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.