all are destined to become solar systems. The energy of already-formed stars in the same neighborhood may evaporate them away before planets have a chance to form. In some cases, the proplyds remain embedded in dense portions of a nebula, which is slowly evaporating, whittled away by the energy of nearby stars. These portions end up resembling gaseous tendrils reaching out from a dark, cloudy mass. Astronomers dubbed these evaporating gaseous globules, or EGGs, after their tendrils were first photographed by the Hubble telescope inside the Eagle Nebula in the constellation Serpens and the Trifid Nebula in the constellation Sagittarius. These images also show that stars tend to form clusters containing hundreds or even thousands of stars of the same age but different masses. This gives astronomers an excellent opportunity to make side-by-side comparisons of how stars of different masses grow old. As the stars of each generation forge new elements out of their own hydrogen, they also recycle elements that were made by earlier stars, gathering them into disks of orbiting matter that eventually solidify into particles of dust and bodies of rock, complex gases, or ice. The atoms that make up planets such as Mars, Venus, and Earth—and everything on them, from rocks and soil, to the ink and paper of this magazine, to you—were formed inside distant stars billions of years ago. In fact, our solar system couldn't have formed without the heavy-metal legacy of previous stars.Although astronomers don't know exactly how many generations down the line our Sun is, it definitely isn’t one of the older stars around. Astronomers tell us that in its early stages the Universe was too hot for matter to exist. Then, by the end of the first couple of hundred thousand years, it cooled to about 5,400 degrees F - a temperature low enough for atomic nuclei to finally capture electrons to form the first atoms. Only the lightest and simplest elements such as hydrogen (the most plentiful element around), helium, and a small amount of lithium emerged from the Big Bang. This means that there wasn’t anything out of which to make terrestrial planets. Several generations of massive stars had to pass, and enough matter had accumulate into the thousands of gaseous nebulae that we see adorning the disks of the Milky Way and other spiral galaxies. Recently, astronomers have hinted that the rate of star formation that we currently observe is only a trickle of what it used to be. Analyzing the Deep Field images taken by the Hubble telescope, the most distant images of the Universe yet obtained, they determined that 90 percent of the light that they should be seeing in these images just isn’t there. This suggests that the light is contained in objects too far away for the telescope to see. If Hubble’s eyes could see far enough, some astronomers say, it would find a Universe ablaze with this missing light. This would be the light of star formation that took place when the Universe was perhaps between a few hundred thousand and a billion years old. Since it constitutes 90 percent of the light that should be seen, this suggests that star formation in the very early Universe may have taken place at a rate about ten times faster than is seen today. With the recent upgrade of Hubble’s instrumentation by space shuttle astronauts, and the planning now underway for the Next Generation Space Telescope, astronomers hope to start looking still deeper into the Universe—and back in time to the awesome flurry of celestial fireworks that gave birth to the first stars. Bing F. Quock is Assistant Chairman of the Morrison Planetarium at the California Academy of Sciences. bquock@calacademy.org |
