Stars are born in darkness and in secret. They form deep within clouds of interstellar gas and dust so dense and opaque that no visible light can escape, and so their birth has been kept well hidden from our earthbound telescopes. Unable to pierce the black veil, astronomers have had to content themselves with constructing, from basic physical principles, their own scenario for how a star takes shape.
The chief actor in their makeshift tale has been gravity: A clump of relatively dense gas in the center of a cloud drags in on top of itself more of the cloud, making the center a bit more dense. As more and more gas falls into the center, it grows and grows until it reaches a certain critical mass and collapses under its own enormous weight.
This appealingly straightforward “infall” theory has endured for decades with hardly any variation. More recently, however, unprecedented observations from a new generation of telescopes—most significantly, the Hubble and Spitzer space telescopes—have made the tale seem increasingly simplistic. For the first time, astronomers are getting glimpses of gas clouds in the process of becoming new stars. What they are finding is far more complicated, and picturesque, than they had expected. Not only does gas fall inward, but vast quantities of gas and dust also stream outward, away from the nascent star. Narrow jets of gas apparently race from the stellar cradle at extremely high speeds and stretch several light-years out into space. Even more remarkably, stars in the throes of birth also seem to exhale giant, peanut-shaped bubbles of gas, called outflows, 100 times more massive than our sun.
These spectacular outpourings of gas are among the most extraordinary surprises of modern astronomy. “The realization that jets and outflows might occur as part of the normal process of star formation just blew everybody away,” says Jon Morse, director of the Astrophysics Division at NASA headquarters. It also presented astronomers and physicists with a puzzling question: Why do newly forming stars simultaneously take matter in and spew it out?
Astronomers weren’t looking for these phenomena when they discovered them; they were looking instead, as they had been for many years, for confirmation of their theory that stars were indeed born from infalling gas. But the problems in obtaining such observational support were daunting. Aside from the opacity of the dark clouds that provide the raw stellar material, the birth of a star creates quite a mess for astronomers to sort through. The dark clouds almost never exist on their own but are usually a small part of an even larger complex of gas and dust known as a giant molecular cloud. These vast, turbulent structures span hundreds, even thousands, of light-years. Throughout such a cloud, stars are continually forming, often handfuls at a time in loosely packed groups. This abundance of activity produces a chaos of wisps, clumps, and knots of gas and dust that makes life very difficult for an observer on Earth.
Beginning in the 1940s, astronomers George Herbig and Guillermo Haro had been systematically sorting through this mess for some sign of infalling gas. In 1951 a particular type of debris caught their attention: bright knots of gas emitting so much visible light that they didn’t know what to make of them. For decades, astronomers watched these phenomena closely, keeping track of their positions in the sky. It wasn’t until 1979 that Herbig and a few colleagues announced the dramatic conclusion they had drawn from their observations, one that made the standard infall theory of star formation seem too pat. The knots of gas were traveling at more than 100 miles a second, far too fast to be propelled by the gravitational collapse of a dark cloud. Moreover, they were moving in the wrong direction—outward, away from the dark cloud, rather than inward.
For the next several decades Herbig, Haro, and other astronomers struggled unsuccessfully to explain the nature of these bright knots of gas, now called HH objects. Finally, in the late 1980s, new observations revealed the HH objects in far greater detail. They appeared not as knots of gas so much as large, distinct blobs that trailed away from the dark clouds like beads on a necklace. Threading these blobs, the telescopes revealed, were smaller, pencil-thin jets of bright gas being driven out of the clouds.
These discoveries posed an even bigger conundrum for astronomers than the original sighting of the HH objects because the geometry of the jets was so peculiar. What physics could cause a thin stream of gas to come squirting out of a basically spherical cloud of collapsing gas? The jets stretched more than a light-year out from the cloud while remaining absurdly narrow. In scale, they were akin to a tight stream of water shot five miles into the sky by a garden hose.
While astronomers pondered the jets, another discovery further deepened the mystery. Astronomers had been working throughout the 1970s on new radio telescopes that operated on millimeter-length waves. Since these telescopes would be able to pierce the dark clouds that had kept stellar birth hidden from direct observation, researchers hoped that they would reveal infalling gas and confirm the timeworn infall theory. Instead they almost immediately uncovered vast outpourings of gas ballooning out on opposite sides of the forming stars and stretching much farther away from them than the jets. Although these bipolar outflows were moving much more slowly than the HH objects—about 10 miles per second—they were 10 times as massive as the jets, with a volume many times larger. They were also much colder and much older. By measuring the velocity of the gas and its distance from the dark cloud where it originated, estimating its age was a simple matter. Whereas the jets appeared to have been around for only 1,000 years or so, the oldest outflows had been streaming from the nascent stars for more than 100,000 years. Since stars take only 100,000 to 1 million years to form, the outflows seemed to play an integral role in the stars’ creation.
The jets were akin to a tight stream of water shot five miles into the sky by a garden hose.
To a scientist, nothing is more troubling, or more exciting, than a diversity of phenomena without a unifying cause. Most astronomers felt that some common physical explanation underlay both the jets and the outflows. The most natural one was also the most obvious: that the jets were somehow powering the much larger, and much more distant, bipolar outflows. As the jets plowed through the cloud surrounding the fetal star, perhaps they swept up molecules of gas or pulled the gas along, piling it up into vast peanut-shaped outflows. A big problem with this theory was that the jets seemed so young. “It was very puzzling,” Morse says. What was even more confusing was that the jets did not seem to carry nearly enough matter or force to create the massive bipolar outflows.
Astronomers set out to test this theory, finding another way of measuring the age and size of the jets. Researchers thought that the answer might come from an analysis in terms of quantum mechanics and the physics of shock waves. A shock wave is nature’s way of slamming on the brakes. It occurs whenever a supersonic stream of fluid, such as hot gas from a forming star, strikes an obstacle, such as older, slower-moving gas, in its path. As the fast-moving atoms of the stream slam into the slower-moving atoms of the obstacle, they dissipate most of their kinetic energy as heat. As a result, the stream undergoes a violent deceleration, and temperatures rise accordingly. At the same time, some of the energy is absorbed by the colliding atoms, then reradiated in the form of photons, or particles of light. The rules of quantum mechanics give atoms discrete ways to absorb energy in collisions or lose it to photons. Thus astronomers can interrogate the photons to reveal the basic physical properties, like velocity and density, of gas passing through shock waves in the jets. Astronomers call the light emitted by shocks “diagnostics” and use information locked in the light they gather to infer conditions in the star formation outflows thousands of light-years away.
Morse and other astronomers have used computer models of shock waves to make detailed predictions of the diagnostics. By comparing their predictions with the actual light as measured with the Hubble Space Telescope, Morse has concluded that the jets are 100 times denser than previously estimated. In other words, the jets spout the equivalent of one Jupiter’s worth of matter into space every day. If this is true, then the jets indeed emit enough mass with enough power to have created the gigantic outflows that project far beyond them into space.
Furthermore, Morse’s observations suggest that the giant outflows form when the faster material from the jets catches up with older, slower material in its path. If so, it would mean that the outflows consist in part of older material that was originally part of the jets, making the jets much older than previously thought. “Having older material already out there means that the jets go much farther out than what we see in their brightest structures,” Morse says. In addition, John Bally, an astronomer at the University of Colorado in Boulder, has found a superjet—a string of HH objects 23 light-years long—that he thinks may be 100,000 years old. Morse believes these findings establish a definite link between jets and outflows.
The gas, rather than simply falling in, spirals around the star, ?spinning faster and faster as it approaches the center.
Showing the relationship between jets and outflows is only the first step toward a new understanding of how stars form. The ultimate goal is to explain what role these phenomena play in the life of the embryonic stars themselves. Before scientists could begin to postulate a mechanism, however, they first needed to come to an understanding of precisely where the old infall model of star formation, in which gravity plays the starring role, needed fixing.
Even though astronomers had failed repeatedly in their efforts to find incontrovertible evidence of a collapsing gas cloud on its way to becoming a star, they made great strides in refining their ideas about what might happen when a gas cloud collapses. First of all, they realized that the dark cloud feeding the star should have been set spinning by small, random motions left over from its formation in the giant molecular cloud of which it was a part. As a result, the gas, rather than simply falling straight in and onto the protostar at the center of the cloud, spirals around the star, spinning faster and faster as it approaches the center. This realization presented yet another theoretical difficulty. Astronomers wondered, what makes the cloud slow its spin enough to allow the matter to drop into the new star?
The conservation of angular momentum, the law that explains why ice-skaters spin faster as they pull their arms in toward their torsos, holds equally true for spinning gas clouds, except that the scale is more dramatic. “Skaters change their size by about a factor of two,” says Lee Hartmann, an astronomer at the University of Michigan, “but these clouds shrink by a factor of a million.” As the cloud contracts and its spin accelerates, eventually it whips around so fast that its outward centrifugal force should be sufficient to cancel out the inward pull of gravity. The gas stops falling toward the star. This equilibrium of gravity and angular momentum seemingly presents a fundamental physical barrier that should freeze the development of a new star, preventing further infall of gas before the star could ever be born.
Observational astronomers partially solved this problem by postulating the existence of gaseous disks. The idea was that the centrifugal force created by the cloud’s spin causes the collapsing cloud to assume the shape of a disk. The effect is similar to what happens when a ball of dough is spun into a pizza crust—centrifugal force pushes material at the poles out to the equator. As the disk spins around, the explanation went, gas gradually inches its way toward the center, eventually reaching the inner edge of the disk and dropping onto the hungry star. “You shouldn’t think of a star starting at a large radius and contracting, but instead think of these very tiny seeds that are built up by accretion of material that first passes through the disk,” says Stephen Strom, an astrophysicist at the National Optical Astronomy Observatory. Because of this gradual accretion of matter to the star, astronomers call these gaseous Frisbees accretion disks. “The disks become reservoirs that hold the cloud’s angular momentum,” says astronomer Suzan Edwards of Smith College. “As the gas in the disk rotates, it has time to shed its angular momentum and slow down enough for gravity to drag it inward in a long, contracting spiral.”
Exactly how the disk manages to shed its angular momentum, however, remained a mystery. Friction among the gas atoms in the disk is not enough to dissipate the huge amount of energy stored in the disk’s rotation. The only way nature can reduce the angular momentum of an accretion disk is to shuck off huge quantities of matter. If a skater spinning his partner suddenly lets go, for instance, the partner gets thrown away, carrying with her most of the angular momentum. As astronomers mulled over the growing evidence for jets and outflows, they began to suspect that the accretion disks were doing something similar. The major impediment to incorporating this idea into a theory of star formation was the problem of the peculiar geometry of the jets. Shouldn’t matter being thrown off an accretion disk travel outward in all directions along the plane of the disk? The jets and outflows, by contrast, seemed to throw matter up and down along the disk’s axis of rotation. The idea that the jets and outflows came directly from the disk and served to rid it of angular momentum seemed as absurd as a spinning ice-skater letting go of his partner and having her shoot straight up into the air.
Yet that absurdity may disappear when you take into account one more cosmic phenomenon: magnetic fields. These, after all, are found almost everywhere in space and are powerful enough to shape much of what goes on there. They create sunspots, control the rippling curtains of Earth’s auroras, and give pulsars their pulse. Giant molecular clouds and the dark clouds contained in them also possess powerful magnetic fields. Could those fields power? the jets and outflows too?
The most promising of the magnetic field theories, developed by astrophysicist Arieh Königl of the University of Chicago, postulates a magneto-centrifugal wind that flings matter up from the disk into the jets. Königl starts with the standard assumptions that the dark cloud from which a star is born possesses a magnetic field and that in the immediate vicinity of the new star the direction of the field is consistent: If you drew lines indicating the orientation of the magnetic field, they would all run parallel. The rotation of the gas in the disk reinforces this field. The gas in the accretion disk is hot enough for some of its atoms to lose electrons and become ionized—that is, to take on a positive electric charge. At the same time, as the cloud collapses, the magnetic field lines get compressed along with the gas and end up embedded in the disk. They form a sort of hourglass shape, much as stalks of wheat would look if you tied them in the middle.
With this magnetic field in place, the stage is set for matter to come streaming off in jets. As the disk accelerates its spin, its centrifugal force increases so it begins to overcome the gravitational pull of the young star at its center, and gas molecules near the disk’s surface are flung off. Since charged particles tend to follow magnetic field lines, moving along them in a sort of corkscrew motion, the gas molecules fly not only outward but also upward and downward along the magnetic field lines.
While this model contains some significant uncertainties, its aesthetic appeal has won it many converts among astronomers. By ridding the disks of angular momentum, the winds solve two problems: They not only power the jets but also slow the rotation of gas in the disk enough to let it make the final hop onto the star. This theory also explains why the jets appear to be beadlike. As the accretion disk spins faster and its centrifugal force stops matter from falling in, a clump of gas gets thrown off the disk and up into the jet. The loss of matter slows the disk down, allowing more matter to move through the disk and toward the center. That transference of mass, like the skater’s arms coming in toward his body, serves to accelerate the disk once more. As this process of fits and starts is repeated, matter is fed into the jets in discrete chunks. In addition, astronomers have conjured some tricky mathematics to show that the magnetic field lines contract and twist as they get farther and farther away from the disk. If so, this would explain why the jets are so tightly focused.
With the disk-wind theory, Königl says, “it no longer seems like an accident that we see all those outflows in connection with star formation.” The theory gives astronomers the feeling that, if they had just thought hard enough about it beforehand, they could have predicted the existence of outflows even before they observed them. This kind of 20-20 hindsight is the mark of a powerfully descriptive theory.
The journey of a new star from the turbulent chaos of the giant molecular clouds to the serene constancy of maturity seems to contain many of the structures found elsewhere in the universe. Accretion disks have been observed around many white dwarfs and neutron stars, and they seem to fuel black holes at the center of quasars. Jets, too, can be seen traveling at close to the speed of light from the center of active galaxies and stretching millions of light-years into space; in fact, the disk-wind theory was developed to explain these extragalactic jets. The similarity between the structures of newborn stars and the eruptions of quasars offers astronomers a golden opportunity to study the mechanisms of these more distant phenomena. And the unity that astronomers seek in their theories of star formation may even provide some clues to a grand synthesis. In the end, star birth, which started off as a dark mystery, may well end up casting light on some of the most dramatic, violent, and poorly understood events in the cosmos.
