Cosmic Dawn

Pinning down the origin and nature of the first stars

Most parents will tell you that the arrival of their firstborn was a life-altering event. Abraham Loeb, a first-time father and cosmologist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., is no exception. Standing in front of an audience of NASA astronomers one recent Friday afternoon, Loeb begins his lecture by flashing a slide of his 1-year-old daughter, Klil. “It’s always fascinating to study the infant universe,” he jokes.

RISING STAR. Top row: Red and yellow filaments depict the formation (left to right) of the first microgalaxy in the universe. Middle row: At the center of the microgalaxy, gas condenses (left to right) from a cloud as massive as 1,000 suns to the first star (white dot in right panel). Bottom row: Three million years after its birth, the first star in the universe dies in a supernova explosion, destroying the microgalaxy in which it formed. Abel, courtesy of The Discovery Channel
BIRTH OF A HEAVYWEIGHT. Simulation shows material from a gas cloud (yellow, red, purple) about to collapse into one of the first stars in the universe. By itself, the densest gas (yellow core) would form a star a few times as massive as the sun. But lower-density gas (red) rapidly piling onto the core results in a star about 100 times as massive as the sun. Bromm, Coppi, and Larson

Loeb says he now views his life in two parts–before and after the birth of his daughter. Similarly, he divides the history of the universe into two eras–before and after the birth of the first stars.

New, sophisticated computer simulations of those baby stars are providing a more accurate estimate of when they formed and how massive they were. Characterizing these first stars is crucial, notes Volker Bromm of Harvard-Smithsonian, because they not only lit up the cosmos, but also profoundly influenced the birth and evolution of galaxies.

Ten or 15 years ago, he notes, most cosmologists assumed that galaxies formed wholesale, during a very limited window of time, and they devoted much of their research to pinpointing that era. Astronomers now accept that galaxy formation is an ongoing process, with smaller galaxies forming first and later merging to build more massive ones. “But there was a group of the very first stars, and they are the beginning of this chain” of galaxy formation, says Bromm. “These stars set the stage for everything that happens afterwards.”

Unveiling the nature of the first stars, he adds, will help solve cosmology’s fundamental riddle: How did an almost perfectly smooth, dark universe become lumpy and littered with stars? Although the first stars lived and died billions of years ago, their explosive demise may have left a calling card that telescopes on Earth can detect, he adds.

Stellar models

Before astronomers can hope to successfully scour the universe for signs of the first stars, they have to know exactly what to look for. That’s where modeling comes in. Star birth, especially as it happens in the tumult of the universe today, is notoriously difficult to model. Several features of the first stars simplify that challenge, notes Tom Abel of Pennsylvania State University in State College. Theorists only have to keep track of hydrogen, helium, and small amounts of a few heavier elements, since these were the only atoms forged in the Big Bang. Unlike later episodes of star formation, there was no dust, stellar winds, magnetic fields, or cosmic rays to complicate star birth.

The fireball that begat the universe some 14 billion years ago left it searingly hot and blindingly bright. After about 300,000 years, however, the expanding universe cooled sufficiently for the radiation left over from the Big Bang to shift to longer, invisible wavelengths and travel freely into space. The cosmos’ brilliant beginning ended, giving way to an era of blackness. Nearly structureless and featureless, the universe remained in the murk for millions of years. But even in the darkness, the tiny, primordial fluctuations in the density of matter were growing bigger.

A supercomputer simulation of the early universe developed by Abel and his collaborators assumes the existence of cold dark matter–slow-moving, invisible material that is believed to make up 90 percent of the mass of the universe. According to the cold-dark-matter scenario, small objects formed first and then coalesced into galaxy-size bodies.

The simulation by Abel, Greg L. Bryan of the Massachusetts Institute of Technology, and Michael L. Norman of the University of California, San Diego begins 13 million years after the Big Bang. That’s when cold dark matter started to clump and form invisible halos in regions that contained slightly higher-than-average concentrations of matter.

After about 100 million years, cold-dark-matter halos weighing about 100,000 times as much as the sun had begun to form, according to the team’s model, which was described in the Jan. 4 Science. That was a watershed, notes Abel,

because cold-dark-matter halos this massive are the first to pull in and confine small amounts of hydrogen and helium gas–the stuff of which the first stars were made.

That was no mean feat because gravity had to overcome the outward pressure exerted by the gases, which became ever more compressed and hot as they were jammed into a smaller volume.

To make stars, compressed gas must grow even denser. To do so, it must cool down to a few degrees above absolute zero. But in order to cool, the gas must first become relatively hot.

Because these first halos of matter weren’t very massive, they compressed the trapped gas only slightly, and the material therefore never reached a temperature much greater than 1,000 kelvins. At that temperature, the gas has enough energy to excite molecular hydrogen but not atomic hydrogen. The excited molecules cool the gases by converting their heat into radiation, which escapes into space. Nevertheless, molecular hydrogen is a much poorer coolant than atomic hydrogen.

The simulations show that the clouds of gas within each of the cold-dark-matter halos weigh as much as 1,000 times the sun–an early, lightweight version of the giant molecular clouds that would later give birth to thousands of stars in the Milky Way and countless other galaxies. At the very core of each cloud, a chunk of material about as massive as the sun rapidly condensed. But before this small, relatively dense bit of material had a chance to turn into a relatively lightweight star, gas as massive as 100 suns piled on top of it. It, too, was cooled by molecular hydrogen.

The entire process, from first halo to first star, happened so rapidly–in no more than about 10,000 years–that there was no time for the material to fragment. Instead, a single massive star was born. As similar stars flashed into existence across the universe a few hundred million years after the Big Bang, the cosmic Dark Ages ended.

These first stars, each weighing 50 to 300 times the mass of the sun, “are very rare beasts,” notes Bromm. “If we could look at the universe then, it would look very, very boring, dark with a very few beacons of light.” Bromm, along with Paolo S. Coppi and Richard B. Larson of Yale University in New Haven, Conn., described their model in several recent articles in the Astrophysical Journal.

But that’s just part of the story told by the new models. Although these stars illuminated the universe, they didn’t last long. Like most massive stars, they shined at a furious rate and burned out quickly, after just 3 million years. The supernova explosions that ended the brief lives of this first generation of extremely massive stars were 100 times more powerful than the famous supernova 1987A, which astronomers witnessed in a nearby galaxy 15 years ago.

The early explosions had several consequences for future generations of stars and the formation of the first galaxies, notes Abel. These early supernovas forged and spewed into space the first elements that astronomers think of as metals–any atom heavier than helium. By polluting space with metals at such an early time, the first stars altered the composition of the raw material available for making new stars. Metals also radiate heat away better than hydrogen can, providing a more efficient way for compressed gas to cool and form stars.

The early seeding of space with metals dovetails with observations of the spectra of light from distant quasars. Light from these brilliant beacons pierces numerous gas clouds and fledgling galaxies as it travels to Earth, and the radiation absorbed by these intervening systems indicates their composition.

So far, no matter how far back in time astronomers have looked using the most distant quasars known, they’ve yet to find a cloud devoid of metals.

Along with the newly forged metals, the first supernovas also blew out 99 percent of the gas that hadn’t yet condensed into stars. The eruptions scattered matter across the universe and halted star formation for several million years.

Our very existence may depend on such an interruption, says Martin Rees of the University of Cambridge in England. If most gas had condensed quickly into stars and stayed there, the universe would have rapidly used up its star-making material. Today the cosmos would be full of elderly, red stars in dwarf galaxies, which were the first galaxies to form. Large spiral galaxies such as our Milky Way, which are rich in gas, would be rarities rather than the rule.

Super cool

According to Abel and his colleagues, the cold-dark-matter halos continued to grow bigger regardless of the supernova explosions or any other nongravitational force. That’s because cold dark matter, say theorists, reacts only to gravity and is impervious to pressure.

A few million years after the first stars exploded, each halo weighed as much as 100 million suns, roughly the mass of a dwarf galaxy today. Halos this massive mark another milestone in star formation, Abel says, because they could pull back the gas dispersed by the supernovas and compress it to temperatures as high as 10,000 kelvins. Gas that hot can cool much more efficiently because it has enough energy to excite atomic hydrogen.

Because atomic hydrogen is such a good coolant, much more of the gas condenses into stars than was possible during the very first episode of star formation.

This time, a few hundred thousand stars form–the equivalent of a globular cluster. It would still take a few billion years for spiral and elliptical galaxies, as seen around us today, to assemble. On the other hand, some of these early objects would resemble the smallest galaxies that now surround the Milky Way. These blobby halos and the stars they contained–some 500 million years after the Big Bang–could be considered the first galaxies in the universe, Bromm says.

These objects are the targets of NASA’s Next Generation Space Telescope (NGST), the proposed successor to the Hubble Space Telescope. When it launches about a decade from now, NGST may glimpse clusters of some of the very first stars in the universe. Groups of these stars would have emitted copious amounts of ultraviolet light, which would have then been absorbed by surrounding hydrogen gas and reemitted at longer wavelengths.

A near-infrared spectrograph on NGST should have the capability to detect this radiation, says Nino Panagia of the Space Telescope Science Institute in Baltimore and the European Space Agency. The spectrograph should also detect the small amounts of carbon and oxygen–one part in a hundred, relative to hydrogen–that these first stars would have produced when they exploded.

But astronomers may not have to wait a decade to find far more dramatic, albeit indirect, signposts of the first stars. Their explosive deaths may have left behind signals that can be detected with current telescopes. According to one popular theory about gamma-ray bursts, these energetic flashes of radiation form when an extremely massive star explodes as a supernova. The explosion is so powerful that the cinder left behind by the supernova turns into a black hole.

The explosive deaths of the first generation of stars could therefore be detected today as gamma-ray bursts.

Several researchers agree that some of the bursts now reaching our planet may have come from the earliest stars to form in the universe and then traveled for nearly 14 billion years. Bromm and Loeb calculate that the first 15 percent of stars that formed in the cosmos could be responsible for a significant fraction of all gamma-ray bursts that may be observed from Earth.

Telescopes such as the Compton Gamma Ray Observatory, which orbited Earth from 1991 to 2000, have detected thousands of bursts. A few of them might already have come from the first stars, says Loeb. Although astronomers can only study the bursts briefly, their afterglows–the radiation emitted at X-ray, visible-light, infrared, and radio wavelengths–can last for weeks to months.

Detecting the afterglow of a distant gamma-ray burst isn’t as daunting a task as it might at first appear. For a steady source of light, such as a quasar, the greater the distance and the farther back in time the light was emitted, the fainter it appears. But the afterglow isn’t steady–it’s brightest just after the burst ends. That turns out to be a fortunate happenstance.

Because of the expansion of the universe, an observer on Earth views light emitted by a distant, receding object as being stretched, or shifted, to longer wavelengths. The more distant the object, the greater the stretch. Likewise, the same observer also views the interval of time between distant light signals as being longer, as if time had slowed.

The afterglow from a distant gamma-ray burst therefore takes longer to fade than the afterglow of a nearby burst, and astronomers have a better chance of catching the remote afterglow when it’s at its brightest. For an extremely distant gamma-ray burst, this time-stretching effect counteracts the decrease in brightness that results from the burst’s great distance.

A property of the early universe may also aid in the detection of gamma-ray bursts it generates, Loeb says. If a burst originated so far back in time that stars and quasars hadn’t yet ionized the vast reserves of atomic hydrogen in the universe, then its afterglow would have a large absorption gap, or trough, in its spectrum. Bursts that occur later, after hydrogen has ionized, don’t have such a gap. That phenomenon, known as the Gunn-Peterson effect, indicates the absorption of the burst’s ultraviolet afterglow by unionized atomic hydrogen.

The spectra of some extremely distant quasars, in fact, show the Gunn-Peterson effect (SN: 8/11/01, p. 84: Light’s Debut: Good Morning, Starshine!).

Next year, NASA plans to launch a gamma-ray-burst observatory called Swift, which will have the capability to record, on average, one burst a day and pinpoint its position in the sky. Among the hundreds of bursts that will be detected by Swift each year, it’s likely some will have come from objects that originated deep in space and far back in time, Loeb says.

“It would be a wonderful thing if we were so lucky that the first stars that formed in the universe also produced the brightest signposts,” says Abel.

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