Violent agents of cosmic change

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Peter Challis is a big bear of a guy. Sitting in the air-conditioned computer room at the Cerro Tololo Inter-American Observatory headquarters in La Serena, Chile, Pete is wearing his "Center for Astrophysics" T-shirt for the third day in a row, cargo shorts, and sneakers. He looks like he just stepped off an Ann Arbor Softball diamond. It's evening and the lights of the coastal city scintillate down below. Pete isn't looking. His attention is riveted to his computer screen Pete is making judgment calls on what he sees there.




Pete is sifting through images of distant galaxies, searching for supernovae as carefully as a prospector looks for the flash of gold in his pan. Brian Schmidt's fancy software has picked out candidates, but not all of them are real stars. Not even most. More like 1 in 10. Somebody has to sift the gravel from the gold. That would be Pete. The pressure is on because the high-z team Pete is playing for tonight needs some supernovae right now. Alex Filippenko is in the air, flying from Berkeley to Hawaii to oUserve at the Keck telescopc tomorrow night.. He'll vibrate to destruction without some targets to work on. I've promised lo provide supernova positions to the control center at. the Space Telescope Science Institute by Tuesday, jusi 60 hours from now. They will proceed with our plan, but if Fete doesn't find some supernovae very soon, the world's most expensive telescope will observe fields without super-novae in them. Bruno Leibundgut has time fin a monster 8-meter telescope at the European Southern Observatory, up in the north of Chile, starting in 22 hours. He won't have much fun if we don't have supernovae.

An hour later, Pete's perseverance furthers our cause.

"Bingo! We got onei"

Pete's colleagues look up briefly from their computer screens in the flat fluorescent light of the computer rtx)m.

"You buy the next pizza," Nick Suntzeff says.

One supernova is good, but they need ihree more by morning. The only way to find them is to grind on through the night. Last night's images are gigabytes spinning on the disks, full of false alarms and a few real nuggets.

Pete keeps looking.

The universe has been changing very slowly over time, so slowly that asking your grandmother to tell you what she remembers from her childhood doesn't help to understand the aging of stars, the accumulation of heavy elements, or cosmic expansion. Supernova explosions are the exception. These violent events play out on the human timescale of days, months, and years. But even if we don't see cosmic change any more clearly than a mayfly sees a redw(X)d age, the whole universe is changing. On the microscopic scale, the atoms that make up the stars and gas of the universe have grown more complex over time as stars fuse light elements into heavier ones to fuel their brilliance. When stars explode as supernovae, the wreckage expels fresh products of nuclear fusion into the gas between the stars.

On the big scale, galaxies mark cosmic expansion. Pete Challis is looking for evidence of this—he is looking for supernovae lialf-way across the universe to see how cosmic expansion has changed since the light was emitted from those distant explosions. Supernovae work well for measuring cosmic distances, but you wouldn't want to use a measuring rod you don't understand. For a long time, Pete has been part of a team trying to learn what supernovae are and how they work. The roots of these investigations go right back to the beginning of modern astronomy.

How do we know which atoms are present in the shreds of a distant star and how do we learn about motion in the universe? This is routine stuff now, but in 1835, authorities thought these things were not knowable. The French philosopher Auguste Comte declared:

On the subject of stars, all investigations which are not ultimately reducible ro simple visual observations are . necessarily denied to us. While we can conceive of the possibility of determining their shapes, their sizes, and their motions, we shall never by any means be able to study their chemical composition . I regard any notion concerning the true mean temperature of the various stars as forever denied to us 1

Scientists love ro quote Comte, because precisely at the time when he was making these pronouncements, the chemistry and the temperatures of stars came into the grasp of astronomy. Comte illustrates the hazards of declaring which aspects of the physical world lie beyond understanding. The zone of the unknowable has been shrinking. In the 1800s, the shrinking realm was the nature of stars; in the 1900s, the shrinking realm was the nature of the universe at large; today, the shrinking realm concerns the first and last, moments and true contents of the universe, which are emerging from pure speculation into the world of observation.

Since 1704, when Newton published his Optiks, physicists had been clear on how to split sunlight, using a prism to form a rainbow from white light. In 1814, Fraunhofer, an optics manufacturer in Munich, used a more elegant spectroscope than Newton's to see thai the spectrum of sunlight was not a continuous rainbow of color from blue to red. There were some narrow gaps in the spectrum— missing colors in the rainbow. The places where there is no light hold the key lo unraveling the mystery of cosmic chemistry. Like detectives, astronomers gather evidence to build a picture of past events. Spectra are the fingerprints that identify elements.

A prism or grating spreads light from a star into the colors of the rainbow. The scientist's job is to take something beautiful and turn it. into a graph. We plot the amount of light at each color Cor wavelength) of light. What Newton didn't see, but Fraunhofer did, are the dark lines or gaps in the spectrum. The dark lines in a stellar spectrum become sharp dips in a graph and bright lines form sharp peaks in a plotted spectrum. These unique patterns identify chemical elements. For example, if you take the element calcium—found in chalk, cheese, and bones—and heat it up as Bunsen did in his burner, it gives off light at very specific wavelengths. If you see those lines, you know you are looking at calcium atoms.

Just as in the curious incident of the dog in the nighttime, we solve the mystery of the chemistry of a distant star by paying attention to places where the spectrum does nothing.2 Calcium in a star's atmosphere absorbs light at exactly the wavelengths where calcium atoms in a terrestrial lab emit their light. Spectroscopy lets us reach across the light-years to measure the chemical composition of distant objects.

Applying spectrum analysis to the stars, beginning in the 1850s, produced a deep change in astronomy. To capture that idea, the new journal started by the American Astronomical Society in 1899 was called The A str(physical Journal—in 1899, "astrophysics" meant precisely the application of spectrum analysis to astronomy. Today "astrophysics" is just a more forbidding synonym for astronomy—if an airline seats you next to a garnilous stranger and you don't want to talk, you tell them you're an astrophysicist and that usually shuts them up. If that doesn't work, you tell them you are a physicist. That always stops the conversation. On the other hand, if you're feeling expansive and you do want to chat, you tell them you're an astronomer. "Oh really, an astronomer? I'm a Leo."

The subatomic world is grainy in a way that, the world of everyday objects is not. Near the positively charged nucleus, the energy of electrons is constrained to certain discrete values. It's like an elevator—you can get on and off at the floors, but not in between. Electrons take quantum leaps between states that correspond to different floors. The spectrum of an atom is set by the energy steps violent agents of cosmic change violent agents of cosmic change

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Figure 2,1. Galaxy Spectra. Astronomers take the light from a galaxy and spread it into a rainbow Then they construct a graph as shown at the top and the bottom The galaxy spectra at the top of the rainbow have absorption lines, those near the bottom have emission lines that come from gas clouds whose atoms are excited by the ultraviolet light from stars. Courtesy of Barbara Carter, Harvard-Smithsonian Center for Astrophysics (Also see color insert)

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Figure 2,1. Galaxy Spectra. Astronomers take the light from a galaxy and spread it into a rainbow Then they construct a graph as shown at the top and the bottom The galaxy spectra at the top of the rainbow have absorption lines, those near the bottom have emission lines that come from gas clouds whose atoms are excited by the ultraviolet light from stars. Courtesy of Barbara Carter, Harvard-Smithsonian Center for Astrophysics (Also see color insert)

between those grainy states—a hydrogen atom can absorb or emit only photons whose energy is tile energy difference between one level and another. The observed spectrum of a star depends on the internal workings of these liny systems.

By understanding the structure of atoms and mastering the counterintuitive rules of quantum mechanics, pioneering astrophysicists transformed the empirical world of astronomical spectra, compiled in giant catalogs, into a powerful tool for analyzing the physical universe.

This is not just qualitative knowledge, hut quantitative, too. Wc know how much of cach clement is present in a typical star's atmosphere. The simplest elements, hydrogen and helium, are by far the most abundant. The next most abundant elements, carbon and oxygen, arc 10,000 times rarer, and all the elements beyond helium taken together add up to only about 1 pcrcent of the mass of a star. In the distant past, the complex atoms were even less abundant— the universe has grown richer in heavy elements over time. Sccond-and third-generation stars such as the sun inherited the family silver from their ancestors. Also the family carbon, calcium, and iron.

Stars are balls of gas, where outward pressure from hot gas in the interior balances the inward pull of gravity. Each star emits light at its surface, and the energy books must balancc, too. If a star didn't replacc the energy that it radiates away, it would shrink and wink out in just a hundred million years. In the middle of the 1800s that cooling time, 100,000,000 years, was the conventional lifetime of the sun. When Lord Kelvin, a prominent theoretical physicist, articulated this argument for the limited duration of the sun in 1862, the message was so clcar and powerful that it intimidated Charles Darwin.5

The first edition of Origin of Species estimated the age of the Earth, based on geological erosion, at 300,000,000 years. Awed by the power of thcorctical physics, which showed this long timescale was not consistent with the sun's lifetime, Darwin omitted his discussion of timescales from later editions and left open a serious question. Had there been enough time for his proposed natural selection to operate? Arguments from fundamental physical theory arc often asserted in a loud voicc with a grave tone of authority, and Lord Kelvin's pronouncements were definitely not the last occasion of this phenomenon. But what Lord Kelvin could not know was that the subatomic world discovered just at the beginning of the 1900s produces both a reliable clock for measuring the age of the Earth and a stupendous and durable source for stellar energy.

Wc now know the Earth is much older than Lord Kelvin declared or than Darwin estimated from the wearing down of land-forms. Our clock is the very slow, but extremely steady, accumulation of radioactive decay products in rocks as one nuclcus changcs into another. Nuclear forces are much stronger than the electrical forces that determine the height of mountains or the bouncc in baseballs. Hven extraordinary variations in temperature or pressure don't affect the rate of change among the neutrons and protons of a nuclcus. As a nucleus emits subatomic particles in radioactive decay, it can become another element. Radioactive uranium becomes stable lead. From the relative abundances of the parent and daughter nuclei we accumulatc cvidcncc that the Harth and the solar system are almost 5 billion years old. Danvin can relax in his grave. There has been plenty of time for natural selection to operate. From the fossil rccord in sedimentary rocks wc know that life started simmering along at the singlc-ccll level 3 billion years ago, and began burgeoning 600,000,000 years ago—the sun has been steadily shining for a much longer time than Lord Kelvin supposed and a good thing, too, because complex life took a long time to evolve here on Larth.

In the 1920s astronomers spcculatcd about the origin of the sun's energy, but their estimates of stellar lifetimes were handicapped by the rudimentary state of nuclear physics. The energy source for the sun is nuclcar fusion in the hot, dense core of the star. But it is a subtle chain of transformations. Deep in the sun's core, several steps of nuclear fusion transform four nuclei of the element hydrogen into a single helium nucleus. Since the sun is made mostly of hydrogen, fusion has an ample source of fuel. Unlike ordinary cooking, the mass of the assembled helium is less than the mass of the ingredients. The balancc shows up as energy according to a very well-known (but not so widely understood) equation: E = mc\

More quantitatively, fusing 4.000 kilograms of hydrogen produces 3.972 kilograms of helium. Hinsteins equation says you get to exchange the missing 0.028 kilogram into energy at the going rate, which is c*. Bccausc c is so big and c2 immense (1017 joules of energy for every kilogram of mass), the energy release from nuclear fusion is astonishing. At currcnt rates charged for electricity, pure energy has a street value of $1 billion per kilogram. Ordinary chemical reactions rearrange electrons in the outer parts of atoms, which are boLind to the nucleus by clectrical forces. The energy release in a candle ultimately comes from electrical forces. But nuclear reactions comc from rearranging neutrons and protons in the nuclei of atoms which are 10,000 times smaller than atoms. The powerful forccs acting on that tiny scale arc larger: the energy released in nuclcar changc is typically a million times the energy released in chemical changc.

Now that astronomers understand the sun's structure and composition and know how nuclear fusion yields energy, we can predict the future of the sun. We use the same authoriative tone of voicc as Lord Kelvin, but this time with better understanding. The sun has ample supplies of hydrogen for another 5 billion years of steady fusion. This provides a useful upper limit to the duration of long-term financial investments,

Eventually, the accumulated ashes of hydrogen fusion, helium nuclei, begin to make a diffcrcncc to the structure of a star. As you combine four hydrogen nuclei into a single helium nucleus, fewer particles barge around in the star's core to provide the gas pressure that balances gravity. A star needs to balance out the internal forces that make a star expand or shrink. About 10 billion years after it formed, that is, 5 billion years from now, the sun will adjust by swelling up to become a luminous but cool red giant star, with a diameter 100 times larger than it has today. Seen from the Earth, the sun will cover almost half the sky. The sun's florid old age will not be a pleasant era for earthlings, if there are any, 5 billion years in the future, because the Earth will heat up, what's left of the oceans will boil, first cooking all the lobsters, then melting the rocks, and eventually evaporating our favorite planet.

Our sun's elder brothers, stars similar to the sun but formed earlier in the history of our Milky Way galaxy, have already had enough time to become red giants. We see red giants of a little less than one solar mass in globular clusters, great clusters of 100,000 stars in our galaxy, with all the stars of very nearly the same age. Based on our understanding of the timescalc for fusion in stars, these globular clustcr stars must be about 12 billion years old. Globular cluster stars formed out of the ambient gas in our galaxy at that time. Spectra of globular cluster stars testify to the changc in the chemistry of our galaxy since these stars formed Old stars of our violent agents of cosmic change 23

violent agents of cosmic change 23

Figure 2.2 The Globular Cluster NGC 6093 A globular cluster contains many thousands of stars that formed at the same time^ early in our galaxy's history. By measuring the properties of stars that have recently become red giants {visible in the color image as reddish, bright stars in the cluster) the age of the cluster can be inferred. The oldest globular dusters have ages of 12 ± I billion years Courtesy of NASA and the Hubble Heritage Team {STScl/AIJRA). (Also see color insert)

Figure 2.2 The Globular Cluster NGC 6093 A globular cluster contains many thousands of stars that formed at the same time^ early in our galaxy's history. By measuring the properties of stars that have recently become red giants {visible in the color image as reddish, bright stars in the cluster) the age of the cluster can be inferred. The oldest globular dusters have ages of 12 ± I billion years Courtesy of NASA and the Hubble Heritage Team {STScl/AIJRA). (Also see color insert)

galaxy have only about 1/100, or in extreme cases 1/1000, the iron abundance of the sun. Something important happened between the time when the first globular cluster stars formed, about 12 billion years ago, and the time when the sun formed, about 5 billion years ago. The galaxy, anemic at first, is now rich with iron and all the other elements heavier than helium.

Red giant stars in globular clusters do not come stamped with the date of their manufacture, but practitioners in the an of determining stellar ages think the precision of this measurement for the oldest stars in our galaxy is about 1 billion years. They are willing to bet $2 to win your SI that they are right within a billion years. That's a lo Cone Greek sigma) result. Based on the statistics of the bell-shaped curve of probabilities worked out by the mathematician

Karl Friedrich Gauss, students of globular clusters should be willing to bet 20 to 1 that they are right within 2a, 2 billion years. Gauss assessed the probability of getting a spurious result by chance. Rare things happen, but they don't happen very often. Gauss tells the globular cluster experts they should be willing to bet 370 to 1 that they are right within 3o, 3 billion years. If you believe Gaussian statistics, you should be willing to bet your goldfish (4o), your house (5o), or your dog (6o). In astronomy, knowing the uncertainly in a measurement can be as important as knowing the number itself because it tells you how much confidence to place in it. For important measurements, we try to give both the value and its to uncertainty. Hut nobody really believes the statistics enough to risk their bull terrier! For the ages of the oldest stars, we write 12 ± 1 billion years, with the "± 1" intended to rcflcct the lo odds that the true answer has a different value through nobody's fault—that is, just by chance. Uncertainty is not a gocxi thing, but knowing the uncertainty is. It keeps you from arrogancc when the data are poor and gives you courage when it is warranted by the facts.

As the sun swells up to become a red giant, the energy source for the sun will shift from fusing hydrogen into helium to an elegant stage of pcrfcct recycling where helium, the waste product of hydrogen fusion, bccomcs the next fuel. There is no stable nudcus with five particles. This fact of subatomic physics means there's no simple way to turn helium (which has four particles in its nucleus: two neutrons and two protons) into the next element by banging one proton into a helium nucleus. They just don't stick. So stars have trouble making the next elements, lithium, beryllium, and boron, out of helium. Instead, red giant stars skip across that gap, as improbably as crossing a stream by stepping on a salmon, to fuse three helium nuclei into a single carbon nucleus. (Carbon has 12 particles in its nucleus: 6 neutrons plus 6 protons, made from 3 helium nuclei with 2 neutrons and 2 protons apiece.) In addition, carbon and helium will fuse to make oxygen in the sun when it is a red giant.

This remarkable stage in stellar energy generation explains important astronomical phenomena through subtleties of nuclear physics. Fred I loyle proposed it and Edwin Salpeter elaborated it in the 1950s.'1 In 1997, these two received the Crafcxird Prize for this work from the hands of the King of Sweden amid trumpet blasts in Stockholm. At the dinner, the King and the prizewinners were at the center, and the guests spiralcd outward in order of importance. My fiancee Jayne Loader, was promoted toward the royal center to balancc out the dearth of women among the academicians. In the outermost circle, I sat with Fred Hoyle's teenage granddaughters. I told them I was an astronomer. A Leo, actually.

After a star has made carbon and oxygen by this prizewinning process, there is still more nuclear energy ro squeeze out of fusion, all the way up to iron with 56 nuclear particles. But the sun will not burn its carbon and oxygen. Only more massive stars, typically 10 times the mass of the sun, can do a thorough job of extracting all the energy from nuclcar fusion.

Iron is the most tightly bound nucleus. Stars cxtract energy from nuclear fusion by building up heavy nuclei from light ones all the way up to iron. This makes iron the end of the road for fusion, but iron is by no means the most complicated nucleus in nature. Lead and gold and uranium are all more elaborate elements whose nuclei have more neutrons and protons than iron does. Uranium-238 has 92 proteins and 146 neutrons, far beyond the total of 56 baryons for iron. Power reactors on the Earth release nuclcar energy from fission—by splitting uranium nuclei into smaller picces. In this case, the combined mass of the smaller picccs is less than the mass of the uranium you started with. The balance is exchanged for energy at the usual extravagant rate. So you can get energy from fusing together light nuclei up to iron and you can get energy from fission by breaking up bigger nuclci down to iron. Iron itself is the nuclear turnip out of which no more blood can be squeezed.

These details of nuclear physics affect the way stars generate energy, and they also affect the chemistry of our galaxy and of every galaxy. Lithium, beryllium, and boron arc rare elements throughout the universe. They are formed by heavier elements that break up when they are whizzing through interstellar space as cosmic rays. These rare light elements are the ones skipped over by stars when they fuse helium into carbon. Carbon and oxygen are a million times more abundant. Everybody has seen carbon in graphite or coal or diamond. And carbon is the basis for the chemistry of life— at least here on Earth. Diamonds may be a girl's best friend but your best girlfriend is carbon.

Stars make the elements in accord with microscopic rules set by nuclear physics. Carbon-based life-forms like us are made of stardust whose composition is determined by subtle details of furious nuclear collisions in the centers of stars. Sometimes people look to the stars for our origins—in this very literal sense, we did come from out there. Bur not in shiny saucers. Wc arrived atom by atom in the gas and dust that formed the solar system 5 billion years ago. The carbon nuclei incorporated into the base pairs of your own DNA were synthesized in the fiery hearths of red giants before the sun formed.

Like loyal alumni, successive generations of stars have donated their atoms to the chemical endowment of our galaxy. While globular cluster stars had to make do with the thin gruel of the early galaxy when they formed, the sun, formed about 7 billion years later, inherited heavy elements from stars that vanished long ago.

After a brief but glorious 1 billion years as a red giant, the sun will begin to puff off its outer atmosphere, while its core hunkers down under the relentless force of gravity to become a dense white dwarf star, about the size of the Earth. During the transition, the star and its departing gas form a beautiful "planetary nebula"—an object that resembled a planet when seen in early telescopes. White dwarf stars have a small surface that doesn't emit much light, and we can see them only when they are quite nearby, as in the case of Sirius B (the white dwarf flea that accompanies the brightest star we see from Earth, Sirius, the dog star). A white dwarf is held up by quantum forces between electrons, not by gas pressure. This "degeneracy pressure" can support a white dwarf, even as it cools into invisibility. But the quantum mcchanical support for a white dwarf is overwhelmed by gravity at a sharp upper mass limit of 1.4 solar masses. This upper mass for a white dwarf was worked out by Subrahmanyan Chandrasekhar fa suitably astronomical name: Chandra means "moon" in Sanskrit), and is known as the Chandrasekhar limit.

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