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The First Star In The Universe

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					                THE
                          FIRST
                           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          omers have long wondered: How did this dramat-
      AND VOLKER BROMM
     ILLUSTRATIONS BY DON DIXON   ic transition from darkness to light come about?

4   SCIENTIFIC AMERICAN                                                      Updated from the December 2001 issue
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EARLIEST COSMIC STRUCTURE most 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).



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     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
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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       T H E S I M U L A T I O N S 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                                                   ars
                                                                                                                                  billion ye
 evolved from the density fluctuations left over from the big bang.                                                    12 to 14
                                                                                                       years
                                                                                             1 billion

                                                                 on years
                                                     100 milli
                                    years
                        1 million
 BIG
BANG



                     Emission of
                  cosmic background
                      radiation                Dark ages
                                                                 First stars

                                                                               First supernovae
 ... TO THE RENAISSANCE                                                               and
                                                                                                     Protogalaxy
 The appearance of the first stars and protogalaxies                              black holes
                                                                                                       mergers
 (perhaps as early as 100 million years after the big bang) set off
                                                                                                                              Modern galaxies
 a chain of events that transformed the universe.

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                            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.




                                                 1   The first star-forming systems— small
                                                     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




2    The cooling of the hydrogen allowed
     the ordinary matter to contract, whereas
the dark matter remained dispersed.
                                                 3     The denser regions of gas contracted
                                                       into star-forming clumps, each hundreds
                                                 of times as massive as the sun. Some of the
                                                                                                 4    Ultraviolet radiation from the stars
                                                                                                      ionized the surrounding neutral hydrogen
                                                                                                 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




5   A few million years later, at the end of
    their brief lives, some of the first stars
exploded as supernovae. The most massive
                                                 6   Gravitational attraction pulled the
                                                     protogalaxies toward one another.
                                                 The collisions most likely triggered star
                                                                                                 7   Black holes possibly merged to form a
                                                                                                     supermassive hole at the protogalaxy’s
                                                                                                 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
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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
                                                 THE AUTHORS




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
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.

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                                                         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
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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.
T H I S H Y P O T H E S I S about early star
formation might help explain some puz-               MORE TO E XPLORE
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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