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Out of the dark ages: the first stars: the earliest stars were powerhouses that paved the way for future stellar generations, along with planets and life.

A hundred million years after the Big Bang, the universe must have offered an utterly stunning and unfamiliar sight: total darkness. This was the cosmic Dark Ages, an epoch still shrouded in mystery, when no planets, stars, or galaxies had yet formed. Slowly and inexorably, gravity would pull clumps of primordial material together, providing the conditions for the first stars. We now think that these power houses exhibited extreme masses and luminosities, and that they transformed the peaceful infant universe into a seething cauldron of activity.

This crucial cosmic transformation occurred because the first stars produced prodigious amounts of ultraviolet light that ionized the universe's hydrogen. And when the stars died, after only 2 or 3 million years of existence, they probably triggered the most powerful explosions our universe has ever witnessed: hypernovae with energies up to 100 times greater than any supernova exploding today.

Population III stars, as the first stars are known, exert a special

kind of allure (see "Populations I, II, and III," page 33). Once you begin to study them, you cannot let go. Theorists like me strive to determine their properties and how they differed from the stars of today. Observers push the limits of instrumentation to hunt these stars down.

The race to observe Pop III stars has been heating up. In September 2005 NASA's Swift satellite detected an extremely distant gamma-ray burst, signaling the death of a massive star about 12.8 billion years ago (January issue, page 48). Late last year scientists using NASA's Spitzer Space Telescope announced that they had found significant fluctuations in the cosmic background glow of infrared light, possibly originating in part from Pop III stars (February issue, page 17). Cosmologists are intensely debating whether these discoveries are signposts of the first stars.

MAKING A POP III STAR

Population III stars formed in an environment very different from typical star-formation sites in our current Milky Way Galaxy. Theorists predict that these early stars formed in roughly spherical clouds with 100,000 solar masses of gas gravitationally held together by dark matter. These mini halos contained only the simplest chemical elements, which were synthesized in the Big Bang: atomic hydrogen (H) and helium with only a trace of molecular hydrogen ([H.sub.2]). Despite a level of only one H2 molecule per 1,000 H atoms, the presence of H2 had momentous consequences.

[H.sub.2] was the only available coolant in the low temperatures of the Dark Ages (H acts as a coolant only at much higher temperatures). It allowed the minihalo gas to cool by radiating heat, reducing a cloud's outward pressure. But the remaining outward pressure was still much larger than the pressure in today's star-forming clouds. The force of gravity, and therefore a cloud's mass, had to be much larger to overwhelm this pressure. Primordial clouds thus collapsed and fragmented into dense clumps, which contained several hundred solar masses. Present-day clumps generally have only a few solar masses.

Earlier this decade two independent teams used sophisticated computer simulations to determine that these dense clumps spawned extremely massive stars, ranging from perhaps 30 to 500 solar masses. One group consisted of Paolo Coppi, Richard Larson, and me, then working at Yale and Harvard. Tom Abel, Greg Bryan, and Michael Norman, then working at the University of Illinois and the Max Planck Institute for Astrophysics in Germany, formed the other group. We engaged in a hot scientific race. With hindsight it's fair to say that the two groups stimulated each other and that we reached agreement on the main result: the first stars were typically very massive. All of us felt the exhilarating thrill of discovery.

Once this first generation of stars had formed, about 150 to 200 million years after the Big Bang, they synthesized heavy elements and then spewed this inventory into space at the ends of their lives. These heavy elements prevented such massive stars from ever forming again, except perhaps in rare stellar mergers (April issue, page 36). When a massive star forms today, its intense outpouring of radiation pushes against infalling material, which is enriched in heavy elements and dust that are more easily blown away by radiation pressure. As a result, this process shuts off further accretion, limiting a star's mass to perhaps 120 Suns at most. But Pop III stars formed in clouds that lacked dust and heavy elements, so radiation pressure did not stop accretion until the objects voraciously accumulated up to 500 solar masses of gas in the most extreme cases.

After our discovery we immediately asked two follow-up questions: How would such a population of very massive stars influence subsequent cosmic history? And how can we observe Pop III stars to put our theories to the test?

TRANSFORMATION

By analyzing spectra of distant quasars found by the Sloan Digital Sky Survey, astronomers know that the universe experienced a critical phase transition within the first billion years of its history (April issue, page 20). Intergalactic gas clouds evolved from a neutral state to an ionized state. In a neutral gas, electrons are bound to atomic nuclei; in an ionized gas, electrons have been stripped from atomic nuclei and are thus free to roam about. The Dark Ages were cold and electrically neutral, because no sources of heat and ionizing photons yet existed. Our present-day universe, on the other hand, consists mostly of ionized gas.

It came as a big surprise when astronomers learned in 2003 from NASA's Wilkinson Microwave Anisotropy Probe (WMAP) that at least some of the ionization reached back to the epoch of the first stars. WMAP detects cosmic micro wave background (CMB) photons, which were emitted before the Dark Ages began, roughly 400,000 years after the Big Bang--when the universe had cooled enough to allow electrons to combine with protons to form atoms for the first time. The CMB photons traveled largely unimpeded for 13.7 billion years before ending up in WMAP's microwave detectors. En route, CMB photons were slightly polarized whenever they collided with free electrons. WMAP measured relatively strong polarization, meaning large numbers of free electrons existed in the early universe, presumably because ultraviolet radiation from Pop III stars had stripped them from hydrogen atoms.

With surface temperatures exceeding 100,000[degrees]C and luminosities of millions of Suns, these massive stars emitted a barrage of ultraviolet photons. Even a spatial density of just a few hundred Population III stars within a sphere 3 million light-years across would have produced enough photons to ionize all of the early universe's hydrogen. But ionization wasn't the only way that Pop III stars transformed the early universe.

THE FIRST SUPERNOVAE

Population III stars profoundly altered the universe when they exploded as hypernovae (hypernovae is a generic term for stellar explosions that are considerably more powerful than supernovae). These extraordinarily violent blasts dispersed heavy elements, preventing additional Pop III stars from forming. But they initiated the crucial process of chemical enrichment that would ultimately make planets and life possible.

Normal, low-mass stars began to form once clouds were sprinkled with heavy elements to a level above 1/10,000 the amount found in our solar system. Hundreds of individual supernovae probably contributed to the heavy elements that make up Earth. But with its colossal energy, a solitary Population III hypernova dispersed enough heavy elements to enrich more than 10 million solar masses of gas to beyond the critical level.

In the 1960s Fred Hoyle, William Fowler, and others realized that stars with about 140 to 260 solar masses would be completely blown apart by a hypernova--without leaving behind a collapsed stellar remnant. In contrast, a normal core-collapse supernova buries a large inventory of its heavy elements in a black hole or neutron star. But the first hypernovae dispersed all their heavy elements into the surrounding gas. This process rapidly transformed the universe to one dominated by low-mass stars.

This type of hypernova is also called a pair-instability supernova because of the mechanism that triggers the explosion. Massive stars are supported against gravity by the outward pressure of photons originally produced by fusion reactions in their cores. During a very massive star's later evolutionary stages, when it begins fusing oxygen in its core, the photons become so energetic that they can literally transform themselves (via Einstein's equation E = [mc.sup.2]) into pairs of matter and antimatter particles (electrons and positrons). Since particles in a very hot star provide less pressure to counterbalance gravity, the star implodes, leading to rapid compression and a corresponding jump in temperature that triggers a thermonuclear explosion. All the remaining nuclear fuel ignites at once, producing a blast that completely destroys the star.

THE SEARCH FOR THE FIRST STARS

Theorists like me feel confident that our ideas on Population III stars are on the right track, but observers are developing two broad strategies to put them to the test. Some astronomers are trying to make direct observations of the first stars and their explosions in the very early universe. Others look for Pop III "fossils" in our cosmic backyard.

Unfortunately, astronomers have not yet built telescopes large enough to detect Population III stars, whose light is extremely faint and highly redshifted by cosmic expansion. Even Hubble's successor, the James Webb Space Telescope (JWST), won't have a mirror large enough to see individual Pop III stars. But one recent study claims the direct detection of collective light from Pop III stars.

Using NASA's Spitzer Space Telescope, Alexander Kashlinsky (Science Systems and Applications, Inc.) and three collaborators studied the cosmic near-infrared background, a glow that pervades the entire sky. They found fluctuations in the background that are much stronger than those expected from known distant galaxies. Kashlinsky's group interprets these fluctuations as circumstantial evidence for Pop III stars. But their case is not yet airtight. The connection between the first stars and the infrared background holds considerable promise, however, and both theorists and observers are working feverishly to ferret out the Pop III component in deep Spitzer images.

Even if we cannot observe an individual Pop III star while it's alive, the hypernova that ends its life should be bright enough that JWST can easily observe it. With its planned 6.5-meter mirror, JWST should find hundreds of Pop III hypernovae per square degree surveyed in one year.

A second method to probe the death of the first stars is being carried out right now with NASA's Swift satellite, which detects gamma-ray bursts (GRBs). Most GRBs are triggered when a rapidly rotating massive star collapses and explodes as a Type Ib or Ic supernova. Abraham Loeb (Harvard-Smithsonian Center for Astrophysics) and I recently calculated that at least 10 percent of the bursts that Swift will detect should originate from the universe's first billion years. The Swift observations so far have confirmed our prediction, with two GRBs from the first billion years out of approximately 20 bursts with measured distances. But eventually Swift should catch extremely distant GRBs from Pop III stars, which will enable us to test various theoretical models for the number of Pop III bursts.

STELLAR ARCHAEOLOGY

Another probe of the Pop III era can be termed "stellar archaeology." The basic idea is to scrutinize the abundances of chemical elements in our galaxy's oldest stars to see if they contain the signature of Pop III hypernovae. In the past few years the Hamburg/European Southern Observatory (ESO) Survey discovered two halo stars that contain less than 1/100,000 the iron (relative to hydrogen) found in the Sun. But these two stars, HE 0107-5240 and HE 1327-2326, have carbon and oxygen abundances 1/10 of the Sun's. This peculiar composition had never before been observed.

Ken'ichi Nomoto and Hideyuki Umeda (University of Tokyo) have proposed that primordial stars with 30 to 40 solar masses each died in weak, black-hole-forming supernovae, which accounts for the weird abundance pattern. Such a star's mass is not large enough to trigger a pair-instability supernova, but it's larger than those that produce standard core-collapse supernovae in today's universe. The explosion stalls as the shock wave plows through the envelope, and most of the star's mass falls back into a black hole. Only the outermost envelope escapes. But that envelope is full of carbon and oxygen, whereas all the heavier nuclei, all the way up to iron, are buried in the black hole. That could account for the chemical abundances observed in HE 0107-5240 and HE 1327-2326.

My collaborators and I have instead proposed that the Hamburg/ESO stars are third generation rather than second generation. Their element distribution would thus contain the signature of second-generation stars, and not that of Pop III stars alone. Also, since we only have two examples right now, it's hard to assess whether these two stars are typical or oddballs. Stellar archaeology has begun to provide deeper understanding of Pop III stars, but its great promise will be fully realized in the near future with even larger surveys of stars with low heavy-element abundances.

INTO THE FUTURE

Reconstructing the universe's history from the Big Bang to the present has been one of the greatest adventures of the human mind. Once we have elucidated what happened at the end of the Dark Ages, which we may achieve with JWST and other future telescopes, we will have closed one of the remaining gaps in our knowledge of cosmic history. Then we will be able to chart the entire cosmic timeline, and in a sense, our age-long quest will have come to an end.

Will that spell the end of cosmology? Not at all. If past experience offers any guide, every answer poses new questions. Science is the ultimate perpetual-motion machine, always propelling us to new frontiers. We can only speculate what kind of exciting new questions we will pursue 20 years from now. Stay tuned!

Populations I, II, and III

The concept of stellar populations has its origins in the World War II blackouts of Los Angeles, California. German emigre Walter Baade (pictured below), who was declared an enemy alien and restricted to the Los Angeles area, used the temporarily dark skies at the Mount Wilson Observatory to make a major discovery. While using his almost unlimited telescope time to observe the Andromeda Galaxy (M31), he resolved two distinct types of stars: blue, young stars in the galaxy's disk and red, old stars in the central bulge. He termed the former "Population I" and the latter "Population II."

Astronomers later realized that this sequence is connected to the buildup of heavy chemical elements over cosmic history. Pop II stars actually formed before Pop I stars, out of material that was not yet strongly enriched with elements heavier than helium. Pop stars, which include our Sun, are born in clouds that have been highly enriched.

Slowly, however, an important fact came into focus: Pop II stars, though underabundant in heavy elements, still contain some of these materials. Truly primordial material from the Big Bang consisted purely of hydrogen and helium, plus a smattering of lithium. A different stellar population must have existed near the beginning, one that formed with no heavy elements at all. In the early 1980s Howard Bond (now at the Space Telescope Science Institute) and others termed this mysterious first generation of stars "Population III."

A Pair-Instability Supernova vs. a Core-Collapse (Type II) Supernova

Pair-instability supernovae and "normal" supernovae are both triggered when the cores of massive stars can no longer produce photons. But after that, the similarity ends. Pair-instability supernovae explode with up to 100 times more energy, and because the entire star is blown apart, no black hole or neutron star is left behind.

Volker Bromm is an astrophysicist at the University of Texas, Austin.
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Author:Bromm, Volker
Publication:Sky & Telescope
Geographic Code:1USA
Date:May 1, 2006
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