THEFIRST STARS IN THE UNIVERSE Exceptionally massive and bright, the earliest stars changed the course of cosmic history

We live in a universe that is full of bright objects. On a clear night one can see thousands of stars with the naked eye. These stars occupy mere- ly a small nearby part of the Milky Way galaxy; tele- scopes reveal a much vaster realm that shines with the light from billions of galaxies. According to our current understanding of cosmology, howev- er, the universe was featureless and dark for a long stretch of its early history. The first stars did not appear until perhaps 100 million years after the big bang, and nearly a billion years passed before galaxies proliferated across the cosmos. Astron- BY RICHARD B. LARSON AND VOLKER BROMM omers have long wondered: How did this dramat- ILLUSTRATIONS BY DON DIXON ic transition from darkness to light come about?

4 SCIENTIFIC AMERICAN Updated from the December 2001 issue COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC. EARLIEST COSMIC STRUCTURE mmostost likely took the form of a network of filaments. The first protogalaxies, small-scale systems about 30 to 100 light-years across, coalesced at the nodes of this network. Inside the protogalaxies, the denser regions of gas collapsed to form the first stars (inset).

COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC. After decades of study, researchers The Dark Ages to longer wavelengths and the universe have recently made great strides toward THE STUDY of the early universe is grew increasingly cold and dark. As- answering this question. Using sophisti- hampered by a lack of direct observa- tronomers have no observations of this cated computer simulation techniques, tions. Astronomers have been able to ex- dark era. But by a billion years after the cosmologists have devised models that amine much of the universe’s history by big bang, some bright galaxies and show how the density fluctuations left training their telescopes on distant galax- quasars had already appeared, so the first over from the big bang could have ies and quasars that emitted their light stars must have formed sometime before. evolved into the first stars. In addition, billions of years ago. The age of each ob- When did these first luminous objects observations of distant quasars have al- ject can be determined by the redshift of arise, and how might they have formed? lowed scientists to probe back in time its light, which shows how much the uni- Many astrophysicists, including Mar- and catch a glimpse of the final days of verse has expanded since the light was tin Rees of the University of Cambridge the “cosmic dark ages.” produced. The oldest galaxies and and Abraham Loeb of Harvard Universi- The new models indicate that the first quasars that have been observed so far ty, have made important contributions to- It seems safe to conclude that the FIRST STARS IN THE UNIVERSE WERE TYPICALLY MANY TIMES more massive and luminous than the sun.

stars were most likely quite massive and date from about a billion years after the ward solving these problems. The recent luminous and that their formation was big bang (assuming a present age for the studies begin with the standard cosmo- an epochal event that fundamentally universe of 13.7 billion years). Re- logical models that describe the evolution changed the universe and its subsequent searchers will need better telescopes to of the universe following the big bang. Al- evolution. These stars altered the dynam- see more distant objects dating from still though the early universe was remarkably ics of the cosmos by heating and ionizing earlier times. smooth, the background radiation shows the surrounding gases. The earliest stars Cosmologists, however, can make evidence of small-scale density fluctua- also produced and dispersed the first deductions about the early universe tions—clumps in the primordial soup. heavy elements, paving the way for the based on the cosmic microwave back- These clumps would gradually evolve eventual formation of solar systems like ground radiation, which was emitted into gravitationally bound structures. our own. And the collapse of some of the about 400,000 years after the big bang. Smaller systems would form first and then first stars may have seeded the growth of The uniformity of this radiation indicates merge into larger agglomerations. The supermassive black holes that formed in that matter was distributed very smooth- denser regions would take the form of a the hearts of galaxies and became the ly at that time. Because there were no network of filaments, and the first star- spectacular power sources of quasars. In large luminous objects to disturb the pri- forming systems—small protogalaxies— short, the earliest stars made possible the mordial soup, it must have remained would coalesce at the nodes of this net- emergence of the universe that we see to- smooth and featureless for millions of work. In a similar way, the protogal- day—everything from galaxies and qua- years afterward. As the cosmos expand- axies would then merge to form galaxies, sars to planets and people. ed, the background radiation redshifted and the galaxies would congregate into galaxy clusters. The process is ongoing: although galaxy formation is now most- Overview/The First Stars ly complete, galaxies are still assembling ■ Computer simulations show that the first stars should have appeared between into clusters, which are in turn aggregat- 100 million and 250 million years after the big bang. They formed in small ing into a vast filamentary network that protogalaxies that evolved from density fluctuations in the early universe. stretches across the universe. ■ Because the protogalaxies contained virtually no elements besides hydrogen According to the models, the first and helium, the physics of star formation favored the creation of bodies that small systems capable of forming stars were many times more massive and luminous than the sun. should have appeared between 100 mil- ■ Radiation from the earliest stars ionized the surrounding hydrogen gas. Some lion and 250 million years after the big stars exploded as supernovae, dispersing heavy elements throughout the bang. These protogalaxies would have universe. The most massive stars collapsed into black holes. As protogalaxies been 100,000 to one million times more merged to form galaxies, the black holes possibly became concentrated massive than the sun and would have in the galactic centers. measured 30 to 100 light-years across. These properties are similar to those of

6 SCIENTIFIC AMERICAN THE SECRET LIVES OF STARS COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC. the molecular gas clouds in which stars stars. The stars with no metals at all—the work of Fumitaka Nakamura and Ma- are currently forming in the Milky Way, very first generation—are sometimes sayuki Umemura (now at Niigata and but the first protogalaxies would have called Population III stars. Tsukuba universities in Japan, respec- differed in fundamental ways. For one, In the absence of metals, the physics tively) has yielded instructive results. All they would have consisted mostly of of the first star-forming systems would these studies have produced similar de- dark matter, the putative elementary par- have been much simpler than that of pres- scriptions of how the earliest stars might ticles that are believed to make up 90 per- ent-day molecular gas clouds. Further- have been born. cent of the universe’s mass. In present- more, the cosmological models can pro- day large galaxies, dark matter is segre- vide, in principle, a complete description Let There Be Light! gated from ordinary matter: over time, of the initial conditions that preceded the THE SIMULATIONS show that the pri- ordinary matter concentrates in the first generation of stars. In contrast, the mordial gas clouds would typically form galaxy’s inner region, whereas the dark stars that arise from molecular gas clouds at the nodes of a small-scale filamentary matter remains scattered throughout an are born in complex environments that network and then begin to contract be- enormous outer halo. But in the proto- have been altered by the effects of previ- cause of their gravity. Compression would galaxies, the ordinary matter would still ous star formation. Several research heat the gas to temperatures above 1,000 have been mixed with the dark matter. groups have used computer simulations to kelvins. Some hydrogen atoms would pair The second important difference is portray the formation of the earliest stars. up in the dense, hot gas, creating trace that the protogalaxies would have con- A team consisting of Tom Abel, Greg amounts of molecular hydrogen. The hy- tained no significant amounts of any ele- Bryan and Michael L. Norman (now at drogen molecules would then start to ments besides hydrogen and helium. The Pennsylvania State University, Columbia cool the densest parts of the gas by emit- big bang produced hydrogen and helium, University and the University of Califor- ting infrared radiation after they collided but most of the heavier elements are cre- nia at San Diego, respectively) has made with hydrogen atoms. The temperature ated only by the thermonuclear fusion re- the most realistic simulations. In collab- in the densest parts would drop to 200 to actions in stars, so they would not have oration with Paolo Coppi of Yale Uni- 300 kelvins, reducing the gas pressure in been present before the first stars had versity, we have done simulations based these regions, allowing them to contract formed. Astronomers use the term “met- on simpler assumptions but intended to into gravitationally bound clumps. als” for all these heavier elements. The explore a wider range of possibilities. This cooling plays an essential role in young metal-rich stars in the Milky Way Toru Tsuribe, now at Osaka University allowing the ordinary matter in the pri- are called Population I stars, and the old in Japan, has made similar calculations mordial system to separate from the dark metal-poor stars are called Population II using more powerful computers. The matter. The cooling hydrogen would set- COSMIC TIMELINE FROM THE DARK AGES ... After the emission of the cosmic microwave background radiation (about 400,000 years after the big bang), the universe grew increasingly cold and dark. But cosmic structure gradually evolved from the density fluctuations left over from the big bang. 12 to 14 billion years 1 billion years

100 million years 1 million years BIG BANG

Emission of cosmic background radiation Dark ages First stars

First supernovae ... TOTHE RENAISSANCE and black holes Protogalaxy The appearance of the first stars and protogalaxies mergers (perhaps as early as 100 million years after the big bang) set off a chain of events that transformed the universe. Modern galaxies www.sciam.com SCIENTIFIC AMERICAN 7 COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC. THE BIRTH AND DEATH OF THE FIRST STARS

PRIMEVAL TURMOIL The process that led to the creation of the first stars was very different from present-day star formation. But the violent deaths of some of these stars paved the way for the emergence of the universe that we see today.

The first star-forming systems—small 1 protogalaxies—consisted mostly of the elementary particles known as dark matter (shown in red). Ordinary matter—mainly hydrogen gas (blue)—was initially mixed with the dark matter.

Ultraviolet radiation

The cooling of the hydrogen allowed The denser regions of gas contracted Ultraviolet radiation from the stars 2 the ordinary matter to contract, whereas 3 into star-forming clumps, each hundreds 4 ionized the surrounding neutral hydrogen the dark matter remained dispersed. of times as massive as the sun. Some of the gas. As more and more stars formed, the The hydrogen settled into a disk at the center clumps of gas collapsed to form very bubbles of ionized gas merged and the of the protogalaxy. massive, luminous stars. intergalactic gas became ionized.

Supernova

Black hole

A few million years later, at the end of Gravitational attraction pulled the Black holes possibly merged to form a 5 their brief lives, some of the first stars 6 protogalaxies toward one another. 7 supermassive hole at the protogalaxy’s exploded as supernovae. The most massive The collisions most likely triggered star center. Gas swirling into this hole might have stars collapsed into black holes. formation, just as galactic mergers do now. generated quasarlike radiation.

8 SCIENTIFIC AMERICAN THE SECRET LIVES OF STARS COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC. tle into a flattened rotating configuration er is that the cooling from molecular hy- stars. In collaboration with Loeb of Har- that was clumpy and filamentary and pos- drogen becomes inefficient at the higher vard, one of us (Bromm) has recently sibly shaped like a disk. But because the densities encountered when the clumps used numerical simulations to study the dark-matter particles would not emit ra- begin to collapse. At these densities the accretion onto a primordial protostar. diation or lose energy, they would remain hydrogen molecules collide with other The calculations show that a Population scattered in the primordial cloud. Thus, atoms before they have time to emit an III star grows to roughly 50 solar mass- the star-forming system would come to infrared photon; this raises the gas tem- es within the first 10,000 years after the resemble a miniature galaxy, with a disk perature and slows the contraction until initial core forms. Although we could not of ordinary matter and a halo of dark the clumps have built up to at least a few follow the accretion further because of matter. Inside the disk, the densest clumps hundred solar masses. numerical limitations, it is likely that the of gas would continue to contract, and Did the first collapsing clumps form star continues to grow, perhaps to 100 to eventually some of them would undergo stars with similarly large masses, or did 200 solar masses. It seems safe to con- a runaway collapse and become stars. they fragment and form many smaller clude that the first stars were typically The first star-forming clumps were stars? The research groups have pushed many times more massive and luminous much warmer than the molecular gas their calculations to the point at which than the sun. clouds in which most stars currently the clumps are well on their way to form- form. Dust grains and molecules con- ing stars, and none of the simulations has The Cosmic Renaissance taining heavy elements cool the present- yet revealed any tendency for the clumps WHAT EFFECTS did these first stars day clouds much more efficiently to tem- to fragment. This agrees with our under- have on the rest of the universe? An im- peratures of only about 10 kelvins. The standing of present-day star formation; portant property of stars with no metals minimum mass that a clump of gas must the fragmentation of clumps is typically is that they have higher surface temper- have to collapse under its gravity is called limited to the formation of binary sys- atures than stars with compositions like the Jeans mass, which is proportional to tems (two stars orbiting around each that of the sun. The production of nu- the square of the gas temperature and in- other). Fragmentation seems even less clear energy at the center of a star is less versely proportional to the square root of likely to occur in the primordial clumps, efficient without metals, and the star the gas pressure. The first star-forming because the inefficiency of molecular hy- would have to be hotter and more com- systems would have had pressures similar drogen cooling would keep the Jeans pact to produce enough energy to coun- to those of present-day molecular clouds. mass high. The simulations, however, teract gravity. Because of the more com- But because the temperatures of the first have not yet determined the final out- pact structure, the surface layers of the collapsing gas clumps were almost 30 come of collapse with certainty, and the star would also be hotter. In collabora- times higher, their Jeans mass would have formation of binary systems cannot be tion with Loeb and Rolf-Peter Kudritzki been almost 1,000 times larger. ruled out. of the University of Hawaii Institute for In molecular clouds in the nearby Precise estimates of just how massive Astronomy, Bromm devised theoretical part of the Milky Way, the Jeans mass is the first stars might have been are diffi- models of such stars with masses between roughly equal to the mass of the sun, and cult because of feedback effects. In gen- 100 and 1,000 solar masses. The models the masses of the prestellar clumps are eral, a star forms from the “inside out,” showed that the stars had surface tem- about the same. If we scale up, we can es- by accreting gas from the surrounding peratures of 100,000 kelvins—about 17 timate that the masses of the first star- clump onto a central protostellar core. times higher than the sun’s surface tem- forming clumps would have been 500 to But when does this accretion process perature. Thus, the first starlight in the 1,000 solar masses. The computer sim- shut off? As the star grows in mass, it universe would have been mainly ultra- ulations mentioned above showed the produces intense radiation and matter violet radiation from very hot stars, and formation of clumps with masses of sev- outflows that may blow away some of it would have begun to heat and ionize eral hundred solar masses or more. the gas in the collapsing clump. Yet these the neutral hydrogen and helium gas Our group’s calculations suggest that effects depend strongly on the presence around these stars soon after they formed. the predicted masses of the first star- of heavy elements, and therefore they We call this event the cosmic renais- forming clumps are not very sensitive to should be less important for the earlier sance. Although astronomers cannot yet the assumed cosmological conditions. The predicted masses depend primarily RICHARD B. LARSON and VOLKER BROMM have worked together to understand the processes on the physics of the hydrogen molecule that ended the “cosmic dark ages” and brought about the birth of the first stars. Larson, a pro- and only secondarily on the cosmologi- fessor of astronomy at Yale University, joined the faculty there in 1968 after receiving his Ph.D. cal model or simulation technique. One from the California Institute of Technology. His research interests include the theory of star for- reason is that molecular hydrogen can- mation as well as the evolution of galaxies. Bromm earned his Ph.D. at Yale in 2000 and is now not cool the gas below 200 kelvins, mak- an assistant professor of astronomy at the University of Texas at Austin, where he focuses THE AUTHORS ing this a lower limit to the temperature on the emergence of cosmic structure. The authors acknowledge the many contributions of of the first star-forming clumps. Anoth- Paolo Coppi, professor of astronomy at Yale, to their joint work on the formation of the first stars. www.sciam.com SCIENTIFIC AMERICAN 9 COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC. STAR STATS COMPARING CHARACTERISTICS Computer simulations have given scientists some indication of the possible masses, sizes and other characteristics of the earliest stars. The lists below compare the best estimates for the first stars with those for the sun.

SUN FIRST STARS MASS: 1.989 × 1030 kilograms MASS: 100 to 1,000 solar masses RADIUS: 696,000 kilometers RADIUS: 4 to 14 solar radii LUMINOSITY: 3.85 × 1023 kilowatts LUMINOSITY: 1 million to 30 million solar units SURFACE TEMPERATURE: 5,780 kelvins SURFACE TEMPERATURE: 100,000 to 110,000 kelvins LIFETIME: 10 billion years LIFETIME: 3 million years estimate how much of the gas in the uni- years after the big bang. In an important would have ionized helium at the same verse condensed into the first stars, even breakthrough, NASA’s Wilkinson Mi- time. On the other hand, if the first stars as little as one part in 100,000 could have crowave Anisotropy Probe (WMAP) has were not quite so massive, the helium been enough for these stars to ionize measured the fundamental properties of must have been ionized later by energetic much of the remaining gas. Once the first the universe with high precision. These radiation from sources such as quasars. stars started shining, a growing bubble of include the age of the universe—precise- Future observations of distant objects ionized gas would have formed around ly 13.7 billion years—the proportions of may help determine when the universe’s each one. As more and more stars formed dark and luminous matter, and dark en- helium was ionized. over hundreds of millions of years, the ergy in the cosmos. The biggest surprise: If the first stars were indeed very mas- bubbles of ionized gas would have scrutinizing the subtle patterns that were sive, they would also have had relatively merged, and the intergalactic gas would imprinted into the photons of the cosmic short lifetimes—only a few million years. have become completely ionized. microwave background, WMAP has in- Some of the stars would have exploded Scientists from the California Insti- dicated that ultraviolet radiation from as supernovae, expelling the metals they tute of Technology and the Sloan Digi- the first stars ionized atomic hydrogen produced. Stars that are between 100 tal Sky Survey have found evidence for and helium, providing an abundance of and 250 times as massive as the sun are the final stages of this ionization process. free electrons early in cosmic history. Mi- predicted to blow up completely in ener- They observed strong absorption of ul- crowave background photons were po- getic explosions, and some of the first traviolet light in the spectra of quasars larized as they interacted with these elec- stars most likely had masses in this range. that date from about 900 million years trons. An early generation of massive Because metals are much more effective after the big bang. The results suggest Population III stars seems to be required than hydrogen in cooling star-forming that the last patches of neutral hydrogen to account for the surprising strength of clouds and allowing them to collapse gas were being ionized at that time. A the polarization patterns. into stars, the production and dispersal different probe has recently provided Helium requires more energy to ion- of even a small amount could have had clues to the earliest stages of reioniza- ize than hydrogen does, but if the first a major effect on star formation. tion, already occurring only 200 million stars were as massive as predicted, they Working in collaboration with An-

10 SCIENTIFIC AMERICAN THE SECRET LIVES OF STARS COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC. drea Ferrara of the Astrophysical Obser- more massive stars; on dying, these stars that are now found in galactic nuclei. vatory of Arcetri in Italy, we have found would have dispersed large amounts of Furthermore, astronomers believe that when the abundance of metals in metals, which would have then been in- that the energy source for quasars is the star-forming clouds rises above one corporated into most of the low-mass gas whirling into the black holes at the thousandth of the metal abundance in stars that we now see. centers of large galaxies. If smaller black the sun, the metals rapidly cool the gas to Another puzzling feature is the high holes had formed at the centers of some the temperature of the cosmic back- metal abundance of the hot x-ray-emit- of the first protogalaxies, the accretion of ground radiation. (This temperature de- ting intergalactic gas in clusters of galax- matter into the holes might have gener- clines as the universe expands, falling to ies. This observation could be accounted ated “mini quasars.” Because these ob- 19 kelvins one billion years after the big for most easily if there had been an early jects could have appeared soon after the bang and to 2.7 kelvins today.) This ef- period of rapid formation of massive first stars, they might have provided an ficient cooling allows the formation of stars and a correspondingly high super- additional source of light and ionizing ra- stars with smaller masses and may also nova rate that chemically enriched the in- diation at early times. considerably boost the rate at which stars tergalactic gas. This case also dovetails Thus, a coherent picture of the uni- are born. It is possible that the pace of with the recent evidence suggesting that verse’s early history is emerging, although star formation did not accelerate until af- most of the ordinary matter and metals certain parts remain speculative. The for- The formation of the first stars and protogalaxies BEGAN A PROCESS OF COSMIC EVOLUTION.

ter the first metals had been produced. In in the universe lies in the diffuse inter- mation of the first stars and protogalax- this case, the second-generation stars galactic medium rather than in galaxies. ies began a process of cosmic evolution. might have been the ones primarily re- To produce such a distribution of matter, Much evidence suggests that the period sponsible for lighting up the universe and galaxy formation must have been a spec- of most intense star formation, galaxy bringing about the cosmic renaissance. tacular process, involving intense bursts building and quasar activity occurred a At the start of this active period of star of massive star formation and barrages few billion years after the big bang and birth, the cosmic background temperature of supernovae that expelled most of the that all these phenomena have continued would have been higher than in present- gas and metals out of the galaxies. at declining rates as the universe has day molecular clouds (10 kelvins). Until Stars that are more than 250 times aged. Most of the cosmic structure build- the temperature dropped to that level— more massive than the sun do not ex- ing has now shifted to larger scales as which happened about two billion years plode at the end of their lives; instead galaxies assemble into clusters. after the big bang—the process of star they collapse into massive black holes. In the coming years, researchers hope formation may still have favored massive Several of the simulations mentioned to learn more about the early stages of the stars. As a result, many such stars may above predict that some of the first stars story, when structures started developing have formed during the early stages of would have had masses this great. Be- on the smallest scales. Because the first galaxy building by successive mergers of cause the first stars formed in the densest stars were most likely very massive and protogalaxies. A similar phenomenon parts of the universe, any black holes re- bright, instruments such as the James may occur in the modern universe when sulting from their collapse would have Webb Space Telescope—the planned suc- two galaxies collide and trigger a star- become incorporated, via successive cessor to the Hubble Space Telescope— burst—a sudden increase in the rate of mergers, into systems of larger and larg- might detect some of these ancient bod- star formation—producing relatively er size. It is possible that some of these ies. Then astronomers may be able to ob- large numbers of massive stars. black holes became concentrated in the serve directly how a dark, featureless inner part of large galaxies and seeded the universe formed the brilliant panoply of Puzzling Evidence growth of the supermassive black holes objects that now give us light and life. THIS HYPOTHESIS about early star formation might help explain some puz- MORE TO EXPLORE zling features of the present universe. One Before the Beginning: Our Universe and Others. Martin J. Rees. Perseus Books, 1998. unsolved problem is that galaxies contain The First Sources of Light. Volker Bromm in Publications of the Astronomical Society of the Pacific, fewer metal-poor stars than would be ex- Vol. 116, pages 103–114; February 2004. Available at www.arxiv.org/abs/astro-ph/0311292 pected if metals were produced at a rate The First Stars. Volker Bromm and Richard B. Larson in Annual Reviews of Astronomy and Astrophysics, proportional to the star formation rate. Vol. 42, pages 79–118; September 2004. Available at www.arxiv.org/abs/astro-ph/0311019 This discrepancy might be resolved if ear- Graphics from computer simulations of the formation of the first luminous objects can be found at ly star formation had produced relatively http://cfa-www.harvard.edu/~vbromm/

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